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Free Ohm's Law Calculator

This Ohm's Law calculator solves for voltage, current, resistance and power in a simple DC circuit. Enter any two of the three core quantities, voltage (V), current (I) or resistance (R), and the calculator computes the missing value along with the electrical power (P). Ohm's Law is one of the most fundamental relationships in electronics: it describes how voltage, current and resistance interact in a conductor, and it underpins almost every circuit calculation you will encounter when working with batteries, resistors, LEDs, motors and power supplies.

Enter any two values — the rest are calculated.

Results
Voltage12 V
Current3 A
Resistance4 Ω
Power36 W

V = I × R · P = V × I = I²R = V²/R. For DC circuits; use amperes (A), not milliamps.

Quick answer

Ohm's Law states that voltage equals current times resistance: V = I x R. Voltage (V) is measured in volts, current (I) in amperes, and resistance (R) in ohms. Power is found with P = V x I, which can also be written as P = I squared x R or P = V squared / R, measured in watts.

Formula & method

V = I × R    •    P = V × I = I²R = V²/R

V = voltage (volts), I = current (amperes), R = resistance (ohms), P = power (watts). For DC circuits.

Examples

Example 1: Solving for current from voltage and resistance
Input
V = 12 V, R = 4 ohms
Result
I = 3 A, P = 36 W
Why
Rearranging V = I x R gives I = V / R = 12 / 4 = 3 amperes. Power is P = V x I = 12 x 3 = 36 watts. This is a typical scenario for a 12-volt DC load such as a small bulb or fan.
Example 2: Solving for resistance from voltage and current
Input
V = 9 V, I = 0.5 A
Result
R = 18 ohms, P = 4.5 W
Why
Rearranging V = I x R gives R = V / I = 9 / 0.5 = 18 ohms. Power is P = V x I = 9 x 0.5 = 4.5 watts. This is useful when sizing a series resistor for a 9-volt battery circuit.
Example 3: Solving for voltage from current and resistance
Input
I = 2 A, R = 5 ohms
Result
V = 10 V, P = 20 W
Why
Using V = I x R = 2 x 5 = 10 volts directly. Power can be found with P = I squared x R = (2 x 2) x 5 = 20 watts, which matches P = V x I = 10 x 2 = 20 watts.

When to use this tool

  • Sizing a current-limiting resistor for an LED or other component on a known supply voltage.
  • Checking how much current a load will draw from a battery or DC power supply before connecting it.
  • Estimating the power (in watts) a resistor or device will dissipate, to choose a part with an adequate power rating.
  • Diagnosing a circuit by comparing measured voltage, current and resistance against the values Ohm's Law predicts.
  • Learning or teaching the fundamental relationship between voltage, current, resistance and power in DC electronics.

Common mistakes

  • Mixing up units: current must be entered in amperes (A), not milliamps (mA). To convert, divide milliamps by 1000 (e.g. 250 mA = 0.25 A).
  • Confusing the positions in the V = I x R triangle. To find a quantity, cover it: V is on top (so V = I x R), while I and R sit side by side (so I = V / R and R = V / I).
  • Forgetting that resistance cannot be zero when dividing. I = V / R is undefined for R = 0, which would represent an idealised short circuit.
  • Assuming Ohm's Law applies to every component. It holds for ohmic (linear) resistors at constant temperature, but not directly for non-ohmic devices like diodes, LEDs or filament lamps whose resistance changes.
  • Treating these DC formulas as exact for AC circuits. With alternating current, reactance and power factor come into play, so impedance replaces simple resistance and P = V x I only gives apparent power.

Frequently asked questions

What is Ohm's Law in simple terms?

Ohm's Law says that the current flowing through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance. In equation form this is V = I x R, where V is voltage in volts, I is current in amperes and R is resistance in ohms. It was named after the German physicist Georg Simon Ohm.

How do I calculate power with Ohm's Law?

Electrical power in watts is the product of voltage and current: P = V x I. By substituting Ohm's Law, you can also write P = I squared x R or P = V squared / R. All three forms give the same answer; you simply choose the one that uses the values you already know.

What are the units for voltage, current, resistance and power?

Voltage is measured in volts (V), current in amperes or amps (A), resistance in ohms (the symbol is the Greek letter omega), and power in watts (W). Keeping consistent base units, so amps rather than milliamps, is essential for the formulas to give correct results.

What is the Ohm's Law triangle?

The V = I x R triangle is a memory aid with V at the top and I and R along the bottom. Cover the quantity you want to find: covering V leaves I x R, covering I leaves V over R, and covering R leaves V over I. It is a quick way to recall the three rearrangements of the formula.

Does Ohm's Law work for AC circuits?

This calculator uses the direct current (DC) form of Ohm's Law. In alternating current (AC) circuits, components such as capacitors and inductors add reactance, so the simple resistance R is replaced by impedance, and real power depends on the power factor. For purely resistive DC circuits, the formulas here apply directly.

Why does my real circuit not match the calculated value?

Several factors can cause differences: components have manufacturing tolerances, resistance changes with temperature, wires and connections add small resistances, and non-ohmic parts like diodes and lamps do not follow a constant resistance. The calculator assumes an ideal ohmic resistor at a fixed temperature.

Sources & references

External references open in a new tab. We are independent and not affiliated with these organizations.

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