The formula
hp = W × 0.00134102209
Where the factor comes from — each unit by its definition, then its size in SI:
1 W is the SI unit of power.1 hp = 550 × ft·lbf / s = 745.699871582 W
Dividing the two SI sizes gives the factor: 0.0013410220896 (rounded for display; the underlying ratio is fixed by the definitions). The reverse factor is 745.699871582.
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: 74,570 W = 100 hp — 74,570 × 0.00134102209 = 100
- An old incandescent bulb: 60 W = 0.0804613 hp — 60 × 0.00134102209 = 0.0804613
- An electric kettle (2 kW): 2,000 W = 2.68204 hp — 2,000 × 0.00134102209 = 2.68204
- A room air conditioner (12000 BTU/h): 3,516.85 W = 4.71618 hp — 3,516.85 × 0.00134102209 = 4.71618
Frequently asked questions
How many horsepower (mechanical) is 1 W?
1 W = 0.0013410220896 hp.
How do I convert horsepower (mechanical) back to watts?
Divide by 0.0013410220896 — or multiply by 745.699871582: 1 hp = 745.699871582 W.