Ohm’s Law Calculator
Compute voltage, current, resistance and power from any two values.
Formula
V = I × R
How to use
- 1Enter the voltage across the component in volts.
- 2Enter the current through it in amperes.
- 3Read resistance (R = V ÷ I) and power (P = V × I).
- 4Check the power figure against the component’s wattage rating before building.
About this calculator
Ohm’s Law is the most useful relationship in electronics: know any two of voltage, current and resistance and you can find the third, plus the power the circuit uses. It states V = I × R and pairs with P = V × I to size resistors, protect components and track down faults.
Formula explained
V = I × R, where V is volts, I is amperes and R is ohms. Rearranged, R = V ÷ I and I = V ÷ R. Power has three identical forms: P = V × I, P = I² × R and P = V² ÷ R, all in watts. Use whichever matches the numbers you already have.
Worked example
R = 12 ÷ 2 = 6 Ω. Power = 12 × 2 = 24 W. That wattage is the headline number, since a quarter-watt resistor would fail here; choose a part rated comfortably above 24 W.
Tips & notes
- Resistors in series add together; in parallel, reciprocals add (1/R = 1/R₁ + 1/R₂ and so on).
- Give power a margin: aim to run resistors at about half their rated wattage in continuous use.
- Convert units first, since 20 mA is 0.02 A. Mixing mA and A is the most common error.