To get an indication of the actual power of an engine the dimension "horsepower" is used. When this unit was used for the first time, it was obvious to compare it with a horse, since there was no other powersource to compare with. So the horsepower was the equivalent of the power that one horse could deliver. Nowadays we consider a hp as the equivalent of 735 Watt or 75 kgm/s.

At first the nominal horsepower was used. This nhp was not measured but calculated by some very questionable dimensions of the engine. The nhp of a single cilinder engine was calculated as D2/10, D being the diameter of the cilinder. For a compound the formula was D2/5. Other formulas define the nhp as 7 x the area of the piston x the equivalent of piston speed/33,000. Obviously there was no mathematical relation with the real power, so there isn't something like a conversiontable. One nhp is something between 2 and 6 modern hp.

There is a definition difference between hp and bhp (brake horsepower), the bhp is measured directly onto the engine, the hp is measured on the wheels. The difference is the lost power in gearboxes, bearings etc. Bhp is very British, hp is more commonly used in the continent.

Nowadays there is equipment available to measure the powerdelivery of an engine.