Ohm's Law & Power Calculator
Solve the whole electrical picture from any two known values. Enter any two of voltage, current, resistance, or power and the calculator works out the other two using Ohm's law and the power formula — the twelve-formula wheel in one input panel.
Enter Any Two
Solved Values
Enter at least two values
How this calculator works
Two identities generate everything on the classic formula wheel:
V = I × R (Ohm’s law) P = V × I (power)
Every other formula — P = I²R, I = √(P/R), R = V²/P, and the rest — is those two combined and rearranged. The calculator takes whichever two values you enter, solves for V and I first through the applicable rearrangement, then derives R and P from them, so all four values always agree with each other. The results hold exactly for DC and for AC across purely resistive loads (heaters, incandescent lamps); for AC circuits with motors, transformers, or electronics, real power P = V × I × PF — apply the power factor separately or the wattage will read high.
Worked example
A 1,500 W resistive heater on a 120 V circuit — what does it draw, and what is its element resistance?
- Current: I = P ÷ V = 1,500 ÷ 120 = 12.5 A.
- Resistance: R = V ÷ I = 120 ÷ 12.5 = 9.6 Ω.
- Cross-check with the wheel: P = V² ÷ R = 14,400 ÷ 9.6 = 1,500 W — consistent, as every solve from this calculator is by construction.
Enter 120 in Voltage and 1500 in Power above to reproduce it. Note the practical read: 12.5 A is over the 80% continuous threshold on a 15 A circuit — which is why 1,500 W is where plug-in heater nameplates stop.
Reference: the formula wheel
All twelve relationships, organized by what you know and what you need:
| Solve for | Form 1 | Form 2 | Form 3 |
|---|---|---|---|
| V (volts) | V = I × R | V = P ÷ I | V = √(P × R) |
| I (amps) | I = V ÷ R | I = P ÷ V | I = √(P ÷ R) |
| R (ohms) | R = V ÷ I | R = V² ÷ P | R = P ÷ I² |
| P (watts) | P = V × I | P = V² ÷ R | P = I² × R |
Frequently asked questions
How do I calculate amps from watts and volts?
Divide: I = P ÷ V. A 1,500 W space heater on a 120 V circuit draws 1500 ÷ 120 = 12.5 A. That holds exactly for DC and for resistive AC loads like heaters and incandescent lamps; for motors and electronics the current runs higher than P ÷ V because of power factor, so use the nameplate amps when you have them.
Which two values do I need to enter?
Any two of the four — voltage, current, resistance, or power — and the calculator solves the other two. The most common field pairs are watts + volts (to get amps) and volts + amps (to get watts and resistance). If you fill in more than two, it uses the first consistent pair; clear a field to change which inputs drive the answer.
Does Ohm's law work for AC circuits?
For purely resistive AC loads, yes — heat trace, resistance heaters, incandescent lighting all follow V = I × R and P = V × I at any instant. Loads with motors, transformers, or electronic power supplies have reactance and a power factor below 1.0, so real power is P = V × I × PF and the simple resistance form understates the current. This calculator is the DC/resistive case; apply power factor separately for reactive AC loads.
Why doesn't my measured resistance match the calculated value?
Temperature, usually. Resistance rises with heat, so a heating element or incandescent filament measured cold with a meter reads far lower than its operating resistance — a 120 V, 60 W incandescent lamp computes to 240 Ω hot but can ohm out around 20 Ω cold. On low-resistance measurements, meter lead resistance also adds a few tenths of an ohm. The calculated value reflects the load at operating conditions.
What's the difference between watts and volt-amps?
Watts are real power — what does work and what the utility meter bills. Volt-amps are apparent power, volts times amps, which is what actually flows in the conductors. They're equal only at power factor 1.0; conductors, breakers, transformers, and generators must be sized for the amps (VA side), which is why equipment is rated in kVA while loads are quoted in kW.
Method: V = I × R and P = V × I, solved simultaneously from any two knowns (DC / resistive AC loads). For reactive AC loads apply power factor separately — the solver’s P is apparent power in that case. Ohm’s law is physics, not code: it has no edition and no AHJ, but the circuits it describes still do.
Need the resistance to plug in? The wire resistance chart has Ω per 1000 ft by size. From there the Voltage Drop Calculator applies this same law to real runs, and the low-voltage DC drop chart covers 12/24 V device circuits.