Ohm's Law Calculator
Calculate voltage, current, resistance or power — V = I × R.
The Ohm's Law Calculator solves for any electrical value from the other two: give it voltage and current and it returns resistance (and power); voltage and resistance gives current; current and resistance gives voltage.
Ohm's law — V = I × R — is the first rule of every circuit, and its companion P = V × I ties in power. A 12 V supply pushing 2 A through a circuit implies 6 Ω of resistance and 24 W of power; a 10 Ω resistor on 5 V draws 0.5 A. The calculator shows the substituted formula so the math stays transparent.
Enter volts, amps and ohms in base units (convert mA to A and kΩ to Ω first) and read the answer plus power. Everything runs locally in your browser.
Resistance
6.000
R = V ÷ I = 12 ÷ 2
Power
24.00 W
P = V × I
Ohm's law: V = I × R. Power follows from P = V × I = I²R = V²/R. Units are volts, amps and ohms — convert mA to A and kΩ to Ω before entering.
How to use the Ohm's Law Calculator
- Choose what you want to calculate: R, I or V.
- Enter the two known values (avoid a zero divisor).
- Read the missing value with the formula shown.
- Check the power dissipation (P = V × I).
- Verify against your circuit's spec sheet.
Frequently asked questions
What is Ohm's law?
V = I × R — voltage equals current times resistance. Rearranged: I = V ÷ R and R = V ÷ I. It governs every resistive circuit and explains why more resistance means less current at the same voltage.
How do I calculate power in a circuit?
P = V × I, and by substitution P = I²R = V² ÷ R. Power tells you heat: a 10 Ω resistor carrying 1 A dissipates 10 W, which is why component wattage ratings matter.
Why are my results off by 1000×?
Unit mismatch — the most common error. Milliamps must become amps (÷1000) and kilohms must become ohms (×1000) before the formulas work. Convert first, then calculate.
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