# HIFreeTools

> Free, fast online tools — calculators, unit converters, text and developer utilities, finance and health calculators, and random generators. Most tools run entirely in your browser with no sign-up and nothing uploaded. Results are general information only and are not professional (financial, medical, legal, or tax) advice.

Site: https://hifreetools.com
Operator: HIFreeTools, an independent project by a solo developer based in South Korea.

## Calculators
- [Basic Calculator](https://hifreetools.com/calculators/basic-calculator): A free online calculator for everyday math — add, subtract, multiply, divide, and use parentheses with correct order of operations. In your browser.
- [Volume, Area & Depth Calculator](https://hifreetools.com/calculators/volume-area-depth): Relate volume (m³) and area (m²) using a depth: Area = Volume ÷ Depth. Enter any two of volume, area, or thickness. Free, in your browser.
- [Wolfram MathWorld — Arithmetic Mean](https://hifreetools.com/calculators/average-calculator): Calculate the mean, median, mode, range, sum, and count of any list of numbers. Free, instant, with the formulas shown. Paste your numbers and go.
- [Wolfram MathWorld — Percent](https://hifreetools.com/calculators/percentage-calculator): Calculate a percentage of a number, what percent one value is of another, and percentage increase or decrease. Free, instant, with the formula shown.

## Unit Converters
- [Acres to Square Feet Converter](https://hifreetools.com/converters/acres-to-square-feet): Convert acres to square feet instantly. 1 acre = 43,560 sq ft. Essential for real estate, land surveys, and property dimensions.
- [Bar to PSI Converter](https://hifreetools.com/converters/bar-to-psi): Convert bar to PSI instantly. 1 bar = 14.5038 psi. Handy for tyre and tube pressures, diving, and equipment rated in bar.
- [Calories (kcal) to Kilojoules Converter](https://hifreetools.com/converters/calories-to-kilojoules): Convert calories (kcal) to kilojoules instantly. 1 kcal = 4.184 kJ. Useful for comparing US/UK calorie figures with kilojoule nutrition labels.
- [Celsius to Kelvin Converter](https://hifreetools.com/converters/celsius-to-kelvin): Convert Celsius to Kelvin easily. Add 273.15 to your Celsius value. Essential for science, chemistry, and thermodynamics.
- [Cubic Meters to Liters Converter](https://hifreetools.com/converters/cubic-meters-to-liters): Convert cubic meters to liters instantly. 1 m³ = 1,000 L. Useful for water tanks, construction volumes, and capacity.
- [Cups (US) to Milliliters Converter](https://hifreetools.com/converters/cups-to-ml): Convert US cups to mL for any recipe. 1 US cup = 236.6 mL. Essential for adapting American recipes to metric measuring equipment.
- [Currency Converter](https://hifreetools.com/converters/currency-converter): Convert between 160+ world currencies with live exchange rates. Swap any pair, save conversions to compare, and copy the result. Free, no sign-up.
- [Fahrenheit to Celsius Converter](https://hifreetools.com/converters/fahrenheit-to-celsius): Convert Fahrenheit to Celsius instantly. Formula: multiply by 5/9 and subtract 32. Perfect for weather, cooking, and fever readings.
- [Fahrenheit to Kelvin Converter](https://hifreetools.com/converters/fahrenheit-to-kelvin): Convert Fahrenheit to Kelvin. Use the formula: (°F − 32) × 5/9 + 273.15. For bridging US readings and scientific work.
- [Feet to Inches Converter](https://hifreetools.com/converters/feet-to-inches): Convert feet to inches. 1 ft = 12 in. Perfect for expressing heights, lumber and construction, and furniture measurements.
- [Feet to Meters Converter](https://hifreetools.com/converters/feet-to-meters): Convert feet to meters instantly. 1 ft = 0.3048 m exactly. Perfect for heights, building dimensions, and US-to-metric conversions.
- [Fluid Ounces (US) to Milliliters Converter](https://hifreetools.com/converters/fluid-ounces-to-ml): Convert US fluid ounces to milliliters. 1 fl oz = 29.57 mL. Perfect for bottled drinks, perfume, cosmetics, and recipes.
- [Gallons (US) to Liters Converter](https://hifreetools.com/converters/gallons-to-liters): Convert US gallons to liters instantly. 1 gallon = 3.785 liters. Perfect for fuel tanks, aquariums, and water usage calculations.
- [Grams to Ounces Converter](https://hifreetools.com/converters/grams-to-ounces): Convert grams to ounces (and back) instantly using 1 oz = 28.349523125 g, with a kitchen-friendly table. Free, accurate, in your browser.
- [Grams to Pounds Converter](https://hifreetools.com/converters/grams-to-pounds): Convert grams to pounds for food, packages, and product specifications. Factor: 0.0022046 per gram.
- [Hectares to Acres Converter](https://hifreetools.com/converters/hectares-to-acres): Convert hectares to acres instantly. 1 hectare = 2.471 acres. Perfect for agricultural land, parks, and international property transactions.
- [Inches to Centimeters Converter](https://hifreetools.com/converters/inches-to-cm): Convert inches to centimeters. 1 inch = 2.54 cm exactly. Ideal for screen sizes, clothing measurements, and DIY projects.
- [Kelvin to Celsius Converter](https://hifreetools.com/converters/kelvin-to-celsius): Convert Kelvin to Celsius by subtracting 273.15. Quick converter for physics, astronomy, and scientific research.
- [Kilograms to Grams Converter](https://hifreetools.com/converters/kg-to-grams): Convert kilograms to grams using the factor 1000. Essential for recipes, labs, and precise measurements.
- [Kilograms to Stones Converter](https://hifreetools.com/converters/kg-to-stones): Convert kilograms to stones and pounds. Essential for UK and Ireland body weight tracking and health discussions.
- [Kilojoules to Calories (kcal) Converter](https://hifreetools.com/converters/kilojoules-to-calories): Convert kilojoules to calories (kcal) instantly. 1 kcal = 4.184 kJ. Ideal for reading nutrition labels that show food energy in kilojoules.
- [Kilometers per hour to Miles per hour Converter](https://hifreetools.com/converters/kmh-to-mph): Convert kilometers per hour to miles per hour instantly. 1 km/h = 0.621 mph. Perfect for car speedometers and road trip planning.
- [Kilometers to Miles Converter](https://hifreetools.com/converters/km-to-miles): Convert kilometers to miles. 1 km = 0.6214 miles. Perfect for running, cycling, marathons, road trips, and car odometers.
- [Kilopascals to PSI Converter](https://hifreetools.com/converters/kpa-to-psi): Convert kilopascals (kPa) to PSI instantly. 1 kPa = 0.145038 psi. Useful for tyre pressure, weather, and engineering specs.
- [Knots to Miles per hour Converter](https://hifreetools.com/converters/knots-to-mph): Convert knots to miles per hour fast. 1 knot = 1.151 mph. Use for boating, sailing, aviation, and hurricane tracking.
- [Liters to Gallons Converter](https://hifreetools.com/converters/liters-to-gallons): Convert liters to US gallons (and back) instantly using 1 US gallon = 3.785411784 L, with a table. Free and accurate. Note: US, not imperial, gallons.
- [Megapascals to PSI Converter](https://hifreetools.com/converters/mpa-to-psi): Convert megapascals (MPa) to PSI instantly. 1 MPa = 145.038 psi. Useful for material strength, hydraulics, and equipment ratings.
- [Meters per second to Kilometers per hour Converter](https://hifreetools.com/converters/ms-to-kmh): Convert meters per second to kilometers per hour instantly. 1 m/s = 3.6 km/h. Essential for physics, wind speed, and sports analytics.
- [Meters to Feet Converter](https://hifreetools.com/converters/meters-to-feet): Convert meters to feet (and back) instantly using 1 m = 3.280839895 ft, with a common-values table. Free, accurate, runs in your browser.
- [Meters to Yards Converter](https://hifreetools.com/converters/meters-to-yards): Convert meters to yards. 1 m = 1.0936 yd. Perfect for swimming pools, athletics tracks, and US sports conversions.
- [Miles per hour to Meters per second Converter](https://hifreetools.com/converters/mph-to-ms): Convert miles per hour to meters per second fast. 1 mph = 0.447 m/s. Perfect for physics homework and SI unit conversions.
- [Miles to Feet Converter](https://hifreetools.com/converters/miles-to-feet): Convert miles to feet. 1 mile = 5,280 feet. Use for walking distance, elevation gain, aviation altitudes, and track laps.
- [Miles to Kilometers Converter](https://hifreetools.com/converters/miles-to-km): Convert miles to kilometers (and back) instantly using 1 mile = 1.609344 km, with a table for common distances. Free and accurate.
- [Milliliters to Cups (US) Converter](https://hifreetools.com/converters/ml-to-cups): Convert milliliters to US cups instantly. 1 cup = 236.6 mL. Perfect for scaling metric recipes to US kitchen measurements.
- [MM to Inches Converter](https://hifreetools.com/converters/mm-to-inches): Convert millimeters to inches (and back) instantly using 1 inch = 25.4 mm, with a common-values table. Free, exact, runs in your browser.
- [MPH to KM/H Converter](https://hifreetools.com/converters/mph-to-kmh): Convert miles per hour to kilometers per hour (and back) instantly using 1 mph = 1.609344 km/h, with a speed table. Free and accurate.
- [Nautical Miles to Kilometers Converter](https://hifreetools.com/converters/nautical-miles-to-km): Convert nautical miles to kilometers. 1 nmi = 1.852 km. Essential for marine and air navigation, sea distances, and maritime speed.
- [NIST — SI units](https://hifreetools.com/converters/kg-to-lbs): Convert kilograms to pounds (and back) instantly, with the exact factor and a common-values table. Free, accurate, runs in your browser.
- [NIST — SI units](https://hifreetools.com/converters/cm-to-inches): Convert centimeters to inches (and back) instantly using the exact factor 1 inch = 2.54 cm, with a common-values table. Free, in-browser, accurate.
- [NIST — SI units](https://hifreetools.com/converters/celsius-to-fahrenheit): Convert Celsius to Fahrenheit (and back) instantly with the exact formula °F = °C × 9/5 + 32 and a reference table. Free and accurate.
- [Ounces to Grams Converter](https://hifreetools.com/converters/ounces-to-grams): Convert ounces to grams instantly with factor 28.3495. Perfect for cooking, baking recipes, and food labels.
- [Pints (US) to Liters Converter](https://hifreetools.com/converters/pints-to-liters): Convert US pints to liters. 1 US pint = 0.473 liters. Perfect for beer servings, dairy packaging, and US recipe quantities.
- [Pounds to Kilograms Converter](https://hifreetools.com/converters/lbs-to-kg): Convert pounds to kilograms using the exact factor 0.4536. Common for body weight, gym equipment, and luggage.
- [PSI to Bar Converter](https://hifreetools.com/converters/psi-to-bar): Convert PSI to bar instantly. 1 psi = 0.0689476 bar (1 bar = 14.5038 psi). Ideal for tyre pressure, compressors, and hydraulics.
- [Quarts (US) to Liters Converter](https://hifreetools.com/converters/quarts-to-liters): Convert US quarts to liters. 1 quart = 0.946 liters. Essential for stockpots, milk, oil changes, and large recipe quantities.
- [Square Feet to Square Meters Converter](https://hifreetools.com/converters/square-feet-to-square-meters): Convert square feet to square meters instantly. 1 sq ft = 0.0929 m². Essential for real estate, renovations, and interior design.
- [Square Kilometers to Square Miles Converter](https://hifreetools.com/converters/square-km-to-square-miles): Convert square kilometers to square miles instantly. 1 km² = 0.386 mi². Perfect for geographic areas, maps, and regional comparisons.
- [Square Meters to Square Feet Converter](https://hifreetools.com/converters/square-meters-to-square-feet): Convert square meters to square feet (and back) using 1 m² = 10.763910417 ft², with a table for room and floor areas. Free and accurate.
- [Stones to Kilograms Converter](https://hifreetools.com/converters/stones-to-kg): Convert stones to kilograms (and back) using 1 stone = 6.35029318 kg, with a body-weight table. Free and accurate. 1 stone = 14 pounds.
- [Tablespoons (US) to Milliliters Converter](https://hifreetools.com/converters/tablespoons-to-ml): Convert US tablespoons to milliliters. 1 tbsp = 14.79 mL. Ideal for precise sauces, oils, and small recipe quantities.
- [Teaspoons (US) to Milliliters Converter](https://hifreetools.com/converters/teaspoons-to-ml): Convert US teaspoons to mL instantly. 1 tsp = 4.93 mL. Essential for spice measurements, extracts, and liquid medicine dosing.
- [Yards to Meters Converter](https://hifreetools.com/converters/yards-to-meters): Convert yards to meters. 1 yard = 0.9144 m. Ideal for American football fields, textiles, and track-and-field events.

## Text Tools
- [Lorem Ipsum Generator](https://hifreetools.com/text/lorem-ipsum-generator): Generate Lorem Ipsum placeholder text by paragraphs, sentences, or words, and copy it instantly. Free, classic dummy text for mockups and layouts.
- [Remove Duplicate Lines](https://hifreetools.com/text/remove-duplicate-lines): Remove duplicate lines from a list or text, with options to ignore case and trim spaces. Free and private — runs entirely in your browser.
- [Remove Extra Spaces](https://hifreetools.com/text/remove-extra-spaces): Clean text by collapsing multiple spaces, trimming each line, and optionally removing blank lines. Instant and 100% in your browser — nothing uploaded.
- [Sort Lines Alphabetically](https://hifreetools.com/text/sort-lines): Sort text lines alphabetically A-Z or Z-A with options for case-insensitive sorting, numeric sort, duplicate removal, and reverse order. Works in your browser.
- [Text Case Converter](https://hifreetools.com/text/text-case-converter): Convert text between UPPERCASE, lowercase, Title Case, and Sentence case instantly. Free and private — your text never leaves your browser.
- [Text to Slug Converter](https://hifreetools.com/text/text-to-slug): Convert text to clean URL-safe slugs instantly. Removes punctuation, strips accents, lowercases, and replaces spaces with hyphens. Works offline.
- [The Unicode Consortium](https://hifreetools.com/text/character-counter): Count characters with and without spaces, plus words and lines, against limits for tweets, meta tags, and bios. Free and private — text stays in your browser.
- [Word Counter](https://hifreetools.com/text/word-counter): Count words, characters, sentences, and paragraphs, plus estimated reading and speaking time. Free and private — your text never leaves your browser.

## Developer Tools
- [Base64 Encoder & Decoder](https://hifreetools.com/developer/base64-encoder): Encode text to Base64 and decode Base64 back to text, instantly and privately in your browser. Supports full Unicode (UTF-8). Free, no uploads.
- [Denso Wave — QR Code (inventor)](https://hifreetools.com/developer/qr-code-generator): Create a QR code for a URL or text and download it as a PNG, right in your browser. Free, no watermark, no sign-up, and what you enter is not uploaded.
- [Hash Generator](https://hifreetools.com/developer/hash-generator): Generate SHA-256, SHA-1, SHA-384, or SHA-512 hashes from text instantly. In-browser tool with no data uploads—100% private.
- [JSON Validator](https://hifreetools.com/developer/json-validator): Validate JSON instantly and see exactly where the error is (line and column). Pretty-prints valid JSON. Runs in your browser — nothing uploaded.
- [JWT Decoder](https://hifreetools.com/developer/jwt-decoder): Decode JSON Web Tokens (JWTs) to view claims and timestamps in plain English. Free, in-browser, no upload. Verify token structure and expiration instantly.
- [Regex Tester](https://hifreetools.com/developer/regex-tester): Free regex tester for JavaScript patterns. See all matches and counts live, with flags like g, i, m. Test before you code — 100% in your browser.
- [RFC 8259 — The JSON Data Interchange Format](https://hifreetools.com/developer/json-formatter): Format, validate, and minify JSON in your browser. Pinpoints syntax errors and pretty-prints with 2-space indents. Nothing you paste is uploaded. Free.
- [Unix Timestamp Converter](https://hifreetools.com/developer/timestamp-converter): Convert Unix timestamps (epoch) to human-readable dates and back, in UTC and your local time. Supports seconds and milliseconds. Free, in-browser.
- [URL Encoder & Decoder](https://hifreetools.com/developer/url-encoder): Encode and decode URLs and query parameters with percent-encoding, in your browser. Handles full Unicode (UTF-8). Free, private, no uploads.

## Finance Tools
- [Compound Interest Calculator](https://hifreetools.com/finance/compound-interest-calculator): Estimate how savings grow with compound interest and optional regular contributions. Free, shows the formula and the interest earned. Estimate only.
- [Discount Calculator](https://hifreetools.com/finance/discount-calculator): Calculate the sale price and how much you save from a percentage discount. Free, instant, with the formula shown. No sign-up required.
- [Hourly to Salary Calculator](https://hifreetools.com/finance/hourly-to-salary-calculator): Convert an hourly wage to annual, monthly, and weekly salary based on hours per week and weeks per year. Free, with the formula shown. Estimate only.
- [Loan Payment Calculator](https://hifreetools.com/finance/loan-payment-calculator): Estimate the monthly payment, total cost, and total interest on a fixed-rate loan or mortgage. Free, shows the amortization formula. Estimate only.
- [Profit Margin Calculator](https://hifreetools.com/finance/profit-margin-calculator): Calculate profit, profit margin, and markup from cost and selling price. Free, with the formulas shown — handy for pricing and ecommerce. Estimate only.
- [Salary to Hourly Calculator](https://hifreetools.com/finance/salary-to-hourly-calculator): Convert an annual salary to an equivalent hourly wage based on hours per week and weeks per year. Free, with the formula shown. Estimate only.
- [Sales Tax Calculator](https://hifreetools.com/finance/sales-tax-calculator): Add sales tax (or VAT/GST) to a price using a rate you enter, and see the tax amount and total. Free and instant. Enter your local rate.
- [Savings Goal Calculator](https://hifreetools.com/finance/savings-goal-calculator): Find how much to save each month to reach a goal by a target date, given a starting amount and interest rate. Free. Estimate only, not advice.
- [Simple Interest Calculator](https://hifreetools.com/finance/simple-interest-calculator): Calculate simple interest and the final amount from principal, rate, and time. Free, with the I = P × r × t formula shown. Estimate only.
- [Tip Calculator](https://hifreetools.com/finance/tip-calculator): Calculate the tip and total for any bill, and split it between people. Free, instant, with the simple formula shown. No sign-up.

## Health & Fitness Tools
- [BMI Calculator](https://hifreetools.com/health/bmi-calculator): Calculate your Body Mass Index in metric or imperial units and see the WHO weight category. Free, shows the formula. General information, not medical advice.
- [BMR Calculator](https://hifreetools.com/health/bmr-calculator): Estimate your Basal Metabolic Rate (BMR) with the Mifflin-St Jeor formula, in metric or imperial units. Free. General information, not medical advice.
- [Body Surface Area (BSA) Calculator](https://hifreetools.com/health/body-surface-area): Estimate body surface area (BSA) in m² from height and weight using the Mosteller formula, with Du Bois as an alternative. Worked examples and sources.
- [Calorie Calculator (TDEE)](https://hifreetools.com/health/calorie-calculator): Estimate your daily calorie needs (TDEE) from BMR and activity level, in metric or imperial. Free. General information, not medical or diet advice.
- [Ideal Body Weight (IBW) Calculator](https://hifreetools.com/health/ideal-body-weight): Estimate ideal body weight from height and sex with the Devine formula. A general population estimate, not medical advice — see how it works and its limits.

## Image Tools
- [Image Compressor](https://hifreetools.com/image/image-compressor): Compress JPG, PNG, and WebP images in your browser. Control quality, see the before/after size, and download — nothing is uploaded. Free, no sign-up.
- [Image Resizer](https://hifreetools.com/image/image-resizer): Resize images to exact pixel dimensions in your browser. Lock the aspect ratio, choose a format, and download — nothing is uploaded. Free, no sign-up.

## Color & Design Tools
- [Color Contrast Checker](https://hifreetools.com/color/color-contrast-checker): Check if text and background colors meet WCAG AA and AAA accessibility standards. Instant contrast ratio calculator with live preview.
- [HEX to RGB Converter](https://hifreetools.com/color/hex-to-rgb): Instantly convert HEX color codes to RGB values. Supports #RRGGBB and #RGB formats. See RGB output, HSL values, and live color preview.
- [RGB to HEX Converter](https://hifreetools.com/color/rgb-to-hex): Convert RGB colors to HEX codes instantly. Enter red, green, blue values (0-255) and get the hex code, plus HSL values and live color swatch.

## Date & Time Tools
- [Age Calculator](https://hifreetools.com/date-time/age-calculator): Calculate your exact age in years, months, and days from your date of birth, plus total days and days until your next birthday. Free, in-browser.
- [Date Difference Calculator](https://hifreetools.com/date-time/date-difference-calculator): Calculate the time between two dates in years, months, and days, plus the total number of days and weeks. Free, exact, runs in your browser.
- [Days Until Calculator](https://hifreetools.com/date-time/days-until-calculator): Count the days until any future date — a holiday, deadline, birthday, or event. Free countdown showing days and weeks. Runs in your browser.

## Random Generators
- [Coin Flip](https://hifreetools.com/random/coin-flip): Flip a virtual coin for a fair heads-or-tails result, with a running tally of your flips. Free, instant, and fair — runs in your browser.
- [Dice Roller](https://hifreetools.com/random/dice-roller): Roll virtual dice online — choose the number of dice and sides (D4, D6, D20, D100). Fair results with a total. Free, in-browser, no app needed.
- [Random Number Generator](https://hifreetools.com/random/random-number-generator): Generate random numbers in any range, one or many, with options for no duplicates and sorting. Fair and secure, runs in your browser. Free.
- [Random Picker](https://hifreetools.com/random/random-picker): Paste a list and pick one or more items at random — fairly and instantly. Options to remove duplicates and pick without repeats. Free, in your browser.
- [Yes or No Generator](https://hifreetools.com/random/yes-no-generator): Get a random yes or no answer to help make a quick decision, with an optional ‘maybe’. Fair 50/50 result in your browser. Free, no sign-up.

## Security Tools
- [NIST SP 800-63B — Digital Identity Guidelines (authentication)](https://hifreetools.com/security/password-generator): Generate strong, random passwords in your browser using the Web Crypto API. Choose length and character types. Nothing is sent or stored. Free.
- [RFC 9562 — Universally Unique IDentifiers (UUIDs)](https://hifreetools.com/security/uuid-generator): Generate random version-4 UUIDs in your browser, one or many at a time, and copy them instantly. Cryptographically random, never uploaded. Free.

## SEO & Marketing Tools
- [Meta Description Length Checker](https://hifreetools.com/seo/meta-description-checker): Check meta description length and see how your text appears in Google search results. Get real-time feedback on optimal SEO length (120-160 characters).
- [Meta Title Length Checker](https://hifreetools.com/seo/meta-title-checker): Check if your SEO title fits Google's display limit (~50–60 characters). See a live SERP preview and get feedback instantly. Free, in-browser, private.
- [UTM Link Builder](https://hifreetools.com/seo/utm-builder): Free UTM parameter builder. Add utm_source, utm_medium, utm_campaign, utm_term, and utm_content to any URL for accurate marketing analytics. Works in browser.

## Science & Engineering Tools
- [Combined Gas Law Calculator](https://hifreetools.com/science/combined-gas-law): Solve P1V1/T1 = P2V2/T2 for any unknown pressure, volume, or temperature. Includes Boyle's, Charles's, and Gay-Lussac's law special cases with worked examples.
- [Ideal Gas Law Calculator](https://hifreetools.com/science/ideal-gas-law): Solve PV = nRT for pressure, volume, moles, or temperature. Uses R = 8.314 J/(mol·K), accepts SI units, and shows worked examples with steps.
- [Ohm's Law Calculator](https://hifreetools.com/science/ohms-law): Free Ohm's Law calculator. Enter any two of voltage, current or resistance to solve the rest plus power, using V = I x R and P = V x I.
- [Scientific Calculator](https://hifreetools.com/science/scientific-calculator): Free online scientific calculator: trigonometry, logarithms, exponents, powers, roots, factorials, π, e, memory, and DEG/RAD/GRAD. In your browser.

## Guides
- [APR vs APY: Understanding the Difference Between Interest Rates](https://hifreetools.com/guides/apr-vs-apy): Learn the difference between APR and APY, how compound interest makes APY higher, and where each applies. Includes formulas and real-world examples.
- [Simple vs Compound Interest: Formulas, Comparison, and Real-World Applications](https://hifreetools.com/guides/simple-vs-compound-interest): The difference between simple and compound interest — with formulas, a 10-year worked example, and why compounding dominates savings and loans.
- [How to Calculate Percentage Change](https://hifreetools.com/guides/how-to-calculate-percentage-change): How to calculate percentage change, increase, and decrease — with formulas, worked examples, and the percent-change vs percentage-points difference.
- [Metric vs Imperial Units: History, Differences, and Conversion Guide](https://hifreetools.com/guides/metric-vs-imperial-units): Learn the key differences between metric and imperial systems, why the US uses imperial, and exact conversion factors for length, weight, temperature, and more.
- [Understanding BMI](https://hifreetools.com/guides/understanding-bmi): Learn what Body Mass Index (BMI) measures, the WHO weight categories, the exact formula, and why it's a screening tool—not a diagnosis. Includes limitations.
- [How Loan Amortization Works](https://hifreetools.com/guides/how-loan-amortization-works): How loan amortization splits each payment between principal and interest, why early payments are mostly interest, and how extra payments save money.
- [Mean vs Median vs Mode: Definitions, Formulas, and When to Use Each](https://hifreetools.com/guides/mean-median-mode): Learn the difference between mean, median, and mode. Understand formulas, worked examples with real salary data, and how outliers affect each measure.
- [What is Base64? Encoding Explained](https://hifreetools.com/guides/what-is-base64): Learn what Base64 encoding is, how it works, and why it's not encryption. Covers 33% size increase, data URIs, JWT, and email attachments.
- [How to Convert Kilograms to Pounds (and Back)](https://hifreetools.com/guides/how-to-convert-kg-to-lbs): Convert kg to lbs with the exact factor (1 kg = 2.20462 lb), a quick ×2.2 mental trick, a reference table, and common mistakes to avoid.
- [JSON Formatter vs JSON Validator: What's the Difference?](https://hifreetools.com/guides/json-formatter-vs-validator): A JSON formatter pretty-prints and indents data; a validator checks syntax and locates errors. When to use each, and what valid JSON requires.
- [How UTM Parameters Work (with Examples)](https://hifreetools.com/guides/how-utm-parameters-work): How the five UTM parameters (source, medium, campaign, term, content) tag links so analytics can credit traffic — with examples and a tag table.
- [How to Calculate a Tip (and Split the Bill)](https://hifreetools.com/guides/how-to-calculate-a-tip): Learn how to calculate a tip, what percentage to leave in different situations, how to split a bill among friends, and whether to tip on pre-tax or total.
- [What Makes a Strong Password](https://hifreetools.com/guides/what-makes-a-strong-password): Learn what actually makes a password strong: why length beats complexity, how passphrases work, and the current NIST SP 800-63B guidance on resets and rules.
- [How to Compress an Image Without Losing Quality](https://hifreetools.com/guides/how-to-compress-images-without-losing-quality): Learn how to compress images without visible quality loss: lossy vs lossless, JPEG vs PNG vs WebP, when to resize, quality sliders, and stripping metadata.
- [HEX, RGB, and HSL Color Codes Explained](https://hifreetools.com/guides/hex-rgb-hsl-color-codes-explained): Understand HEX, RGB, and HSL color notation: what each one means, how they convert, when to use each, and how alpha controls opacity.
- [Kitchen Conversions: Cups, Milliliters, Liters, and Gallons](https://hifreetools.com/guides/kitchen-volume-conversions): Convert cups, tablespoons, fluid ounces, milliliters, liters, and gallons. Learn US vs metric vs imperial units and why kitchen measurements differ.
- [URL Encoding (Percent-Encoding) Explained](https://hifreetools.com/guides/url-encoding-explained): Learn how URL percent-encoding works: reserved vs unreserved characters, when to encode query parameters, and common gotchas with spaces, &, +, and #.

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## En Español (Spanish)
- [Conversores (índice)](https://hifreetools.com/es/converters)
- [miles-to-feet](https://hifreetools.com/es/converters/miles-to-feet)
- [nautical-miles-to-km](https://hifreetools.com/es/converters/nautical-miles-to-km)
- [meters-to-yards](https://hifreetools.com/es/converters/meters-to-yards)
- [celsius-to-kelvin](https://hifreetools.com/es/converters/celsius-to-kelvin)
- [kelvin-to-celsius](https://hifreetools.com/es/converters/kelvin-to-celsius)
- [fahrenheit-to-kelvin](https://hifreetools.com/es/converters/fahrenheit-to-kelvin)
- [knots-to-mph](https://hifreetools.com/es/converters/knots-to-mph)
- [ms-to-kmh](https://hifreetools.com/es/converters/ms-to-kmh)
- [acres-to-square-feet](https://hifreetools.com/es/converters/acres-to-square-feet)
- [hectares-to-acres](https://hifreetools.com/es/converters/hectares-to-acres)
- [square-km-to-square-miles](https://hifreetools.com/es/converters/square-km-to-square-miles)
- [calories-to-kilojoules](https://hifreetools.com/es/converters/calories-to-kilojoules)
- [bar-to-psi](https://hifreetools.com/es/converters/bar-to-psi)
- [kpa-to-psi](https://hifreetools.com/es/converters/kpa-to-psi)
- [mpa-to-psi](https://hifreetools.com/es/converters/mpa-to-psi)
- [cubic-meters-to-liters](https://hifreetools.com/es/converters/cubic-meters-to-liters)
- [feet-to-inches](https://hifreetools.com/es/converters/feet-to-inches)
- [ml-to-cups](https://hifreetools.com/es/converters/ml-to-cups)
- [tablespoons-to-ml](https://hifreetools.com/es/converters/tablespoons-to-ml)
- [teaspoons-to-ml](https://hifreetools.com/es/converters/teaspoons-to-ml)
- [fluid-ounces-to-ml](https://hifreetools.com/es/converters/fluid-ounces-to-ml)
- [pints-to-liters](https://hifreetools.com/es/converters/pints-to-liters)
- [quarts-to-liters](https://hifreetools.com/es/converters/quarts-to-liters)
- [kg-to-stones](https://hifreetools.com/es/converters/kg-to-stones)
- [grams-to-pounds](https://hifreetools.com/es/converters/grams-to-pounds)
- [yards-to-meters](https://hifreetools.com/es/converters/yards-to-meters)
- [psi-to-bar](https://hifreetools.com/es/converters/psi-to-bar)
- [kilojoules-to-calories](https://hifreetools.com/es/converters/kilojoules-to-calories)
- [lbs-to-kg](https://hifreetools.com/es/converters/lbs-to-kg)
- [inches-to-cm](https://hifreetools.com/es/converters/inches-to-cm)
- [fahrenheit-to-celsius](https://hifreetools.com/es/converters/fahrenheit-to-celsius)
- [km-to-miles](https://hifreetools.com/es/converters/km-to-miles)
- [feet-to-meters](https://hifreetools.com/es/converters/feet-to-meters)
- [ounces-to-grams](https://hifreetools.com/es/converters/ounces-to-grams)
- [gallons-to-liters](https://hifreetools.com/es/converters/gallons-to-liters)
- [kmh-to-mph](https://hifreetools.com/es/converters/kmh-to-mph)
- [cups-to-ml](https://hifreetools.com/es/converters/cups-to-ml)
- [kg-to-grams](https://hifreetools.com/es/converters/kg-to-grams)
- [mph-to-ms](https://hifreetools.com/es/converters/mph-to-ms)
- [square-feet-to-square-meters](https://hifreetools.com/es/converters/square-feet-to-square-meters)

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