Contents 1 History 2 Calculating power 3 Definitions 3.1 Mechanical horsepower 3.2 Metric horsepower (PS, cv, hk, pk, ks, ch) 3.3 Tax horsepower 3.4 Electrical horsepower 3.5 Hydraulic horsepower 3.6 Boiler horsepower 3.7 Drawbar horsepower 3.8 RAC horsepower (taxable horsepower) 3.9 Compressed air horsepower 4 Measurement 4.1 Nominal (or rated) horsepower 4.2 Indicated horsepower 4.3 Brake horsepower 4.4 Shaft horsepower 4.5 Wheel horsepower 5 Engine power test standards 5.1 Society of Automotive Engineers/SAE International 5.1.1 Early "SAE horsepower" (see RAC horsepower) 5.1.2 SAE gross power 5.1.3 SAE net power 5.1.4 SAE certified power 5.2 Deutsches Institut für Normung 70020 (DIN 70020) 5.3 CUNA 5.4 Economic Commission for Europe R24 5.5 Economic Commission for Europe R85 5.6 80/1269/EEC 5.7 International Organization for Standardization 5.8 Japanese Industrial Standard D 1001 6 See also 7 References 8 External links

Calculating power When torque T {\displaystyle T} is in pound-foot units, rotational speed ( N ) {\displaystyle (N)} is in rpm and power is required in horsepower: P / hp = T / ( ft ⋅ lbf ) × N / rpm 5252 {\displaystyle P/{\text{hp}}={\frac {T/({\text{ft}}\cdot {\text{lbf}})\times N/{\text{rpm}}}{5252}}} The constant 5252 is the rounded value of (33,000 ft·lbf/min)/(2π rad/rev). When torque T {\displaystyle T} is in inch pounds: P / hp = T / ( in ⋅ lbf ) × N / rpm 63,025 {\displaystyle P/{\text{hp}}={\frac {T/({\text{in}}\cdot {\text{lbf}})\times N/{\text{rpm}}}{63{,}025}}} The constant 63,025 is the approximation of 33,000 ft ⋅ lbf min ⋅ 12 in ft 2 π   rad ≈ 63,025 {\displaystyle 33{,}000\,{\frac {{\text{ft}}\cdot {\text{lbf}}}{\text{min}}}\cdot {\frac {12\,{\frac {\text{in}}{\text{ft}}}}{2\pi ~{\text{rad}}}}\approx 63{,}025} . If torque and rotational speed are expressed in coherent SI units, the power is calculated by ; P = τ ⋅ ω {\displaystyle P=\tau \cdot \omega } where P {\displaystyle P} is power in watts when τ {\displaystyle \tau } is torque in newton-metres, and ω {\displaystyle \omega } is angular speed in radians per second. When using other units or if the speed is in revolutions per unit time rather than radians, a conversion factor has to be included.

See also Brake specific fuel consumption—how much fuel an engine consumes per unit energy output Dynamometer engine testing European units of measurement directives Mean effective pressure Horsepower-hour Torque