This motor horsepower calculator works out an electric or mechanical motor's HP from either torque and RPM, or from electrical inputs (voltage, current, efficiency, and power factor) for single-phase and three-phase supplies. It's built for sizing motors, verifying nameplate ratings, and converting between mechanical and electrical power.
Motor HP Formulas
Mechanical vs Electrical Horsepower
There are two ways to arrive at a motor's HP. The torque-and-RPM method gives mechanical shaft horsepower — the useful output at the motor's shaft. The electrical method estimates the same shaft HP from the power drawn at the terminals, after accounting for efficiency and power factor losses. Both should land near the motor's nameplate rating; if your electrical calculation comes out much higher than the nameplate, the motor may be overloaded.
Sizing a Motor and NEMA Standard Ratings
Motors are manufactured in standard NEMA increments — 1, 1.5, 2, 3, 5, 7.5, 10, 15, 20, 25, 30, 40, 50 HP and up. Calculate your load requirement, then choose the next size above it so the motor isn't run at its absolute limit. Continuous loads should leave headroom, and electrical feeders are sized at 125% of full-load current per the NEC. Always read efficiency and power factor from the nameplate for accurate results.
Three-Phase Motor Full-Load Current (approx.)
| HP | 230 V FLA | 460 V FLA |
|---|---|---|
| 5 | 15.2 A | 7.6 A |
| 10 | 28 A | 14 A |
| 25 | 68 A | 34 A |
| 50 | 130 A | 65 A |
| 100 | 248 A | 124 A |
Values approximate NEC Table 430.250. Use the code tables for actual wire and breaker sizing.
Worked Examples
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
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.
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.
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.
Use HP = (torque in lb-ft × RPM) ÷ 5252 for the mechanical output. For electric motors, calculate from voltage, current, efficiency, and power factor instead.
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.