Enter any two values - the other two are calculated automatically.
All formulas
| Known | V, volts | I, amps | R, ohms | P, watts |
|---|---|---|---|---|
| V and I | — | — | V / I | V · I |
| V and R | — | V / R | — | V² / R |
| V and P | — | P / V | V² / P | — |
| I and R | I · R | — | — | I² · R |
| I and P | P / I | — | P / I² | — |
| R and P | √(P·R) | √(P/R) | — | — |
Ohm's law
The current through a conductor is directly proportional
to the voltage and inversely proportional to the resistance: I = V / R.
Power is the product of voltage and current: P = V · I. Every other
combination in the table above is derived from these two formulas.
Example
A 2000 W kettle on a 230 V supply draws
I = P / V ≈ 8.7 A, and its heating element has a resistance of
R = V² / P ≈ 26.5 Ω. The current is what you size the
wire cross-section and circuit breaker for -
the wiring calculator does that in one place.
AC circuits
The formulas are exact for DC and for resistive AC
loads (heaters, incandescent lamps, kettles). For loads with a reactive component
(motors, transformers, switching power supplies) the apparent power is higher than
the active power, and the current calculation must include the power factor cos φ:
I = P / (V · cos φ).