# How many amps does it draw?

Works out the current in amps from watts and volts (DC or single-phase AC with a power factor), from volts and ohms, or from watts and ohms, with the other circuit values.

- Page: https://www.acalculator.org/physics/amp-calculator
- JSON spec: https://www.acalculator.org/physics/amp-calculator.json
- Version: eb3b3813a0e7

## Default answer

Example with the default inputs (Work out amps from Watts and volts, Current AC, Power factor 1, Power 1,500 W, Voltage 120 V): The load draws 12.5 A at 120 V and 1,500 W.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| from | Work out amps from | Which two values you know. |
| cur | Current | Direct current (batteries, USB, solar) or single-phase alternating current (wall outlets). It matters only for watts and volts. |
| pf | Power factor | The share of the volt-amperes that does work: 1 for heaters and bulbs, about 0.8 to 0.95 for motors. |
| p | Power | The real power the load uses. |
| v | Voltage | The voltage across the load (RMS volts for AC). |
| r | Resistance | The resistance of the load. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| amps | Current | The current through the load, in amperes. |
| milliamps | Current (mA) | The current × 1,000. |
| watts | Power | The real power: V × I (× power factor for AC). |
| volts | Voltage | The voltage across the load. |
| ohms | Resistance | V ÷ I for a DC or resistive load. |
| va | Apparent power | Volts × amps, in volt-amperes. |

## Method

Watts and volts: I = P ÷ V (DC), I = P ÷ (V × pf) (single-phase AC). Volts and ohms: I = V ÷ R, P = V² ÷ R. Watts and ohms: I = √(P ÷ R), V = √(P × R).

## Assumptions

- AC values are RMS (the usual outlet and nameplate values) on a single-phase supply; three-phase circuits are not covered.
- The resistance modes treat the load as a pure resistance (power factor 1).

## Worked examples

1. from = pv, cur = ac, pf = 1, p = 1,500, v = 120 gives amps = 12.5, r = 9.6. Source: OpenStax, University Physics Volume 2, §9.5 Electrical Energy and Power (P = IV = I²R = V²/R), https://openstax.org/books/university-physics-volume-2/pages/9-5-electrical-energy-and-power.
2. from = pv, cur = dc, p = 2,300, v = 115 gives amps = 20. Source: OpenStax, University Physics Volume 2, §9.5 Electrical Energy and Power (P = IV = I²R = V²/R), https://openstax.org/books/university-physics-volume-2/pages/9-5-electrical-energy-and-power (Example 9.9: 20.00 A at 115 V is 2,300 W).
3. from = pv, cur = ac, pf = 0.8, p = 960, v = 120 gives amps = 10, va = 1,200. Source: OpenStax, University Physics Volume 2, §15.4 Power in an AC Circuit (average power = I_rms V_rms cos φ), https://openstax.org/books/university-physics-volume-2/pages/15-4-power-in-an-ac-circuit.
4. from = vr, cur = dc, v = 12, r = 4 gives amps = 3, p = 36. Source: OpenStax, University Physics Volume 2, §9.4 Ohm’s Law (V = IR), https://openstax.org/books/university-physics-volume-2/pages/9-4-ohms-law.
5. from = pr, cur = dc, p = 100, r = 25 gives amps = 2, v = 50. Source: OpenStax, University Physics Volume 2, §9.5 Electrical Energy and Power (P = IV = I²R = V²/R), https://openstax.org/books/university-physics-volume-2/pages/9-5-electrical-energy-and-power (P = I²R).

## FAQ

### How do I convert watts to amps?

Divide the watts by the volts: I = P ÷ V. A 1,500 W heater on a 120 V outlet draws 1,500 ÷ 120 = 12.5 A. For an AC motor, also divide by the power factor.

### How many amps is 1,000 watts?

It depends on the voltage: 1,000 W is 8.33 A at 120 V, 4.35 A at 230 V and 83.3 A at 12 V DC. Lower voltage needs more current for the same power.

### How do I find amps from volts and ohms?

Use Ohm’s law, I = V ÷ R. 12 V across a 4 Ω load drives 3 A, and the load uses V² ÷ R = 36 W.

### What is the power factor?

In AC circuits, the power factor is the share of volts × amps that does real work: P = V × I × power factor. It is 1 for heaters and incandescent bulbs, and often 0.8 to 0.95 for motors. A 960 W motor at 0.8 on 120 V draws 10 A, not 8 A.

### What is the difference between watts and volt-amperes?

Watts are the real power; volt-amperes (VA) are simply volts × amps. They are equal for DC and for resistive AC loads. With a power factor below 1, the VA is higher than the watts, and wiring and breakers must carry the full current.

### Does this work for three-phase power?

No. This page covers DC and single-phase AC, the supply in most homes. Three-phase circuits use a different formula with √3, so ask an electrician or use the equipment’s data plate.

## Sources

- OpenStax, University Physics Volume 2, §9.4 Ohm’s Law (V = IR). https://openstax.org/books/university-physics-volume-2/pages/9-4-ohms-law (retrieved 2026-10-01)
- OpenStax, University Physics Volume 2, §9.5 Electrical Energy and Power (P = IV = I²R = V²/R; Example 9.9). https://openstax.org/books/university-physics-volume-2/pages/9-5-electrical-energy-and-power (retrieved 2026-10-01)
- OpenStax, University Physics Volume 2, §15.4 Power in an AC Circuit (average power = I_rms V_rms cos φ, where cos φ is the power factor). https://openstax.org/books/university-physics-volume-2/pages/15-4-power-in-an-ac-circuit (retrieved 2026-10-01)