Ohm's Law Calculator
Calculate voltage, current, resistance and power with this free online Ohm's law calculator — enter any two values and get the other two in mA, kΩ or kW.
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Part of the toolkit
How to use Ohm's Law Calculator
- 1Type any two of voltage, current, resistance and power, choosing units such as mA or kΩ
- 2Leave the other two fields empty and press Calculate
- 3Read all four values with automatic unit prefixes and copy the result
Frequently asked questions
What is Ohm's law?▾
Ohm's law says voltage equals current times resistance: V = I × R. Combined with the power formula P = V × I, any two of voltage, current, resistance and power determine the other two.
How do I calculate current from voltage and resistance?▾
Divide voltage by resistance: I = V ÷ R. For 12 V across 220 Ω, I = 12 ÷ 220 = 54.5 mA, and the resistor dissipates P = V² ÷ R = 0.655 W.
How do I choose a resistor for an LED?▾
R = (supply voltage − LED forward voltage) ÷ LED current. For a 5 V supply, a 2 V red LED and 20 mA: (5 − 2) ÷ 0.02 = 150 Ω. Enter 3 V and 20 mA here to get the same result and the power rating.
How many amps is 1000 watts?▾
Current = power ÷ voltage: about 8.3 A at 120 V or 4.3 A at 230 V.
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