Does anyone know the sum how to work out what bhp you have at the fly when you go on rollers and get the figure at the wheels?

Does this sum change for different models of car ie add 15% for a MK4 and 25% for a mk6, figures just made up for example I have no idea

Also how can you convert bhp to ps?

Cheers for any help, also if there is a link I have missed for this please point me in the right direction

Steve.

By the way does anyone know what a standard 1800 zetec bhp & torque should be ?