Ohm's Law Calculator
Leave exactly one field blank and the calculator solves it from the other two — and shows power as a bonus. The classic V = I × R with all its rearrangements.
Last updated: 2026-09-15
How the calculation works
- Exactly one field must be blank — that is the unknown the calculator solves for.
- The tool applies the corresponding rearrangement of Ohm's law and reports all four quantities.
- Power is computed from P = V × I, which is equivalent to I²R and V²/R.
Formula
V = I × R I = V / R R = V / I P = V × I = I² × R = V² / R
| Symbol | Meaning | Unit |
|---|---|---|
V | Voltage | V |
I | Current | A |
R | Resistance | Ω |
P | Power | W |
Worked example
A 230 V heater draws 4.5 A. Current and voltage known, resistance blank: R = 230 / 4.5 = 51.1 Ω. Power = 230 × 4.5 = 1035 W — about a 1 kW heater.
Interpreting the result
Current rises proportionally with voltage and inversely with resistance. If the computed current exceeds the rating of the wiring or device, something must give — a breaker trips, a fuse blows or a conductor overheats. Power tells you the heat or work rate: in resistive loads it all becomes heat.
Assumptions
- DC circuit or purely resistive AC load (heaters, incandescent lamps).
- Fixed resistance — it does not model how resistance changes with temperature.
Limitations
- Reactive AC loads (motors, transformers, electronics) need impedance and power factor, not plain resistance.
- Non-linear devices (diodes, LED drivers) do not obey Ohm's law in this simple form.
Frequently asked questions
How do I calculate power from voltage and resistance?
Use P = V² / R. For example, 120 V across a 24 Ω element gives 120² / 24 = 600 W.
Does Ohm's law work for AC?
Only for purely resistive AC loads. With inductive or capacitive loads, replace R with impedance Z and account for power factor: P = V × I × PF.