The horsepower (hp) is the name of several non-metric units of power. In scientific discourse the horsepower is rarely used due to the various definitions and the existence of an SI unit for power, the watt (W). However, the idea of horsepower persists as a handy term in many languages, particularly in the automotive industry for listing the maximum power of internal-combustion engines.

The various types of horsepower are:

Table of contents
1 hp
2 RAC horsepower
3 PS
4 pk
5 CV
6 ch
7 bhp
8 ihp

hp

According to the most common definition of horsepower, one horsepower is defined as exactly:

1 hp = 745.69987158227022 W

The horsepower was first used by James Watt during a business venture where his steam engines substituted horses. It was defined that a horse can lift 33,000 pounds with a speed of 1 foot per minute: 33,000 ft·lb·min-1. (This is equivalent to approximately 15,000 kg at 30 cm.)

RAC horsepower

This measure was instituted by the Royal Automobile Club in Britain and used to denote the power of early 20th century British cars. Many cars hence had names such as "40/50hp", which indicated the RAC figure followed by the true measured power.

RAC horsepower cannot be given as a proportion to metric power. Instead, it is derived from dimensions of the engine and certain assumptions about engine efficiency. When invented, it gave a rough guide to its true power rating; as new engines were designed with ever-increasing efficiency, it was no longer a useful measure, but was kept in use by UK regulations which used the rating for tax purposes.

D is the diameter (or bore) of the cylinder in inches
n is the number of cylinders

PS

This unit (German: Pferdestärke = horse strength) is still commonly used in Germany and central Europe, although not an lawful unit any more. It is defined by the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig as:

1 PS = 735.49875 W

pk

A dutch paardekracht equals the german Pferdestärke hence
1 pk = 735.49875 W

CV

A French unit for tax horsepower.

ch

This is a French unit, short for cheval vapeur. Some sources give it as 735.5 W, but it is generally used interchangeably with the German 'PS'.

bhp

Brake horsepower was a term commonly used before the 1970s in the USA, and is still common in the UK. It indicates the brake, the device for measuring the true power of the engine. Stating power in 'bhp' gives some indication this is a true reading, rather than a calculated or predicted one. However, several manufacturers started to strip their engines of essential ancilliaries for the purposes of getting a high horsepower figure to use in marketing the car.

In the USA the term fell into disuse after the American Society of Automotive Engineers (SAE) recommended manufacturers use "hp (SAE)" to indicate the power of the engine, given that particular car's complete engine installation. The British market seemed not to need the correction.

ihp

Indicated horsepower is the theoretical power of a reciprocating engine assuming that it is completely efficient in converting the energy contained in the expanding gases in the cylinders. It is calculated from the pressures developed in the cylinders, measured by a device called an engine indicator - hence indicated horsepower. It was the figure normally used for steam engines in the 19th Century but is misleading because the mechanical efficiency an engine means that the actual power output may be only 70-90% of the indicated horsepower.