Wattsto Horsepower Converter
Instantly convert wattsto horsepower with our free converter. See conversion tables, formulas, and step-by-step explanations.
Formula
1 mechanical HP = 745.7 W | 1 metric HP (PS) = 735.5 W | 1 electrical HP = 746 W
Horsepower has multiple definitions based on historical context. Mechanical HP was defined by James Watt (550 ft-lbf/s = 745.7 W). Metric HP uses SI-adjacent units (75 kgf*m/s = 735.5 W). Electrical HP is a round 746 W. Boiler HP is 9,809.5 W. This converter handles all variants through a common base unit of watts.
Worked Examples
Example 1: Car Engine Power Comparison
Problem: A European car is rated at 150 PS (metric HP). Convert to mechanical HP and kilowatts.
Solution: To watts: 150 x 735.49875 = 110,324.8 W\nTo kW: 110,324.8 / 1,000 = 110.32 kW\nTo mechanical HP: 110,324.8 / 745.69987 = 147.9 hp
Result: 150 PS = 147.9 mechanical hp = 110.32 kW
Example 2: Electric Motor Specification
Problem: Convert a 5 HP electric motor rating to watts and BTU/h output.
Solution: To watts: 5 x 746 = 3,730 W = 3.73 kW\nTo BTU/h: 3,730 / 0.29307107 = 12,728 BTU/h\nNote: This is shaft output power; electrical input is higher
Result: 5 electrical HP = 3,730 W = 12,728 BTU/h
Frequently Asked Questions
How many watts are in one horsepower?
The answer depends on which type of horsepower you mean. Mechanical (Imperial) horsepower equals 745.7 watts, metric horsepower (PS or CV) equals 735.5 watts, and electrical horsepower equals exactly 746 watts. The differences stem from their distinct historical definitions. James Watt defined mechanical horsepower as 550 foot-pounds per second. Metric horsepower was defined as 75 kilogram-force-meters per second.
What is the difference between mechanical and metric horsepower?
Mechanical horsepower (hp) was defined by James Watt as 33,000 foot-pounds per minute (550 ft-lbf/s), which equals 745.7 watts. Metric horsepower (PS in German, CV in French) is defined as 75 kilogram-force-meters per second, equaling 735.5 watts. Metric HP is about 1.4% smaller than mechanical HP. European car specifications typically use metric HP (PS), while American specs use mechanical HP. A 100 PS engine is approximately 98.6 mechanical HP.
What is boiler horsepower?
Boiler horsepower is a much larger unit than mechanical horsepower, equaling 9,809.5 watts or about 13.15 mechanical HP. It was originally defined as the power needed to evaporate 34.5 pounds of water per hour at 212 degrees F from and at 100 degrees C. This equals approximately 33,475 BTU/h. Though largely obsolete for new equipment, boiler horsepower is still occasionally referenced in steam boiler specifications in North America.
Why do electric motors use a different horsepower definition?
Electrical horsepower is defined as exactly 746 watts, a rounded value chosen for convenience in electrical engineering calculations. The slight difference from mechanical HP (745.7 W) is negligible for practical purposes but simplifies motor nameplate ratings and electrical calculations. When you see a motor rated at 1 HP, it means its output shaft delivers 746 watts of mechanical power, though the electrical input will be higher due to motor efficiency losses.
How do I calculate the horsepower of an engine from torque and RPM?
Mechanical horsepower can be calculated from torque and rotational speed using the formula: HP = (Torque in ft-lbf x RPM) / 5,252. This derives from the definition of horsepower as 33,000 ft-lbf per minute combined with the relationship between torque and rotational work. For metric units: kW = (Torque in Nm x RPM) / 9,549. At 5,252 RPM, horsepower and torque in ft-lbf are numerically equal.
What formula does Wattsto Horsepower Converter use?
The formula used is described in the Formula section on this page. It is based on widely accepted standards in the relevant field. If you need a specific reference or citation, the References section provides links to authoritative sources.