Ohm's Law Calculator
Solve voltage, current, resistance and power from any two known values. Press Calculate to see the results.
Enter any two values. The other two are calculated automatically (the two fields you edited last are used as inputs).
Step-by-step
- Enter two known values to begin.
Results assume a purely resistive load (power factor = 1) or DC. Always verify against the applicable NEC edition and the manufacturer’s data before installing.
Ohm’s Law: The Math Behind Every Circuit You Will Ever Touch
Core Engineering Principles
Every wire I have ever pulled comes back to one idea: push a voltage across a resistance and a current flows. Double the voltage and the current doubles. Double the resistance and it halves. That’s Ohm’s Law, V = I × R, and on a DC circuit or a resistive AC load like a heater or an incandescent lamp it’s all you need. Power is the other half. Whatever the circuit does with that current shows up as watts, P = V × I, and the part that doesn’t do useful work comes out as heat in the wire. That’s Joule’s law, P = I² × R, and it explains why a loose lug cooks itself: a bad connection adds resistance, resistance times current squared gives you heat, and heat makes the connection worse.
On AC you have to be careful. Motors, transformers and drivers have inductive reactance, so current and voltage fall out of step and the simple formula only describes the resistive part. For those loads you work with impedance and power factor instead, which is why the voltage drop and motor calculators on this site look a bit different. Use this page for DC, heaters, lamps, resistor networks, and as a sanity check on everything else.
Enter any two values and the other two follow.
NEC & Standard References
Ohm’s Law isn’t an NEC article, but the Code is built on it. The ampacity tables in Table 310.16 exist because I²R heating has to stay below what the insulation can survive. 210.19(A) and 215.2(A) carry the 125% continuous-load rule, and the informational notes there recommend keeping voltage drop to 3% on a branch circuit and 5% overall. Chapter 9, Table 8 gives you the dc resistance of the actual conductors, which is what you plug in for R when you want to check a long run by hand.
| You know | You want | Use |
|---|---|---|
| Volts and amps | Resistance | R = V / I |
| Watts and volts | Amps | I = P / V |
| Watts and amps | Resistance | R = P / I² |
| Volts and ohms | Watts | P = V² / R |
1. Current: I = P / V = 1,500 / 120 = 12.5 A.
2. Resistance of the element: R = V / I = 120 / 12.5 = 9.6 Ω.
3. Check: P = I² × R = 12.5² × 9.6 = 1,500 W, so the numbers agree.
12.5 A is 83% of the 15 A breaker. A heater runs for hours, so it’s a continuous load, and the Code only lets you load a breaker to 80% (12 A) for that. This heater is slightly over what the circuit should carry continuously, and that’s exactly the kind of thing that trips an old breaker at 2 a.m.
- Don’t apply plain Ohm’s Law to motors or LED drivers. Power factor and inrush change the picture. Use the motor and cable-size calculators for those.
- Heat is where it fails. Hot receptacles and warm breaker lugs are almost always a resistance problem. Torque every terminal to the number printed on the device.