What is the difference between mechanical and metric horsepower?

Mechanical (imperial) horsepower is 745.7 W and is standard on US nameplates. Metric horsepower, also called PS or cheval, is 735.5 W and appears on many European and Asian motors. They differ by about 1.4 percent, so a 100 HP rating means slightly less power if it is metric.

Why is electrical input power higher than the rating?

The nameplate horsepower or kW is the mechanical power delivered at the shaft. The motor draws more electrical power because of copper, iron, and friction losses. Dividing shaft power by efficiency, for example 0.90, gives the real input draw used for wiring and breaker sizing.

How is shaft torque calculated?

Power equals torque times angular speed. In SI the tool uses T = 9.5488 × kW × 1000 / RPM for newton-metres. In imperial units the classic relation T = 5252 × HP / RPM gives pound-feet, where 5252 is 33,000 divided by 2π.

Which horsepower should I use for sizing wire?

Size conductors and overloads from the electrical input, not the shaft rating. Use the motor's full-load current from its nameplate or the NEC motor tables. This converter helps you cross-check the input kW that current implies.

Is electrical HP the same as mechanical HP?

Electrical horsepower is defined as exactly 746 W, a rounded figure used in some electrical contexts, while mechanical horsepower is 745.7 W. The difference is tiny, about 0.04 percent, and rarely matters in practice.

What is the Motor Horsepower to kW Converter?

Convert between mechanical horsepower (745.7 W), metric horsepower (735.5 W), and kilowatts, derive electrical input power from motor efficiency, and compute shaft torque in N·m and lb·ft at a given RPM. A motor-nameplate tool for electricians. Runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

Motor Horsepower to kW Converter

Name: Motor Horsepower to kW Converter
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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Motor nameplates come in mechanical horsepower, metric horsepower (PS), or kilowatts depending on where the motor was made, and comparing them needs the exact conversion constants. This tool converts between all of them, derives the electrical input power from efficiency, and computes shaft torque at any speed.

How it works

Each unit maps to watts through its defining constant, then back to the others:

1 mechanical HP = 745.700 W
1 metric HP (PS) = 735.499 W
1 electrical HP  = 746.000 W

Electrical input power accounts for motor losses, and torque follows from the power-speed relationship:

input kW = shaft kW / efficiency
T (N·m)  = 9.5488 × kW × 1000 / RPM
T (lb·ft)= 5252 × HP / RPM

Example and notes

A 10 HP mechanical motor delivers 7.457 kW at the shaft. At 90 percent efficiency it draws about 7.457 / 0.90 ≈ 8.29 kW electrically. Running at 1750 RPM it produces roughly 40.7 N·m, or 30 lb·ft, of torque. Always size the supply circuit from the electrical input and the nameplate full-load current, not from the mechanical rating, because the loss component is real load on the wiring.

Why the three horsepower types matter in practice

If you size wiring, breakers, or variable-frequency drives from the wrong horsepower definition, you introduce errors that propagate through the whole installation design.

Mechanical HP (745.7 W) is the standard on North American motor nameplates and in NEMA specifications. This is the value used in the US National Electrical Code (NEC) motor tables. When in doubt about a US or Canadian motor, assume mechanical HP.

Metric HP (735.5 W) appears on motors made or rated to European or Japanese standards. The abbreviation PS (from German Pferdestärke) or cv (French cheval-vapeur) identifies this variant. On a 100 HP nameplate: a NEMA motor produces 74.57 kW at the shaft, while a 100 PS DIN motor produces 73.55 kW. The difference is about 1.4% — small but meaningful when sizing contactors or selecting VFD output ratings.

Electrical HP (746 W exactly) appears in some utility and electrical-engineering contexts in the US as a round-number approximation. It is close enough to mechanical HP for most purposes.

Torque and speed — the practical link

Understanding the HP-to-torque relationship helps when selecting motors for specific applications:

Low-speed, high-torque applications (conveyors, mixers, compressors): A given HP at 900 RPM produces twice the torque of the same HP at 1800 RPM. If the application needs high torque, gear reduction or a low-speed motor is more efficient than brute-force HP.
High-speed applications (pumps, fans, machining spindles): Speed matters more than raw torque; the HP determines how much fluid or air can be moved.

The torque formula T = 5252 × HP / RPM (in lb·ft) or T = 9549 × kW / RPM (in N·m) shows that for a fixed HP rating, torque is inversely proportional to speed. Doubling RPM halves the available torque.

Common motor nameplate scenarios

Nameplate	Type	Shaft kW	Note
5 HP (NEMA)	Mechanical	3.73 kW	Standard US motor
5 PS (DIN)	Metric	3.68 kW	Common on European imports
7.5 kW	Already in kW	7.5 kW	Enter directly; no conversion needed
10 HP, 90% eff	Mechanical	7.46 kW shaft / 8.29 kW electrical	Size wiring from 8.29 kW

Always verify with the full-load current (FLC) on the motor nameplate when sizing protection — the current accounts for motor power factor and efficiency more precisely than the HP-to-kW conversion alone.