Horsepower variants — mechanical, metric, electrical, boiler
Horsepower is not one unit. Four common variants differ by 1–1300%.
The horsepower is a family of units, not a single one. Each variant traces back to a different historical definition — Watt's mechanical hp, continental Europe's metric hp, the electrical industry's rounded value, and the boiler industry's steam-based hp.
The exact factors
Mechanical hp (US, imperial): 745.6999 W > Metric hp / PS / CV: 735.4988 W (1.4% smaller) > Electrical hp: 746 W (rounded, motor nameplate standard) > Boiler hp: 9809.5 W (evaporation of 34.5 lb water/hr at 212 °F)
Where each is used
Mechanical: US automotive, US pump specs. Metric (PS/CV): European cars, motorcycles. Electrical: US and international motor nameplates. Boiler: steam boiler ratings, industrial heating.
Worked example — European vs US car spec
'300 PS turbo' = 220.65 kW = 295.9 hp mechanical. US press quotes 300 hp — same car, 4 hp discrepancy. On boutique high-output cars, this compounds through engine mods and warranty limits.
Recap
hp without a qualifier is ambiguous. Mechanical vs metric differs by 1.4%; boiler is a completely different scale.
Try the converters mentioned in this article
FAQ
Is 1 hp = 550 ft·lb/s always?+
Only for mechanical hp. Metric hp defined as 75 kgf·m/s = 735.5 W.
Which one does a motor nameplate use?+
Electrical hp (746 W) — a convenient rounded value for AC motor ratings.
This article was written by The Turbo Unit Converter engineering desk (Applied metrology & engineering units) and last reviewed on 2026-07-07 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.