Ohm's Law Calculator
Your estimate
- Power
- 24 W
Know any two values — volts, amps, ohms or watts — and get the other two.
Updated 2026-10-20 · Based on: Ohm’s law, NEC Chapter 9 Table 8, 210.20 and 310.16; DIN VDE 0100-520 for metric.
How to use the Ohm's Law Calculator
- Pick the two values you know with the tabs (for example volts + ohms).
- Type them in volts (V), amps (A), ohms (Ω) or watts (W). Use decimals for milliamps: 20 mA = 0.02 A.
- Read the two missing values. Large resistances are shown in kΩ or MΩ.
Formula
Ohm's law: V = I × R, so I = V ÷ R and R = V ÷ I.
Power: P = V × I = I² × R = V² ÷ R.
From power and resistance: I = √(P ÷ R) and V = √(P × R).
Worked example
With the values preset in the calculator above: (Volts + amps)
- Voltage (V)
- 12 V
- Current (A)
- 2 A
- Current
- 2 A
- Resistance
- 6 Ω
- Voltage
- 12 V
- Power
- 24 W
The Ohm's law wheel — all 12 formulas
- Voltage V = I × R = P ÷ I = √(P × R)
- Current I = V ÷ R = P ÷ V = √(P ÷ R)
- Resistance R = V ÷ I = V² ÷ P = P ÷ I²
- Power P = V × I = I² × R = V² ÷ R
Ohm's law examples
A 12 V source across 6 Ω drives 2 A and turns 24 W into heat. A 120 V circuit through a 1 kΩ resistor carries 0.12 A.
A 60 W bulb on 120 V draws 0.5 A and has a hot-filament resistance of 240 Ω.
Frequently asked questions
What is Ohm's law?
The current through a conductor equals the voltage across it divided by its resistance: I = V ÷ R. It holds for resistors, heaters and wires at a steady temperature.
How do I calculate resistance from volts and amps?
Divide volts by amps: 12 V at 2 A is 6 Ω.
How do I calculate amps from watts and ohms?
I = √(P ÷ R). 24 W in 6 Ω is 2 A.
Does Ohm’s law work for LEDs and motors?
Not directly — their resistance changes with current. For an LED, size the series resistor as (supply V − LED V) ÷ LED current.
More calculators
Estimates only — check quantities with your supplier and local building code before you order.