HP ↔ kW Converter
Convert between horsepower definitions, kW and watts, with torque and nearest standard motor size.
Step-by-step
- Enter valid values to begin.
1 HP (mechanical) = 0.745699872 kW; 1 PS = 0.73549875 kW; 1 electrical HP = 0.746 kW exactly. The value is treated as shaft output power. Torque uses T = 9550 × kW / rpm. Input kW = shaft kW / efficiency and is optional context (set efficiency to 100 to see output only). Standard-size matching picks the closest listed rating, it does not choose a motor for you.
HP ↔ kW: Three Horsepowers and One Kilowatt
Core Engineering Principles
A kilowatt is a kilowatt everywhere on earth, but a horsepower is not. We carry three of them around. Mechanical (imperial) HP is 550 ft·lbf/s, which works out to 745.699872 W. Metric horsepower, the PS or CV on European and Japanese nameplates, is 75 kgf·m/s, or 735.49875 W. Electrical HP was rounded by the electrical trade to exactly 746 W. They look close enough to ignore, and that’s the trap: metric versus mechanical is a 1.4% gap.
The next thing to keep straight is which side of the motor the number describes. A motor’s rating, in kW or HP, is shaft output: the mechanical power it delivers to the coupling. The utility meter sees electrical input, which is output divided by efficiency, so a 37 kW motor at 93% draws about 40 kW. Speed ties it to torque, because power is torque times angular speed. At 1480 rpm a given output needs far more torque than at 2960 rpm, so convert power and then check the coupling, key and gearbox against the torque, not the horsepower figure.
T (N·m) = 9550 × kW / rpm • Pin = Pshaft / η
NEC & Standard References
IEC 60034-1 rates motors in kilowatts of shaft output and defines the duty types, and IEC 60034-30-1 sets the efficiency classes. NEMA MG-1 rates in horsepower, including the service factor, and lists the standard HP ratings. NEC Article 430 sizes conductors and protection from the full-load current tables (such as 430.250 for three-phase motors), which are indexed by horsepower, never by kilowatts and never by nameplate amps for the conductor itself. So a kW rating must be converted to the nearest table HP before you look anything up. Check the adopted code edition.1. kW = 50 × 0.745699872 = 37.285 kW, which is 37,285 W.
2. Metric: 37.285 / 0.73549875 = 50.69 PS.
3. Electrical HP: 37.285 / 0.746 = 49.98 HP.
4. Torque = 9550 × 37.285 / 1480 = 240.6 N·m, or 177.4 lb·ft.
5. Electrical input = 37.285 / 0.93 = 40.09 kW.
6. Nearest standard sizes: 37 kW on the IEC series and 50 HP on the NEMA series. A 37 kW replacement is 0.8% smaller than the original.
- Metric hp is not HP. Reading 50 PS as 50 HP, or the reverse, hides a 1.4% error on imported machines, enough to matter on a tightly sized drive.
- kW is output, not input. Size cables, breakers and generators on electrical input and current, not on the shaft rating.
- Service factor is not extra nameplate. A 1.15 service factor is reserve for abnormal conditions, and running there continuously shortens insulation life.
- Altitude and heat derate the motor. Standard ratings assume 1000 m and 40 °C ambient, and above that cooling air thins, so check the manufacturer curve.
- Pick the next size up when between ratings. Rounding down to the nearest standard size overloads the motor.