Ohm's law calculator
Fill in any two of the four fields and leave the rest empty, and the tool works out the missing values. Handy for students and technicians checking a circuit or choosing a resistor.
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How to use it
- Enter voltage, current, resistance and power.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
V = I × R and P = V × I, which give: I = V ÷ R, R = V ÷ I, P = I² × R = V² ÷ R, V = √(P × R), I = √(P ÷ R).
Example
Inputs: Voltage = 12 V, Current = 2 A. Result: Resistance: 6 Ω; Voltage: 12 V; Current: 2 A.
Limits and caveats
- Assumes DC or a purely resistive load; with an inductive or capacitive AC load you need impedance and power factor.
- Enter values in base units, so 20 mA is typed as 0.02 A.
Questions about this tool
How many fields should I fill in?
Exactly two. If you fill in one or three, a message asks you to leave two fields empty.
What is the difference between Ohm's law and the power law?
Ohm's law links voltage, current and resistance (V = I × R); the power law gives the energy used per second (P = V × I). The tool combines both.
Can I use it for an LED resistor?
Not directly, because an LED is non-linear with a roughly fixed forward voltage. Use the LED resistor calculator, which subtracts that voltage from the supply.