How many amps does it draw?
Pick the two values you know, watts and volts, volts and ohms, or watts and ohms. The amp calculator works out the current the load draws, with the other circuit values.
- Current
- 12.5
The load draws 12.5 A at 120 V and 1,500 W.
- Current (mA)mA
- 12,500
- PowerW
- 1,500
- VoltageV
- 120
- ResistanceΩ
- 9.6
- Apparent powerVA
- 1,500
Current: 12.5. The load draws 12.5 A at 120 V and 1,500 W.
How to calculate
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.
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.
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).
- 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
Each example is checked against the calculator on every build.
- Work out amps from Watts and volts, Current AC, Power factor 1, Power 1,500, Voltage 120 gives Current 12.5, Resistance 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
- Work out amps from Watts and volts, Current DC, Power 2,300, Voltage 115 gives Current 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)
- Work out amps from Watts and volts, Current AC, Power factor 0.8, Power 960, Voltage 120 gives Current 10, Apparent power 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
- Work out amps from Volts and ohms, Current DC, Voltage 12, Resistance 4 gives Current 3, Power 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
- Work out amps from Watts and ohms, Current DC, Power 100, Resistance 25 gives Current 2, Voltage 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)
How it works
From watts and volts.
- DC: I = P ÷ V.
- Single-phase AC: I = P ÷ (V × pf), where pf is the power factor (from 0.01 to 1; 1 by default).
- Resistance R = V ÷ I is shown only when the power factor is 1 (DC, or a resistive AC load).
From volts and ohms (Ohm’s law): I = V ÷ R, and P = V² ÷ R.
From watts and ohms: I = √(P ÷ R), and V = √(P × R).
In every mode: apparent power VA = V × I, and milliamps = I × 1,000. The DC or AC choice matters only when working from watts and volts.
Rules. Power, voltage and resistance are each from 1 micro-unit to 1,000,000 units (0.000001 W to 1,000,000 W, and so on); the power factor is from 0.01 to 1. Every pair of values in range has an answer.
Output format. Decimals and significant figures below are the most shown; trailing zeros are dropped (23.0 shows as 23, money keeps its cents). Every value shows 6 significant figures: amps in A, power in W, voltage in V, resistance in Ω and apparent power in VA. Values are computed in double precision from the SI units you type (1 kW = 1,000 W, 1 mV = 0.001 V, 1 kΩ = 1,000 Ω).
Assumptions
- AC values are RMS, the values on outlets and nameplates, on a single-phase supply.
- The resistance modes treat the load as a pure resistance.
Worked examples by hand
A 1,500 W heater on 120 V. I = 1,500 ÷ 120 = 12.5 A; R = 120 ÷ 12.5 = 9.6 Ω.
OpenStax’s winch motor (Example 9.9). 2,300 W at 115 V DC: I = 2,300 ÷ 115 = 20 A.
A motor at power factor 0.8. 960 W on 120 V: I = 960 ÷ (120 × 0.8) = 10 A; apparent power 120 × 10 = 1,200 VA.
Ohm’s law. 12 V across 4 Ω: I = 3 A; P = 144 ÷ 4 = 36 W.
Watts and ohms. 100 W in 25 Ω: I = √4 = 2 A; V = √2,500 = 50 V.
Other questions people ask
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.