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For other uses, see Horse power (disambiguation).
A team of six horses mowing hay in Lancaster County, Pennsylvania

Horsepower (hp) is a unit of measurement of power (the rate at which work is done). There are many different standards and types of horsepower. Two common definitions being used today are the mechanical horsepower (or imperial horsepower), which is approximately 746 watts, and the metric horsepower, which is approximately 735.5 watts.


Definitions of term[edit]


Units called "horsepower" have differing definitions:

  • The mechanical horsepower, also known as imperial horsepower equals approximately 745.7 watts. It was defined originally as exactly 550 foot-pounds per second).
  • The metric horsepower equals approximately 735.5 watts (98.6% of an imperial mechanical horsepower). It was defined originally as 75 kgf-m per second is approximately equivalent to 735.5 watts.
  • The boiler horsepower equals 9809.5 watts. It was used for rating steam boilers and is equivalent to 34.5 pounds (about 15.6 kg) of water evaporated per hour at 212 degrees Fahrenheit (100 degrees Celsius).
  • One horsepower for rating Continental European electric motors is equal to 735 watts. Continental European electric motors used to have dual ratings.
  • One British Royal Automobile Club (RAC) horsepower can equal a range of values based on estimates of several engine dimensions. It is one of the tax horsepower systems adopted around Europe.

History of the unit[edit]










When considering human-powered equipment, a healthy human can produce about 1.2 hp briefly (see orders of magnitude) and sustain about 0.1 hp indefinitely; trained athletes can manage up to about 2.5 hp briefly[16] and 0.3 hp for a period of several hours. The Jamaican sprinter Usain Bolt produced a maximum of 3.5 hp 0.89 seconds into his 9.58 second 100m dash world record in 2009.[17]

Calculating power[edit]

When torque is in pound-foot units, rotational speed is in rpm and power is required in horsepower:


When torque is in inch pounds:

The constant 63,025 is the approximation of



where is power in watts when is torque in newton-metres, and 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.

Current definitions[edit]

The following definitions have been widely used:

Mechanical horsepower
≡ 33,000 ft lbf/min

= 550 ft·lbf/s
≈ 17696 lb·ft2/s3
= 745.69987158227022 W

Metric horsepower
hp(M) - also PS, cv, hk, pk, ks or ch
≡ 75 kgf·m/s

≡ 75 kg · 9.80665 m/s2 · 1 m/s
≡ 735.49875 W

Electrical horsepower
≡ 746 W
Boiler horsepower
≡ 33,475 BTU/h

= 9,812.5 W


= 550 ft·lbf/s
= 745.69987158227022 W


= 550 ft·lbf/s
= 745.69987158227022 W



Mechanical horsepower[edit]


1 hp ≡ 33,000 ft-lbf/min by definition
= 550 ft·lbf/s since 1 min  = 60 s
= 76.0402249068 kgf·m/s




DIN 66036 defines one metric horsepower as the power to raise a mass of 75 kilograms against the earth's gravitational force over a distance of one metre in one second;[18] this is equivalent to 735.49875 W or 98.6% of an imperial mechanical horsepower.

In 1972, the PS was rendered obsolete by EEC directives, when it was replaced by the kilowatt as the official power measuring unit.[19] It is still in use for commercial and advertising purposes, in addition to the kW rating, as many customers are still not familiar with the use of kilowatts for engines.



French and Italian tax horsepower (CV)[edit]

Main article: Tax horsepower


Electrical horsepower[edit]

The horsepower used for electrical machines is defined as exactly 746 W.[21] In the US, nameplates on electrical motors show their power output in hp, not their power input. Outside the United States watts or kilowatts are generally used for electric motor ratings and in such usage it is the output power that is stated.

Hydraulic horsepower[edit]

Hydraulic horsepower can represent the power available within hydraulic machinery, power through the down-hole nozzle of a drilling rig,[22] or can be used to estimate the mechanical power needed to generate a known hydraulic flow rate.

It may be calculated as:[22]

Pressure is in psi
Flow rate is in US gallons per minute

Drilling rigs are powered mechanically by rotating the drill pipe from above. Hydraulic power is still needed though, as between 2 and 7 hp are required to push mud through the drill bit in order to clear waste rock. This hydraulic power, considerably more than this, may also be used to drive a down-hole mud motor to power directional drilling.[22]

Boiler horsepower[edit]




Drawbar horsepower[edit]

See also: Power at rail


If the drawbar force () is measured in pounds-force (lbf) and speed () is measured in miles per hour (mph), then the drawbar power () in horsepower (hp) is:





RAC horsepower (taxable horsepower)[edit]

See also: Tax horsepower

This measure was instituted by the Royal Automobile Club in Britain and was used to denote the power of early 20th-century British cars. (An identical measure, known as ALAM horsepower or NACC horsepower, was used for early U.S. automobiles.) Many cars took their names from this figure (hence the Austin Seven and Riley Nine), while others had names such as "40/50 hp", which indicated the RAC figure followed by the true measured power.


n is the number of cylinders [24]







In general:

Nominal or rated horsepower is derived from the size of the engine and the piston speed and is only accurate at a pressure of 48 kPa (7 psi).[26][clarification needed]



Nominal (or rated) horsepower (nhp or rhp)[edit]



For paddle ships the piston speed was estimated as 129.7 x (stroke)1/3.35.[26]

The stroke was the distance moved by the piston.


The French Navy used the same definition of nominal horse power as Britain.[26]

Indicated horsepower (ihp) [edit]



Brake horsepower (bhp)[edit]

Brake horsepower (bhp) is the power measured at the crankshaft just outside the engine, before the losses of power caused by the gearbox and drive train.

In Europe, the DIN standard tested the engine fitted with all ancillaries and exhaust system as used in the car. The older American standard (SAE gross horsepower, referred to as bhp) used an engine without alternator, water pump, and other auxiliary components such as power steering pump, muffled exhaust system, etc., so the figures were higher than the European figures for the same engine. The newer American standard (referred to as SAE net horsepower, not as bhp) tests an engine with all the auxiliary components (see "Engine power test standards" below).

Brake refers to the device which was used to load an engine and hold it at a desired rotational speed. During testing, the output torque and rotational speed were measured to determine the brake horsepower. Horsepower was originally measured and calculated by use of the "indicator diagram" (a James Watt invention of the late 18th century), and later by means of a Prony brake connected to the engine's output shaft. More recently, an electrical brake dynamometer is used instead of a Prony brake. Although the output delivered to the drive wheels is less than that obtainable at the engine's crankshaft, use of a chassis dynamometer gives an indication of an engine's "real world" horsepower after losses in the drive train and gearbox.

Shaft horsepower (shp)[edit]

Shaft horsepower (shp) is the power delivered to a propeller shaft, a turbine shaft — or to an output shaft of an automotive transmission.[27] This shaft horsepower can be measured with a torque (torsion) meter, or estimated from the horsepower at the crankshaft and a standard figure for the losses in the transmission (typical figures are around 10%). Shaft horsepower is not commonly used in the internal-combustion-engine automobile industry because of the need to estimate losses in the transmission; instead, this industry in the USA typically uses SAE certified net power, which is measured at the engine's crankshaft, and so does not account for losses in the transmission (see "Engine power test standards", below).

Wheel horsepower (whp)[edit]

Motor vehicle dynamometers can measure wheel horsepower, which is the effective, true horsepower delivered to the driving wheel(s), representing the actual power available to accelerate the vehicle after all losses in the drive train, and all parasitic losses such as pumps, fans, alternator, muffled exhaust, etc. The vehicle is generally attached to the dynamometer and accelerates a large roller and Power Absorbing Unit which is driven by the vehicle's drive wheel(s). The actual power is then computer calculated based on the rotational inertia of the roller, its resultant acceleration rates and power applied by the Power Absorbing Unit. Some motor vehicle (and motorbike) dynamometers can also be purely inertia-based where the power output is calculated from measuring the acceleration of a roller drum with a known rotational inertia and known parasitic frictional losses of the roller drum's bearings.

Engine power test standards[edit]


Society of Automotive Engineers/SAE International[edit]

Early "SAE horsepower" (see RAC horsepower)[edit]

In the early twentieth century, a so-called "SAE horsepower" was sometimes quoted for U.S. automobiles. This long predates the SAE horsepower measurement standards and was really just another term for the widely used ALAM or NACC horsepower figure, which was the same as the British RAC horsepower, used for tax purposes.

SAE gross power[edit]

Prior to the 1972 model year, American automakers rated and advertised their engines in brake horsepower, bhp, which was a version of brake horsepower called SAE gross horsepower because it was measured according to SAE standards (J245 and J1995) that call for a stock test engine without accessories (such as dynamo/alternator, radiator fan, water pump),[29] and sometimes fitted with long tube test headers in lieu of the OEM exhaust manifolds. The atmospheric correction standards for barometric pressure, humidity and temperature for testing were relatively idealistic.

SAE net power[edit]


SAE certified power[edit]





Deutsches Institut für Normung 70020[edit]

DIN 70020 is a standard from German DIN regarding road vehicles. DIN testing, unlike SAE gross power testing, tested the engine as installed in the vehicle, with cooling system, charging system and stock exhaust system all connected. Because the German word for horsepower is Pferdestärke, in Germany it is commonly abbreviated to PS. DIN hp is measured at the engine's output shaft, and is usually expressed in metric (Pferdestärke) rather than mechanical horsepower.



Economic Commission for Europe R24[edit]

ECE R24 is a UN standard for the approval of compression ignition engine emissions, installation and measurement of engine power.[32] It is similar to DIN 70020 standard, but with different requirements for connecting an engine's fan during testing causing it to absorb less power from the engine.[33]

Economic Commission for Europe R85[edit]




International Organization for Standardization[edit]

  • ISO 1585 is an engine net power test code intended for road vehicles.[36]
  • ISO 2534 is an engine gross power test code intended for road vehicles.[37]
  • ISO 4164 is an engine net power test code intended for mopeds.[38]
  • ISO 4106 is an engine net power test code intended for motorcycles.[39]
  • ISO 9249 is an engine net power test code intended for earth moving machines.[40]

Japanese Industrial Standard D 1001[edit]


See also[edit]


  1. ^ "Horsepower", Encyclopædia Britannica Online. Retrieved 2012-06-24.
  2. ^ "International System of Units" (SI), Encyclopædia Britannica Online. Retrieved 2012-06-24.
  7. ^ "The miner's friend". Rochester history department website:. Archived from the original on 2009-05-11. Retrieved July 21, 2011. 
  14. ^ Popular Mechanics. September 1912, page 394
  17. ^ "Scientists model "extraordinary" performance of Bolt". Institute of Physics. 26 July 2013. Retrieved 9 March 2016. 
  22. ^ a b c "Hydraulic Horsepower". Oilfield Glossary. Schlumberger. 
  23. ^ Robert McCain Johnston Elements of Applied Thermodynamics, Naval Institute Press, 1992 ISBN 1557502269, p.503
  27. ^ Oxford Dictionary. Retrieved 2016-12-06. Unabridged, Random House Inc. Retrieved 2016-12-06.
  29. ^ a b c Lucchesi, Domenico (2004). Corso di tecnica automobilistica, vol. 1º—Il motore (in Italian) (6th ed.). Ulrico Hoepli Editore S.p.A. p. 550. ISBN 88-203-1493-2. 
  31. ^ Jeff Plungis, Asians Oversell Horsepower, Detroit News
  32. ^ ECE Regulation 24, Revision 2, Annex 10
  33. ^ Breen, Jim (2003-03-22). "Farmers Journal: Tractor and machine comparison: what's the 'true' measure - 22 March 2003". [dead link]

External links[edit]