Ohm's Law Calculator
Solve for voltage, current, resistance or power from any two known values.
Processing: This tool runs entirely in your browser. Your input and any file you open stay on your device — nothing is uploaded to a server.
How to use the ohm's law calculator
- Fill in the fields — they start with a worked example.
- Read the answer, and the arithmetic shown beneath it.
- Copy the results, or adjust the inputs to compare scenarios.
About this tool
Ohm's law states V = I × R, and power adds P = V × I. Between them, any two known values determine the other two — which is why this takes whichever pair you have rather than asking for a specific combination.
The practical uses are checking that a resistor is sized correctly for an LED, confirming a power supply can deliver enough current, and working out how much heat a component will dissipate. That last one is where circuits fail: a resistor rated at a quarter watt in a circuit dissipating half will not last.
Common uses
- Sizing a resistor for an LED.
- Checking whether a power supply is adequate.
- Working out heat dissipation in a component.
Frequently asked questions
- Which two values should I enter?
- Any two. Leave the others at zero and they are calculated. Entering three lets you check consistency.
- Does this apply to AC circuits?
- Only roughly. AC involves impedance and power factor, so the simple relationships here describe DC accurately and AC approximately.
Related tools
Power Converter
Convert between watts, kilowatts, mechanical and metric horsepower, and BTU per hour.
Energy Converter
Convert between joules, kilowatt hours, calories, food Calories and BTU.
Percentage Calculator
Work out a percentage of a number, what percentage one number is of another, and percentage change.
Unit Price Comparison
Compare products by price per unit to find which is genuinely cheaper.