Power is calculated by dividing the amount of work done by the time it takes to do it. Power is often given in units of horsepower or watts, although other units such as ft-lbf/min, calories/hour and BTU/sec are also used. The unit “horsepower” is defined differently depending on what is being measured and how and where it is measured, so conversions must be done carefully.
Determine exactly which bhp you are starting with. If you are dealing with a boiler, it is boiler horsepower. If you are dealing with a figure from an auto magazine, it is probably the Society of Automotive Engineers, or SAE, net horsepower. Otherwise, you are probably dealing with brake horsepower.
Determine the exact unit of horsepower you are starting with. Boiler horsepower is a unit. SAE net horsepower is a testing method, not a unit, but its result is in mechanical horsepower units. Brake horsepower is also a testing method, but its result can be in any unit, so you will have to find out which one.
Determine exactly which horsepower, or hp, unit you want to end with — mechanical horsepower or metric horsepower. Mechanical horsepower is generally used in the United States and the United Kingdom. Metric horsepower, or an equivalent unit, is generally used throughout the rest of the world.
Check whether or not you really need to convert. For example, if you started with a bhp figure from an auto magazine and you want mechanical horsepower, you don’t need to convert. If you do need to convert, find one of the many Internet sites that will convert between the two different horsepower units. References 4 and 5 specify two such sites.
Enter the numeric value of the unit of horsepower you started from. You may also need to select the unit of horsepower you are starting from and/or the unit to which you are converting. Different conversion sites operate slightly differently. Some sites offer additional parameters, such as number of significant digits. You either hit “Enter” or click on a button to display the converted numeric value.
You can also look up the required conversion factor in an engineering text or other reliable document and do the conversion by hand. You may have to do two conversions, one from your original horsepower unit to watts and a second from watts to your desired horsepower unit. Use the same number of significant digits for all figures in your calculations, rounding after each step.
Mechanical horsepower is also known as imperial horsepower, U.K. horsepower or U. S. customary horsepower. Metric horsepower, abbreviated as either mhp or hp, is also known as continental horsepower or SI (Systéme Internationale) horsepower. Pferdestärke horsepower, abbreviated as either PS or DIN (Deutsches für Normung) horsepower is now the same as metric horsepower, although it was originally a little different.