Horsepower (International) to BTU (IT)/second

Mechanical horsepower uses 550 ft·lbf/s, equal to exactly 745.69987158227022 W with the international foot and standard pound-force. Metric horsepower instead uses 75 kgf·m/s, equal to 735.49875 W, while electric horsepower is defined as 746 W. The British and international selections in this converter share the mechanical definition; the labels do not imply different conversion factors.

SAE J1349 specifies measuring net horsepower with all production accessories (alternator, water pump, AC compressor) attached, at standard atmospheric conditions. Before 1972, US manufacturers used gross hp (engine on a test stand with minimal accessories), which inflated numbers by 15–25%. The switch to SAE net ratings famously caused "overnight" power drops: a Corvette went from "350 hp" (gross) to "255 hp" (net) in 1972 — same engine, honest measurement.

Japan officially uses metric PS (called 馬力, "horse power," abbreviated PS after the German). Japanese car specs list PS, and JIS standards define power in PS. However, for international export, Japanese manufacturers convert to international hp or kW depending on the destination market. A Nissan GT-R produces 570 PS for the Japanese market and 565 hp for the US market — the same engine, different unit systems, and the ~1% gap occasionally causes forum arguments.

The Wärtsilä-Sulzer RTA96-C, a marine diesel engine used in the largest container ships, produces about 109,000 hp (international) — 80,080 kW from 14 cylinders each the size of a small apartment. It's 13.5 meters tall and weighs 2,300 tonnes. At 102 RPM, it turns propellers the size of houses. For comparison, a Saturn V rocket's five F-1 engines produced about 217 million hp combined, but only for 2.5 minutes.

Probably, but slowly. The EU already legally requires kW; China uses kW; scientific and engineering communities prefer kW. But cultural inertia is powerful — Americans have been buying cars by horsepower for over a century, and "how many horses under the hood" is deeply embedded in car culture. The transition to EVs may accelerate the shift, since electric motors are naturally rated in kW. Give it 20–30 years, and hp may join the furlong and the gill in the museum of obsolete units.

In US combustion engineering and power plant heat rate analysis, fuel energy content is natively specified in BTU (natural gas is sold per therm = 100,000 BTU). Expressing burner output in BTU/s keeps the calculation in one unit system, avoiding constant conversions. When your fuel flow is in BTU/min and your efficiency calculations use BTU, switching to watts mid-calculation just creates errors.

One BTU/s ≈ 1,055 watts — roughly a single-bar electric fire or a small hair dryer. It's a surprisingly human-scale unit. A typical US home gas furnace running at full blast produces about 28 BTU/s (100,000 BTU/h ÷ 3,600). A gas stovetop burner on high delivers about 3–5 BTU/s. So BTU/s lands right in the range where you can feel the heat on your face.

Power plant thermal engineering (heat rate analysis), industrial furnace and kiln design, jet engine combustion analysis, and rocket propulsion engineering. NASA specifications for rocket engines often include BTU/s figures. The Space Shuttle Main Engine produced about 12 million BTU/s of thermal power. Steelmaking blast furnaces operate at 50,000–200,000 BTU/s of heat input.

One BTU/s = 1.415 mechanical horsepower, or roughly 1.4 hp. This is useful in automotive and engine testing where dynamometers may report in BTU/s for thermal measurements but engineers think in horsepower. A 400 hp engine rejects about 280 BTU/s through its cooling system at full power (assuming 60% of fuel energy becomes waste heat). The conversion factor is easy to remember: multiply BTU/s by 1.4 to get hp.

The BTU originated as a water-heating energy unit, with several temperature-dependent variants. This converter uses the fixed International Table variant: 1 BTU(IT) = exactly 1,055.05585262 J. BTU-based energy and power units remain common in U.S. thermal engineering, equipment ratings and fuel-energy quantities. BTU alone measures energy; BTU per second, minute or hour measures power.