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Watts Amps Volts Calculator

By · Updated Jul 2026

Solve watts, amps, or volts instantly by entering the other two values.

W
A
V

Your Result

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Solved Value
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Kilowatts
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Current
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Voltage
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Horsepower (approx.)
What This Result Means
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How to Use
  1. Pick a target - Choose whether you want watts, amps, or volts.
  2. Enter two values - Provide the other two fields required for the formula.
  3. Calculate - Results update in multiple units for convenience.
  4. Check assumptions - This uses basic relationships and does not apply power factor.
  5. Use load calculator for PF - If you need PF-aware current estimates, use Electrical Load Calculator.
Quick answer

Enter any two of watts, amps, and volts and this solves the third with W = V × A. A 1,500 watt space heater on 120 volts draws 12.5 amps - and because a heater runs continuously, that 12.5 amps is 83 percent of a 15 amp circuit, over the 80 percent limit for continuous loads, so it belongs on a 20 amp circuit. Run the same 1,500 watts at 240 volts and the current halves to 6.25 amps. This tool uses the basic relationship and does not apply power factor; for motors or three-phase, use the electrical load calculator.

Double the Volts, Half the Amps

120 V240 V12.5 A12.5 A6.25 ASame 1500 W loaddouble the volts, half the amps
The same 1,500 watt load draws 12.5 amps at 120 volts but only 6.25 amps at 240 volts. The power is unchanged, so raising the voltage lowers the current - which is why large appliances run on 240 volts and can use smaller wire.

What's the Relationship Between Watts, Amps, and Volts?

The basic relationship is W = V × A. If you know any two of watts, volts, and amps, you can solve for the third. This is most accurate for DC and resistive AC loads where power factor is close to 1.

For motors and many electronics, apparent power (VA) can differ from real power (W). If you only have watts and no power factor, treat the result as an estimate. For more detailed planning, use the Electrical Load Calculator.

Example: a 1,500W space heater on 120V draws about 12.5A. The same wattage at 240V draws about half the current, which is why higher voltage circuits often use smaller current for the same power.

Watts/Amps/Volts vs Electrical Load: What's the Difference?

This calculator is for one quick conversion. Electrical Load is for totals across multiple items.

Use this tool to convert a single device (watts ↔ amps) at a known voltage.

Use Electrical Load when you are adding many loads together (rooms, circuits, or a whole panel).

Once you know the target amps, use Breaker Size and Wire Size for practical planning.

Quick Reference (120V vs 240V)

Watts (W)Amps @ 120VAmps @ 240V
5004.22.1
1,0008.34.2
1,50012.56.3
2,00016.78.3
3,00025.012.5
5,00041.720.8

Assumes power factor ≈ 1 (typical resistive loads). Use as a quick estimate.

Formulas Used

These are the formulas this calculator applies:

Watts = W = V × A
Amps = A = W ÷ V
Volts = V = W ÷ A

Code Note

  • Continuous loads - anything expected to run three hours or more, like a space heater or EV charger - should not exceed 80 percent of the circuit rating. That is the same rule as sizing the breaker at 125 percent of the load, seen from the other side.NFPA 70 (NEC) 210.20(A) - Continuous Loads

Next Steps

Once you have the amps, size the circuit:

  1. Size the breakerAdd the 125 percent continuous factor and land on a standard breaker.
  2. Size the wireTurn the amps into a copper or aluminum gauge.
  3. Size a generatorAdd up running watts and starting surge for backup power.

Related Calculators

Ohms Law CalculatorVolts, amps, resistance.Electrical Load CalculatorTotal circuit current.Amp to Wire Size CalculatorWire gauge for a breaker size.Box Fill CalculatorBox cubic inches per NEC 314.16.

Use Ohm's Law to convert between watts, amps, and volts. Browse the electrical calculator collection.

FAQ

Why does this not ask for power factor?
This calculator uses the basic W=V×A relationship. For PF-aware current estimates, use the Electrical Load Calculator.
What is the difference between watts and volt-amps?
Watts represent real power. Volt-amps represent apparent power. They match when power factor is 1.0.
How many amps does a 1500 watt heater draw?
On a 120V circuit, 1500W / 120V = 12.5 amps; on 240V it is 6.25 amps. Watch the continuous-load rule when sizing the breaker: at 12.5 amps the heater fills 83 percent of a 15 amp circuit, over the 80 percent limit for a load that runs continuously, so it really belongs on a dedicated 20 amp circuit.
What is the formula for calculating watts from amps and volts?
Watts = Amps × Volts (W = A × V). This is the basic DC power formula and works for resistive AC loads. For inductive loads like motors and transformers, multiply by power factor: W = A × V × PF. For most household planning purposes the simplified formula is sufficient.
How many watts can a 20 amp circuit handle?
A 20A circuit at 120V can handle 2,400 watts at full load (20 × 120 = 2,400W). For continuous loads the NEC recommends using no more than 80% of circuit capacity, which is 1,920 watts. At 240V a 20A circuit can handle 4,800W total, or 3,840W continuous.
Updated Jul 2026 · See our Methodology
This calculator uses standard electrical formulas and published reference tables. Local codes and amendments vary. Use for planning estimates only. See our Data Sources and Methodology.