The formula
hp = PS × 0.9863200706
Where the factor comes from — each unit by its definition, then its size in SI:
1 PS = 75 × kgf × m / s = 735.49875 W1 hp = 550 × ft·lbf / s = 745.699871582 W
Dividing the two SI sizes gives the factor: 0.98632007062 (rounded for display; the underlying ratio is fixed by the definitions). The reverse factor is 1.01386966542.
Power is energy per second; the watt is one joule per second. Engine ratings in horsepower and air-conditioner ratings in BTU/h convert to watts with fixed factors.
Worked examples
- A small car engine: 101.387 PS = 100 hp — 101.387 × 0.9863200706 = 100
- An old incandescent bulb (60 W): 0.0815773 PS = 0.0804613 hp — 0.0815773 × 0.9863200706 = 0.0804613
- An electric kettle (2 kW): 2.71924 PS = 2.68204 hp — 2.71924 × 0.9863200706 = 2.68204
- A room air conditioner (12000 BTU/h): 4.78159 PS = 4.71618 hp — 4.78159 × 0.9863200706 = 4.71618
Frequently asked questions
How many horsepower (mechanical) is 1 PS?
1 PS = 0.98632007062 hp.
How do I convert horsepower (mechanical) back to metric horsepower?
Divide by 0.98632007062 — or multiply by 1.01386966542: 1 hp = 1.01386966542 PS.