Watts Amps Volts Calculator
Solve watts, amps, or volts instantly by entering the other two values.
Your Result
- Pick a target — Choose whether you want watts, amps, or volts.
- Enter two values — Provide the other two fields required for the formula.
- Calculate — Results update in multiple units for convenience.
- Check assumptions — This uses basic relationships and does not apply power factor.
- Use load calculator for PF — If you need PF-aware current estimates, use Electrical Load Calculator.
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 @ 120V | Amps @ 240V |
|---|---|---|
| 500 | 4.2 | 2.1 |
| 1,000 | 8.3 | 4.2 |
| 1,500 | 12.5 | 6.3 |
| 2,000 | 16.7 | 8.3 |
| 3,000 | 25.0 | 12.5 |
| 5,000 | 41.7 | 20.8 |
Assumes power factor ≈ 1 (typical resistive loads). Use as a quick estimate.
Formulas Used
These are the formulas this calculator applies:
Amps = A = W ÷ V
Volts = V = W ÷ A
Related Calculators
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 standard 120V circuit: 1500W ÷ 120V = 12.5 amps. On a 240V circuit: 1500W ÷ 240V = 6.25 amps. This is a common check when sizing a circuit breaker — a 1500W heater on 120V needs at least a 15A breaker, and should ideally be on its own dedicated 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.