Amp Calculator — Watts, kW, kVA and HP to Amps

Electrical & Energy

Convert watts, kilowatts, kVA or motor horsepower to amps — or amps back to watts — for DC, single-phase and three-phase circuits.

Last updated: October 2026

Current (amps), voltage (volts) and power (watts) are linked: for a DC circuit, watts = volts × amps. This amp calculator applies that relationship for DC, single-phase AC and three-phase AC. It includes power factor for AC loads and efficiency for motors. For AC, power factor matters. A resistive load such as a heater or incandescent bulb has a power factor of 1, so amps = watts ÷ volts. Motors, compressors and many electronic power supplies draw more current than their watt rating suggests. Their power factor is below 1, so divide by it as well. Three-phase circuits spread the load across three conductors, which brings in a factor of √3 (about 1.732) when you use the line-to-line voltage. If you know apparent power in kVA (common on transformers and generators), power factor doesn't apply: current = VA ÷ volts (÷ √3 for three-phase). For a motor rated in horsepower, the calculator converts output HP to the electrical input power using the efficiency you enter.

How to Convert Watts to Amps

1. Choose the system: DC, single-phase AC, or three-phase AC. For three-phase, use the line-to-line voltage (e.g. 208, 480). 2. Convert the power to watts: kW × 1,000; HP × 746 ÷ efficiency for a motor's electrical input. 3. DC: amps = watts ÷ volts. 4. Single-phase AC: amps = watts ÷ (volts × power factor). 5. Three-phase AC: amps = watts ÷ (√3 × volts × power factor). For kVA, skip the power factor: amps = VA ÷ (√3 × volts).

Formula

DC:            I = P ÷ V
Single-phase:  I = P ÷ (V × PF)
Three-phase:   I = P ÷ (√3 × V_LL × PF)

From apparent power (AC):
  Single-phase:  I = S ÷ V
  Three-phase:   I = S ÷ (√3 × V_LL)

Motor input:   P = HP × 746 ÷ Efficiency

I — current (A)   P — real power (W)   S — apparent power (VA)
V — voltage (V)   V_LL — line-to-line voltage   PF — power factor (0–1)

Amp Calculation Examples

Example 1 — 1,800 W space heater on 120 V

Resistive load, PF = 1
I = 1,800 ÷ 120 = 15.0 A

Example 2 — 10 kW three-phase load at 480 V, PF 0.9

I = 10,000 ÷ (1.732 × 480 × 0.9)
  = 10,000 ÷ 748.2
  = 13.37 A per phase

Example 3 — 75 kVA transformer secondary, 208 V three-phase

I = 75,000 ÷ (1.732 × 208)
  = 75,000 ÷ 360.3
  = 208.2 A full-load current

Practical Notes

› For sizing conductors, take the current from here to the wire size calculator. It checks ampacity and voltage drop.
› Nameplate current beats a calculated value. Motors and appliances list full-load amps on the nameplate, and that is what you should design to.
› To see what a load costs to run, enter its watts in the energy cost calculator.
› In the US, 120 V and 240 V are nominal. Actual supply voltage varies, and lower voltage means higher current for the same power.

Watts to Amps at Common Voltages (PF = 1)

Watts	12 V DC	120 V	240 V	208 V 3φ	480 V 3φ
100	8.3 A	0.83 A	0.42 A	0.28 A	0.12 A
500	41.7 A	4.17 A	2.08 A	1.39 A	0.60 A
1,000	83.3 A	8.33 A	4.17 A	2.78 A	1.20 A
1,500	125.0 A	12.50 A	6.25 A	4.16 A	1.80 A
2,000	166.7 A	16.67 A	8.33 A	5.55 A	2.41 A
5,000	416.7 A	41.67 A	20.83 A	13.88 A	6.01 A
10,000	833.3 A	83.33 A	41.67 A	27.76 A	12.03 A

Computed from the formulas above with power factor 1 (resistive load). Loads with a lower power factor draw proportionally more current.

Assumptions & Limitations

What this calculator assumes

Steady-state, sinusoidal AC with balanced three-phase loads. Three-phase results are per-phase line current.
Power factor is entered by you. It is 1 for resistive loads; for motors and electronic loads, use the nameplate or datasheet value.
Horsepower is mechanical output, converted at 746 W per HP and divided by the efficiency you enter to get electrical input.
Voltage is the actual supply voltage at the load.

Not included

Motor branch-circuit sizing. In the US, the NEC sizes motor circuits from its full-load current tables (Article 430), not from nameplate horsepower math.
Inrush or starting current, harmonics, and unbalanced three-phase loads.
Breaker or fuse selection and conductor sizing.

Frequently Asked Questions

How many amps is 1,500 watts? +

At 120 V with a resistive load, 1,500 ÷ 120 = 12.5 A. At 240 V it is 6.25 A. A motor or other inductive load with a power factor below 1 draws more current for the same wattage.

Why does three-phase use √3? +

In a balanced three-phase system, total power is shared across three conductors whose voltages are 120° apart. Measured with line-to-line voltage, total power works out to √3 × V × I × PF, so current per phase is power ÷ (√3 × V × PF).

Should I use watts or kVA? +

Use watts (real power) when you know what the load consumes, with its power factor. Use kVA (apparent power) when the equipment is rated that way, as transformers, generators and UPS units usually are. Power factor is already included in kVA.