How the calculation works
V = I·R, P = V·I = I²·R = V²/R. Enter any two values and leave the others blank (if more than two are filled, the first two in V, I, R, P order are used). Exact for resistive loads; use impedance and power factor for reactive AC loads.
Worked example
120 V and 10 A → R = 12 Ω, P = 1200 W.
Common mistakes to avoid
- Mixing units (mA with A, or kΩ with Ω) before entering values.
- Applying the formulas to reactive AC loads such as motors without power factor.
- Entering values that disagree; the calculator uses the first two it finds in V, I, R, P order.
Frequently asked questions
Does this work on AC?
Yes for resistive loads (heaters, incandescent). Motors and transformers need power factor.
What is the Ohm's Law wheel?
A memory aid that expresses V, I, R and P in terms of any two of the others. This tool computes all twelve relationships for you.
How many amps does a 1500 W heater draw at 120 V?
I = P ÷ V = 1500 ÷ 120 = 12.5 A. It is a continuous load, so size the circuit at 125% (15.6 A), which means a 20 A circuit.
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