Ohm's Law Calculator
Enter any two of volts, amps, ohms and watts for the other two. Below it, convert AC watts to amps with a power factor for single- or three-phase, which is the number that actually sizes a wire or a breaker.
Ohm's law (DC or resistive AC)
Volts (V)
120.00
Amps (A)
0.500
Ohms (Ω)
240.00
Watts (W)
60.00
AC power: watts ↔ amps with power factor
1.0 resistive (heaters); 0.8-0.9 motors, AC units.
Amps
22.22 A
4800 W at 240 V, PF 0.90
Apparent power
5333 VA
what the wire and breaker see
22.2 A back to watts
4800 W
check
Watts to amps at common voltages
Resistive load (power factor 1.0). For motors and AC units multiply by about 1.1 to 1.25.
| Watts | 12 V | 24 V | 120 V | 208 V | 240 V | 480 V | 480 V 3φ |
|---|---|---|---|---|---|---|---|
| 100 | 8.3 A | 4.2 A | 0.8 A | 0.5 A | 0.4 A | 0.2 A | 0.1 A |
| 500 | 41.7 A | 20.8 A | 4.2 A | 2.4 A | 2.1 A | 1.0 A | 0.6 A |
| 1,000 | 83.3 A | 41.7 A | 8.3 A | 4.8 A | 4.2 A | 2.1 A | 1.2 A |
| 1,500 | 125 A | 62.5 A | 12.5 A | 7.2 A | 6.3 A | 3.1 A | 1.8 A |
| 2,000 | 167 A | 83.3 A | 16.7 A | 9.6 A | 8.3 A | 4.2 A | 2.4 A |
| 3,000 | 250 A | 125 A | 25.0 A | 14.4 A | 12.5 A | 6.3 A | 3.6 A |
| 5,000 | 417 A | 208 A | 41.7 A | 24.0 A | 20.8 A | 10.4 A | 6.0 A |
| 7,500 | 625 A | 313 A | 62.5 A | 36.1 A | 31.3 A | 15.6 A | 9.0 A |
| 10,000 | 833 A | 417 A | 83.3 A | 48.1 A | 41.7 A | 20.8 A | 12.0 A |
The twelve formulas
| To find | from V and I | from V and R | from I and R | from P and … |
|---|---|---|---|---|
| Volts (V) | — | — | I × R | P ÷ I, or √(P × R) |
| Amps (I) | — | V ÷ R | — | P ÷ V, or √(P ÷ R) |
| Ohms (R) | V ÷ I | — | — | V² ÷ P, or P ÷ I² |
| Watts (P) | V × I | V² ÷ R | I² × R | — |
Typical power factors
| Load | Power factor | Note |
|---|---|---|
| Resistance heaters, incandescent lamps, water heaters | 1.0 | watts = volt-amps |
| LED drivers, modern electronics with PFC | 0.9–0.99 | cheap supplies can be 0.5–0.7 |
| Air conditioners, heat pumps | 0.85–0.95 | nameplate RLA is the amps to size for |
| Induction motors, loaded | 0.8–0.9 | much lower when lightly loaded |
| Induction motors, idling | 0.2–0.4 | amps stay high while watts fall |
| Microwave ovens | 0.9–0.95 |
Frequently Asked Questions
What is Ohm's law?
V = I × R: voltage equals current times resistance. With power, P = V × I, the four quantities are linked so that any two give the other two. A 60 W bulb on 120 V draws 0.5 A and has a hot resistance of 240 Ω; a 1,500 W heater on 120 V draws 12.5 A and is 9.6 Ω.
How do I convert watts to amps?
Amps = watts ÷ volts for DC or a resistive AC load like a heater. For AC loads with a power factor below 1 (motors, air conditioners, most electronics), amps = watts ÷ (volts × power factor); for three-phase, amps = watts ÷ (1.732 × volts × power factor). A 4,800 W single-phase load at 240 V and 0.9 PF draws 22.2 A, not 20.
What is power factor and why does it matter for wire sizing?
The ratio of real power (watts) to apparent power (volt-amps). Motors and electronic supplies draw current that is partly out of phase with voltage, so the wire carries more amps than the wattage suggests. Wires, breakers and generators must be sized for the amps, which is why nameplates list amps or VA rather than watts.
Is Ohm's law exact for AC?
For resistive loads, yes. For loads with inductance or capacitance, resistance becomes impedance (Z) and the phase angle matters; V = I × Z still holds but the arithmetic involves complex numbers. For sizing household circuits, the power-factor version above is the practical form.
These calculators use National Electrical Code tables and industry sizing charts and are for planning. They are not a substitute for a licensed electrician, an HVAC contractor's Manual J load calculation, or your local code, which governs. Electrical work can kill you and may require a permit; when in doubt, hire it out.