Ohm's Law Calculator

Enter any two values — voltage, current, resistance, or power — and the other two are calculated instantly.

Voltage

Current

Resistance

Power

Enter any two values — the other two are calculated. Pick units from the dropdowns, or type SI prefixes directly: 4.7k, 20m, 100u (µ).

How this calculator works

Ohm's law ties together the four quantities of a simple DC circuit: voltage V (volts), current I (amps), resistance R (ohms), and power P (watts). Knowing any two fixes the other two. Fill in the two you know — in any of the four boxes — and this calculator picks the right formulas and shows its work, including the exact equations it used with your values substituted in.

The twelve formulas

Find	From V and I	From V and R	From I and R	From P
Voltage V	—	—	V = I × R	V = P / I or V = √(P × R)
Current I	—	I = V / R	—	I = P / V or I = √(P / R)
Resistance R	R = V / I	—	—	R = V² / P or R = P / I²
Power P	P = V × I	P = V² / R	P = I² × R	—

Worked examples

An LED circuit: a resistor drops 3 V while passing 20 mA. Its resistance is R = V / I = 3 / 0.02 = 150 Ω, and it dissipates P = V × I = 3 × 0.02 = 60 mW — comfortably inside a standard ¼ W resistor's rating.

A heater element: a 240 V heater rated 1500 W draws I = P / V = 1500 / 240 ≈ 6.25 A, so its hot resistance is R = V / I ≈ 38.4 Ω.

Frequently asked questions

Which two values should I enter?: Any two. The calculator watches which boxes you filled in and computes the other two. If you change a third box, the oldest entry is released and recalculated instead.
What do the m, k, and M suffixes mean?: Standard SI prefixes: p pico (10⁻¹²), n nano, u/µ micro, m milli, k kilo, M mega, G giga. So 20m is 0.02 and 4.7k is 4700. Case matters only for m/M.
Does this work for AC circuits?: For purely resistive AC loads, yes — use RMS voltage and current. For loads with inductance or capacitance you need impedance and power factor, which this calculator doesn't model.
Why is my computed power so large?: Check prefixes. A common slip is typing 20M (20 mega) when you meant 20m (20 milli) — a factor of a billion.
What about choosing a resistor for an LED?: That's a special case with its own steps — use the LED series resistor calculator, which also picks the nearest standard value and power rating.