ProjectFigure Exact material estimates for every home & DIY project.

Amperage Calculator

An amperage calculator works out the current a load draws — from its wattage, or from a motor's horsepower — at a given voltage, for DC and AC single- and three-phase. It's how you check whether a circuit, breaker or extension cord can handle an appliance or motor.

Your measurements

Result

Motors are the tricky case: they're rated in horsepower, not watts, and draw a big surge at start-up. This converts HP to watts, applies efficiency and power factor, and gives the running amps to size from.

How it is calculated

Amps = watts ÷ (volts × power factor) for AC single-phase; three-phase divides by √3 × volts × PF; DC is simply watts ÷ volts. Horsepower is converted at 1 HP = 746 W and divided by motor efficiency. The result is running current — motor inrush can be 3-7× higher for a moment.

Steps

Choose whether your load is in watts or motor horsepower.
Enter the value (and efficiency for a motor).
Enter the voltage and choose DC, single- or three-phase.
Set the power factor for AC loads.
Read the running current in amps.

Worked example

A 1,500 W appliance on a 120 V circuit draws about 12.5 amps (watts ÷ volts, PF ≈ 1).

Common questions

How do I calculate amps?

Divide watts by volts (and by power factor for AC). A 1,500 W load at 120 V is about 12.5 A. The calculator covers DC, single- and three-phase.

How many amps does a motor draw?

Convert horsepower to watts (×746), divide by efficiency, then by volts × power factor. Remember start-up current is several times the running amps.

What power factor should I use?

≈1.0 for heaters and resistive loads, ~0.8-0.9 for motors and many appliances. Check the nameplate.

Why does my breaker trip on start-up?

Motors pull a large inrush current for a fraction of a second. Size the circuit for running amps but use breakers/fuses rated to tolerate the surge.