Amp Calculator

By:
Patrick Myers
Patrick Myers Patrick Myers is the CEO and Founder of Calculator Academy. He has over eight years of experience developing online calculators and holds a B.S. in Mechanical Engineering. His work focuses on formula implementation, calculator accuracy, unit conversions, and creating practical tools that are easy to use.
Last Updated: October 14, 2026

Convert watts or kW to amps at any voltage for single-phase AC, three-phase or DC, or find amps from kVA or ohms and watts from amps.

DC bus voltage or AC RMS voltage. For balanced three-phase, use line-to-line voltage.

Optional load arithmetic
Arithmetic comparison only. It does not select conductors, breakers or permitted loading.

Amp Formula

The current in amps depends on what you know and the circuit type. The calculator solves for current from real power, apparent power, or voltage and resistance. Real and apparent-power modes offer DC, single-phase AC and balanced three-phase AC; the three-phase input is line-to-line RMS voltage.

Amps from power (watts):

DC: I = W / V
 AC single phase: I = W / (V * PF)
 Balanced AC three phase (line to line): Iline = Wtotal / (sqrt(3) * Vline * PF)

Amps from voltage and resistance (Ohm’s law):

I = V / R

Amps from apparent power (kVA):

Single phase: I = (kVA * 1000) / V
 Balanced three phase: Iline = (kVA_total * 1000) / (sqrt(3) * Vline)

Where:

  • I is the current in amps (A)
  • W is the real power in watts (W)
  • V is the voltage in volts (V)
  • R is the resistance in ohms (Ω)
  • PF is the power factor (greater than 0 and no more than 1 for this positive-power model)
  • kVA is the apparent power in kilovolt-amps
  • 1.732 is a rounded approximation to the square root of 3; the calculator uses Math.sqrt(3)

For steady DC, P = VI. For AC, power factor is real power divided by apparent power. Use RMS voltage and current. Balanced three-phase formulas take total power and return line current. The VA/kVA mode does not determine AC real power without power factor. Ohm’s law uses DC resistance or a purely resistive AC load. Schneider Electric’s formula reference gives the single- and three-phase VA relationships.

Current by Circuit Type and Voltage

Use the formula that matches your circuit type. The table shows which equation the calculator applies for each selection.

Circuit typeAmps from wattsAmps from kVA
DCW / V(kVA * 1000) / V
AC single phaseW / (V * PF)(kVA * 1000) / V
AC three phase (line to line)Wtotal / (sqrt(3) * Vline * PF)(kVA_total * 1000) / (sqrt(3) * Vline)

Illustrative power factor ranges. The calculator starts at 1 (a resistive load); change it for other loads. Actual values vary by equipment, loading, waveform and power-factor correction; use a measured or manufacturer-specified value. An ideal resistive load has PF = 1.

Load typeTypical power factor
Resistive heater or incandescent lamp1.0
LED or fluorescent lighting0.9 to 0.95
Induction motor (loaded)0.8 to 0.9
Induction motor (lightly loaded)0.5 to 0.7

Example Problems

Example 1. Find the current of a 1500 watt heater on a 120 volt single phase supply. A resistive heater has a power factor of 1.0.

I = W / (V * PF) = 1500 / (120 * 1.0) = 12.5 A

The heater draws 12.5 amps.

Example 2. Find line current for a motor drawing 10 kW of electrical real input power from a balanced 400 volt three-phase line-to-line supply with power factor 0.85. A mechanical-output rating would also require motor efficiency to obtain electrical input power.

Iline = Wtotal / (sqrt(3) * Vline * PF) = 10000 / (sqrt(3) * 400 * 0.85) ≈ 16.980890 A

The motor draws about 16.98 amps per line.

Frequently Asked Questions

Why do I need the power factor to find amps?

Power factor is the ratio of real power (watts) to apparent power (volt-amps) in an AC circuit. When the load is not purely resistive, some current does no useful work, so the same real power requires more current. Dividing by power factor accounts for that extra current. For DC circuits and purely resistive AC loads, the power factor is 1 and has no effect.

What voltage should I use for three phase?

Enter line-to-line RMS voltage, for example 208 V, 400 V or 480 V, for balanced three-phase inputs. This widget has no line-to-neutral selector. For a balanced Y system, Vline-to-line = sqrt(3) × Vline-to-neutral; convert to line-to-line voltage before entering it. The displayed Y-equivalent phase impedance magnitude uses Vline-to-line / (sqrt(3) × Iline) under a balanced sinusoidal model, not a delta-branch impedance. MIT’s polyphase notes explain these line and phase relationships.

Can I convert kVA to amps without the power factor?

Yes. kVA is apparent power, which already includes the effect of power factor, so you do not apply it again. For single phase, divide volt-amps by voltage. For three phase, divide by voltage times the square root of 3. You only need power factor when converting between kVA (apparent power) and kW (real power).

Amp Calculator screenshot