Ohm's Law calculator
Enter two values among voltage, current, resistance and power and calculate the other two with Ohm's Law (V = R × I) and the power formula (P = V × I).
How to useVoltage, current, resistance and power
Fill in exactly two fields and leave the others blank.
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Ohm's Law calculator: what it is for
Fill in two of the four fields and the calculator finds the other two using V = R × I and P = V × I. Results are shown with prefixes like mA, kΩ and mW to make them easier to read in electronic and electrical circuits.
How to use
- Fill in two values.
- Leave the other fields blank.
- Read the voltage, current, resistance and power.
When to use it
- Studying electricity and physics.
- Sizing electronic components.
- Checking a resistor's power rating.
Frequently asked questions
What does Ohm's Law say?
That the voltage across a resistor equals the resistance times the current: V = R × I. With power P = V × I, you only need two of the four values to find the others.
Which units should I use?
Voltage in volts (V), current in amperes (A), resistance in ohms (Ω) and power in watts (W). For milliamps, divide by 1000: 20 mA is 0.02 A.
Is it really free?
Yes, 100% free. No sign-up, no subscription and no usage limits. All Tools123 tools are free and run right in your browser.