Motor Horsepower Calculator
Output (shaft) horsepower equals electrical input power times efficiency. This calculator handles single-phase and three-phase AC.
Common conversions
| Input | Result |
|---|---|
| 480 V, 10 A, 3φ, PF .85, η .90 | 8.53 HP |
| 480 V, 20 A, 3φ, PF .88, η .92 | 18.05 HP |
| 400 V, 30 A, 3φ, PF .85, η .91 | 21.55 HP |
| 230 V, 28 A, 1φ, PF .80, η .85 | 5.87 HP |
| 240 V, 40 A, 1φ, PF .85, η .87 | 9.52 HP |
| 600 V, 50 A, 3φ, PF .87, η .93 | 56.36 HP |
The math behind it
- HP = 1.732 × 415 × 25 × 0.86 × 0.90 / 746
- HP = 13,910.68 / 746
- HP ≈ 18.65 HP
Everything you need to know
Nameplate horsepower is mechanical output at the shaft, calculated here from the motor's electrical input (voltage, current, power factor) multiplied by efficiency. NEMA Premium motors typically run 91% to 96% efficient depending on frame size, while standard-efficiency motors can fall to 84-89% at the low end of that range.
Sizing HP for pumps, compressors, and fans
Centrifugal pumps and fans follow the affinity laws: required HP scales roughly with the cube of speed or flow, so doubling flow on a centrifugal fan needs about eight times the horsepower, not two. Positive-displacement compressors instead scale closer to linearly with pressure and flow, so their HP requirement is more predictable across the operating range. As a rule of thumb, size pump and fan motors to the actual duty point on the curve, not the shutoff or maximum-flow point, and add headroom only for documented system losses, not a blanket safety margin.
Service factor: how much overload is really available
Service factor (SF) is the multiplier a motor can handle briefly above its nameplate rating without immediate damage. An SF of 1.15 on a 10 HP motor allows short excursions to about 11.5 HP, but running there continuously shortens insulation life and can push efficiency and power factor outside their rated range. Standard practice is to select a motor whose base nameplate HP, not its service-factor HP, covers the expected continuous load, and to treat SF as a buffer for temporary conditions like voltage sag or a clogged filter rather than extra capacity to design around.
Single-phase vs three-phase HP math
The three-phase formula includes a √3 (about 1.732) multiplier that single-phase math doesn't need, because three-phase line current splits across conductors 120° apart. Two motors with identical voltage, current, PF, and efficiency produce very different HP depending on how many phases supply them, which is one reason single-phase motors are rarely built above 5-10 HP: the current needed for the same power becomes impractical to switch and wire.
Common applications
Pump HP follows the affinity laws: doubling flow on a fixed system can require roughly eight times the horsepower. Size the motor to the pump's actual operating point on the curve, not its shutoff head.
Compressors need enough starting torque to overcome trapped pressure on restart. Running HP from this calculator covers steady-state duty; add a soft starter or VFD if the load restarts under pressure.
Fan HP also follows the affinity laws, so verify the motor's calculated HP against the fan curve at the actual duct static pressure, not just the free-air rating.
Rewiring a dual-voltage motor from 240 V to 480 V halves the current for the same HP. Recalculate with the new voltage and amperage to confirm the output HP hasn't changed.
Common mistakes
Applying the single-phase formula to a three-phase reading understates HP by about 42%, since the √3 (1.732) multiplier is missing.
Continuous loads sized to the SF 1.15 rating run the motor hot every hour, not just occasionally, cutting winding life well below its rated design target.
Running HP from this calculator doesn't reflect locked-rotor demand, which can run 6 to 8 times higher. Undersized starters and breakers fail on the first start, not during normal operation.
Entering a line-to-neutral voltage into the three-phase line-to-line formula overstates HP by about 73%. Confirm which voltage the meter is reading before entering it.
Frequently asked questions
1 HP equals how many watts?+
745.7 W for mechanical HP, 746 W for electrical HP, and 735.5 W for metric PS. This calculator uses 746 W, the standard for AC motor work in North America.
How much HP does a 480 V, 20 A three-phase motor produce?+
About 18.05 HP at a power factor of 0.88 and efficiency of 0.92, using the three-phase formula with the √3 multiplier.
Is single-phase motor HP calculated differently from three-phase?+
Yes. Three-phase HP includes a √3 (1.732) multiplier that single-phase math drops, so identical voltage, current, PF, and efficiency numbers yield a smaller HP figure on single-phase.
What service factor should I use when selecting HP for a pump?+
Size to the base nameplate HP, not the service-factor HP. Treat a 1.15 SF as a margin for temporary conditions, not as 15% of usable continuous capacity.
Do compressors need more starting HP than running HP?+
Yes. Locked-rotor current, and the torque that comes with it, can run 6 to 8 times the running value, which is why compressor motors often need soft starters or VFDs even when the running HP looks modest.
How does efficiency change the HP calculated from the same current?+
Directly and proportionally. Raising efficiency from 0.85 to 0.95 at the same voltage, current, and PF increases the calculated output HP by about 12%, since more of the electrical input reaches the shaft.
Can I run a motor continuously at its service-factor HP?+
No. Continuous operation above the base nameplate rating shortens winding insulation life and typically voids the manufacturer's warranty at that load.
Why do fans need a bigger HP margin than compressors?+
Because fan and pump load follows the affinity laws. Required HP scales with roughly the cube of speed or flow, so small increases in system resistance or flow demand disproportionately more horsepower.
Is nameplate HP the same as HP calculated from volts and amps?+
It should be close. Nameplate HP is measured at rated voltage, current, PF, and efficiency; this calculator reproduces that figure when the same nameplate values are entered.
Does power factor change the mechanical horsepower a motor delivers?+
Yes, indirectly. A lower PF for the same voltage and current means less real power is being converted, so the calculated output HP drops even though the current reading looks unchanged.