Free Ohm's law calculator · No signup
Enter any two of voltage, current, resistance, or power — get the other two instantly, including the full power formulas (P = VI, I²R, V²/R).
Enter any two values to solve for the other two.
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Ohm's law ties together the three fundamentals of any circuit: voltage (V), current (I), and resistance (R) — V = I × R. Add power (P) and you have the four-quantity "Ohm's law wheel," where knowing any two lets you solve the rest.
The twelve relationships: V = IR = P/I = √(PR). I = V/R = P/V = √(P/R). R = V/I = V²/P = P/I². P = VI = I²R = V²/R. This calculator picks the right pair automatically based on which two boxes you fill.
On real installations this is the math behind sizing a breaker for a known load, checking conductor heating (I²R losses), and sanity-checking a reading in the field. For loads on a project, Field PM tracks the equipment and circuits alongside your project documentation — note that motors and other reactive AC loads also need power factor for true watts.
Ohm's law states that voltage equals current times resistance: V = I × R. Rearranged, current I = V ÷ R and resistance R = V ÷ I. Add power and you get the full "Ohm's law wheel": P = V × I, P = I² × R, and P = V² ÷ R.
Power in watts equals voltage times current: P = V × I. A 120 V circuit drawing 10 A is using 1,200 watts. If you know current and resistance instead, P = I² × R; with voltage and resistance, P = V² ÷ R.
Current equals power divided by voltage: I = P ÷ V. A 1,500 W heater on 120 V draws 12.5 A. This is the calculation behind sizing breakers and conductors for a known load.
For purely resistive AC loads (heaters, incandescent lighting), yes — use RMS values. For motors and other reactive loads, true power also depends on the power factor (P = V × I × PF), so the simple wheel gives apparent power (VA), not real watts.
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