How do I calculate HP from amps for a single-phase motor?

HP = (Voltage × Amps × Efficiency × Power Factor) / 746. A 240V motor drawing 10A at 90% efficiency and 0.85 PF produces: 240 × 10 × 0.90 × 0.85 / 746 = 2.46 HP.

What efficiency should I use for a motor HP calculation?

Typical AC motor efficiencies range from 85% to 95%. Small motors (<1 HP) are often 80-85% efficient. Standard industrial motors are typically 88-92%. Premium efficiency motors reach 93-96%. Check the motor nameplate for the exact value.

Motor Horsepower Calculator

Q: How do I calculate motor HP from torque and RPM?

Use the formula: HP = Torque (lb-ft) × RPM / 5252. For example, 200 lb-ft at 3000 RPM gives 200 × 3000 / 5252 = 114.2 HP.

Q: How do I calculate motor horsepower from torque?

Use HP = (torque in lb-ft × RPM) ÷ 5252 for the mechanical output. For electric motors, calculate from voltage, current, efficiency, and power factor instead.

Q: What's the difference between input and output horsepower for a motor?

Input is the electrical power drawn from the supply; output (shaft) horsepower is what the motor actually delivers, lower by the motor's efficiency factor.

Calculate motor HP from torque & RPM, or from electrical input for single-phase and three-phase AC motors. Covers automotive engines and industrial electric motors.

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Motor HP Calculator

Torque lb-ft

RPM

Voltage (V)

Current (Amps)

Motor Efficiency %

Power Factor

Line Voltage (V)

Current (Amps)

Motor Efficiency %

Power Factor

Motor Horsepower

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Motor HP Formulas

From Torque & RPM

HP = Torque (lb-ft) × RPM / 5252

Where 5252 = 33,000 ft-lb/min ÷ (2π) — the fundamental mechanical constant.

Single-Phase Electrical

HP = (V × I × Eff × PF) / 746

V = voltage (volts) · I = current (amps) · Eff = efficiency (decimal) · PF = power factor

Three-Phase Electrical

HP = (√3 × V × I × Eff × PF) / 746

√3 ≈ 1.7321 · V = line-to-line voltage · I = line current in amps

Worked Examples

Example 1 — From Torque & RPM

Engine makes 350 lb-ft at 4,500 RPM

HP = 350 × 4500 / 5252 = 1,575,000 / 5252

= 299.9 HP

Example 2 — Three-Phase Motor

480V three-phase, 15A FLA, 91% efficiency, 0.87 PF

HP = (1.732 × 480 × 15 × 0.91 × 0.87) / 746

= 8,745.9 / 746 = 11.72 HP ≈ 10 HP motor (nearest standard size)

Always read efficiency and power factor from the motor nameplate. Calculated HP is input mechanical shaft HP. For motor sizing, choose the next standard NEMA HP rating above your calculated requirement.

Frequently Asked Questions

How do I calculate motor HP from torque and RPM?

Use HP = Torque (lb-ft) × RPM / 5252. The constant 5252 comes from converting RPM to radians per second and reconciling the units. For example, 200 lb-ft at 3,000 RPM = 200 × 3000 / 5252 = 114.2 HP.

What efficiency should I use for my motor?

Check the motor nameplate for the exact efficiency. As a general guide: small motors under 1 HP are 80-85% efficient; standard industrial motors (1-50 HP) are 88-92% efficient; premium/NEMA Premium motors exceed 93%. Older motors may be 80-85% even at larger sizes.

What is power factor in motor calculations?

Power factor (PF) is the ratio of real power to apparent power in an AC circuit. It accounts for the phase difference between voltage and current due to inductive loads. Induction motors typically have PF between 0.80 and 0.90 at full load. A lower PF means more current is drawn to deliver the same real power. Find PF on the motor nameplate.

How do I calculate motor horsepower from torque?