Ohm's Law Calculator: Voltage, Current, Resistance and Power
Ohm's law states that voltage equals current multiplied by resistance: V = I × R. Give the calculator any two of those values and it returns the third, plus the power dissipated — the four numbers behind almost every practical circuit decision.
The four rearrangements
- V = I × R — voltage from current and resistance.
- I = V ÷ R — current from voltage and resistance.
- R = V ÷ I — resistance from voltage and current.
- P = V × I = I²R = V²/R — power in watts.
A worked LED example
To run a 2 V LED at 20 mA from a 12 V supply you must drop 10 V across a series resistor. R = 10 ÷ 0.02 = 500 Ω, so you fit the nearest standard value, 510 Ω. Power in that resistor is 10 × 0.02 = 0.2 W, so a 0.25 W part is marginal and a 0.5 W part is the sensible choice.
Where Ohm's law stops applying
The law holds for ohmic components such as resistors and most conductors at stable temperature. Diodes, transistors, LEDs and filament lamps are non-linear — their resistance changes with voltage or temperature — so you use their characteristic curves or datasheet values instead. AC circuits with capacitance or inductance need impedance rather than plain resistance.
Series and parallel
Resistors in series add directly: R = R₁ + R₂. In parallel, conductances add, so 1/R = 1/R₁ + 1/R₂ and the total is always lower than the smallest branch. Current divides between parallel paths in inverse proportion to their resistance.
Key takeaways — Ohm's Law Calculator
- V = IR, and power P = VI ties the electrical quantities together.
- Size components for power dissipation, not just resistance.
- Non-linear and reactive components need more than Ohm's law.