Amps to HP Calculator

About the Amps to HP Calculator

This amps to HP calculator converts measured motor amperage back to horsepower for any AC or DC motor. Use it to verify nameplate HP from a clamp meter reading, identify mystery motors with worn nameplates, or estimate output capacity from utility metering data.

Defaults assume typical industrial efficiency (90%) and power factor (0.85). Adjust for high-efficiency Premium motors or motors operating at light load.

How amps to HP conversion works

3-phase: HP = (V × I × η × PF × √3) ÷ 746
Single-phase: HP = (V × I × η × PF) ÷ 746
DC: HP = (V × I × η) ÷ 746

This is the inverse of the HP-to-amps formula. Watts (V × I) is converted to HP by dividing by 746, then adjusted for motor losses (efficiency) and reactive power (power factor). The result is the mechanical shaft output, not the electrical input.

Worked example

A 460V three-phase motor measures 6.5 amps at 90% efficiency and 0.85 PF: HP = (460 × 6.5 × 0.9 × 0.85 × 1.732) ÷ 746 = ~5 HP. This matches a typical 5 HP NEMA motor.

When to use this calculator

• Verifying a motor’s HP rating when the nameplate is illegible or missing
• Cross-checking measured amperage against expected motor output
• Estimating motor capacity from utility kWh data
• Field troubleshooting: confirming a replacement motor matches the original
• Calculating actual delivered HP at part-load conditions
• Validating energy audit measurements

Frequently Asked Questions

How do you convert amps to HP for a 3-phase motor?

HP = (V × I × η × PF × √3) ÷ 746. For 460V at 6.5 A, 90% efficiency, 0.85 PF: HP = (460 × 6.5 × 0.9 × 0.85 × 1.732) ÷ 746 ≈ 5 HP.

Is this measured at full load or actual operating load?

The calculator gives output HP at the measured amperage. If you measure with the motor running at full load, you get full-load HP. If the motor is lightly loaded, you get the actual reduced HP, not the nameplate rating.

What efficiency value should I use?

For nameplate-load conditions: use the motor’s nameplate efficiency or 90% as a default. At part-load, efficiency drops — 85% at 50% load is typical. NEMA Premium motors stay efficient (90%+) down to 25% load.

Why don’t single-phase and DC use the √3 factor?

The √3 factor specifically accounts for the 120-degree phase angles in a three-phase system. Single-phase has only one current path; DC has no AC components, so no phase factor is needed.

How accurate is this calculation?

Within ±5% if you use accurate efficiency and PF values. The biggest sources of error are using nameplate efficiency at part-load, or using a generic 0.85 PF when the actual value is 0.65–0.75 (common for lightly loaded motors).

Can I use this with a clamp meter reading?

Yes — that’s exactly what it’s designed for. Measure RMS current (true RMS meter recommended for VFD-driven motors), enter voltage from a voltmeter or nameplate, and the calculator gives the corresponding HP.

Related calculators and motor categories

Identifying or replacing a motor? Browse General Purpose motors, or use the motor search to find by HP, voltage, and frame.

Related calculators: HP to Amps Calculator · Motor FLA Calculator · kW to HP Calculator · Motor Efficiency Calculator.