Formula
Any two of voltage, current, resistance, and power fix the other two. The page multiplies the number you type by the prefix on that field: p is 10⁻¹², n is 10⁻⁹, µ is 10⁻⁶, m is 10⁻³, k is 10³, M is 10⁶, and G is 10⁹. A blank or a zero on a required input leaves the readout as a dash.
V = I R
P = V I = I² R = V² / R
This is a DC resistance, or an AC resistance at a power factor of 1. The amp calculator is the same product with power factor and a three-phase √3. The current from this page is the load the wire size calculator and the voltage drop calculator size. Watts carried for hours are the battery runtime and the kWh calculator.
Worked examples
12 V and 0.5 A. Resistance is 24 Ω. Power is 6 W.
12 V and 100 Ω. Current is 0.12 A. Power is 1.44 W.
1 kΩ dissipating 1 W. Voltage is √(P R) = 31.62 V. Current is 31.62 mA.
Reference
Prefixes are exact powers of ten. The formulas use the prefixed SI value, not the digits alone.
Questions
What is 12 volts at half an amp?
24 Ω and 6 W. Resistance is voltage divided by current. Power is the product.
Where does the power factor go?
This page is a resistance. The amp calculator multiplies by power factor and, for three-phase, by √3. Use that one for a motor nameplate.
Does the answer pick a wire?
No. Take the current to the wire size calculator and the voltage drop calculator. Watt-hours of that load belong on the battery runtime and kWh pages.
Sources
- Ohm’s law, V = I R, and Joule’s law, P = V I.
- SI prefixes: p 10⁻¹², n 10⁻⁹, µ 10⁻⁶, m 10⁻³, k 10³, M 10⁶, G 10⁹.