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Watts, amps and volts, solve for any one.
Fill in two of the three values and get the third. Works for DC and for resistive AC loads such as heaters and incandescent lamps.
Short answer
Amps = watts / volts. A 1,800 W appliance on a 120 V outlet draws 15 A. The reverse is watts = amps x volts, so 20 A at 240 V is 4,800 W.
Values
Result
Worked example
A 1,800 W appliance on a 120 V outlet: 1,800 / 120 = 15 A. A 1,500 W load on 240 V: 1,500 / 240 = 6.25 A, a lower current for the same power. Going the other way, a 20 A circuit at 240 V can deliver up to 20 x 240 = 4,800 W.
Formula, assumptions and sources
HyperPhysics gives the electric power in a DC circuit as the product of voltage and current, P = VI, and notes the relationships stay valid for AC when voltage and current are RMS values and the load is a resistor. For AC loads in general the power expression includes a power factor, which this tool does not apply, so for motors, compressors and power supplies the real current is higher than watts / volts. Use the nameplate amps where one is printed. Single-phase and DC only; three-phase formulas are not included.
Source: HyperPhysics (Georgia State University), Electric Power (opened 2026-10-08)
The three formulas
Electric power in watts is the product of voltage and current: P = V × I. HyperPhysics (Georgia State University) gives this for a resistor in a DC circuit and notes the relationships stay valid for AC when the voltage and current are RMS values. Rearranged, the calculator solves whichever of the three quantities you leave blank:
- Amps = watts ÷ volts
- Watts = amps × volts
- Volts = watts ÷ amps
The wattage is how fast the load converts electrical energy to heat, motion or light; the amperage is how much current flows; the voltage is the supply. The same wattage means half the current at twice the voltage, which is why large appliances such as ranges and dryers use 240 V circuits.
Quick reference
| Power | Amps at 12 V | Amps at 120 V | Amps at 240 V |
|---|---|---|---|
| 100 W | 8.33 | 0.83 | 0.42 |
| 500 W | 41.67 | 4.17 | 2.08 |
| 1,000 W | 83.33 | 8.33 | 4.17 |
| 1,500 W | 125.00 | 12.50 | 6.25 |
| 1,800 W | 150.00 | 15.00 | 7.50 |
| 2,400 W | 200.00 | 20.00 | 10.00 |
| 3,000 W | 250.00 | 25.00 | 12.50 |
Worked examples
- Hair dryer or space heater, 1,500 W at 120 V: 1,500 ÷ 120 = 12.5 A. The same 1,500 W on 240 V is 6.25 A.
- Circuit capacity: a 20 A circuit at 240 V can deliver up to 20 × 240 = 4,800 W; at 120 V it is 2,400 W.
- 12 V DC in a van: a 100 W device draws 100 ÷ 12 = 8.33 A, much more current than the same watts at 120 V, so wire and fuse sizes matter.
- Finding voltage: if a 1,500 W heater is rated 12.5 A, the supply is 1,500 ÷ 12.5 = 120 V.
Power factor: why motors draw more than watts ÷ volts
HyperPhysics points out that the resistor relationship is a special case. For AC loads in general the power expression includes a power factor that accounts for phase differences between voltage and current. For single-phase AC, current = watts ÷ (volts × power factor). As an illustration, a motor load of 1,200 W on 120 V with a power factor of 0.8 draws 1,200 ÷ (120 × 0.8) = 12.5 A, not the 10 A that watts ÷ volts suggests. This calculator applies no power factor, so for motors, compressors and many power supplies use the nameplate amperage instead. Heaters and incandescent lamps are close to resistive, where the simple formula is right.
Common mistakes
- Confusing watts with volt-amperes. Some labels give VA, which is volts × amps without power factor. For real power in watts, multiply VA by the power factor.
- Assuming the nominal voltage. A supply stated as 120 V can differ in practice. Use the voltage on the equipment's label.
- Treating a breaker rating as the load. A 20 A breaker is a limit, not what the circuit carries. Check the continuous-load planning rules in the breaker watts calculator.
- Using the single-phase formula for three-phase equipment. Three-phase loads need a different formula; see the kW to amps three-phase calculator.
Related checks
To see how amps relate to resistance, use the Ohm's law calculator. To size a cord for a given current, use the extension cord gauge calculator. Electrical work can cause fire and shock; this tool is a planning aid, not a substitute for a licensed electrician.
Sources
Retrieved
Every link below was opened and read in October 2026. The note after each link says which statement on this page it supports.
- HyperPhysics (Georgia State University), Electric Power (retrieved 2026-10): P = V x I for a resistor; valid for AC with RMS values; the general AC expression includes a power factor
- U.S. EIA, Measuring electricity (retrieved 2026-10): definitions of watt and kilowatt-hour
Safety and limits
This is a planning aid, not professional advice. Electrical and HVAC work can cause fire, shock and carbon monoxide hazards; have a licensed professional size, install and inspect equipment.
FAQ
How many amps is 1,800 watts?
At 120 V, 1,800 / 120 = 15 A. At 240 V it is 7.5 A.
How many watts is 20 amps?
At 240 V, 20 x 240 = 4,800 W. At 120 V it is 2,400 W.
Is watts divided by volts always the current?
For DC and resistive AC loads, yes. Motors and electronics have a power factor, so their current is higher than watts / volts; read the nameplate.
How do I convert watts to amps at 240 volts?
Divide the watts by 240. A 3,000 W load draws 12.5 A at 240 V and 25 A at 120 V. For a motor, also account for power factor or use the nameplate current.
Why does my motor draw more amps than watts divided by volts?
Motors and many power supplies have a power factor below 1, so current is watts divided by volts times power factor. A 1,200 W motor on 120 V at a power factor of 0.8 draws 12.5 A, not 10 A.
Does the same formula work for DC and 12 V systems?
Yes, for DC amps = watts ÷ volts exactly. A 100 W load on a 12 V battery draws 8.33 A. That current is why 12 V wiring must be thicker than 120 V wiring for the same power.
Can I use this for three-phase power?
No. This tool is for single-phase AC and DC. Three-phase calculations include a factor of the square root of 3, so use a three-phase kW to amps calculator.