Horsepower (British) to Foot pounds-force second

A horse can sustain about 0.7 hp over a working day, and briefly peak at 10–15 hp during a gallop or heavy pull. Watt's definition was deliberately generous — he wanted his steam engines to look good compared to the horses they replaced. A fit human can sustain about 0.1 hp and peak at ~1–2 hp briefly. So a horse is roughly 7× a human in sustained output, which aligns well with historical accounts of animal labor replacing human workers.

Because American engineering inherited the British unit directly — the US was a British colony when Watt defined horsepower in the 1780s. Both equal 550 ft·lbf/s = 33,000 ft·lbf/min = 745.7 W. The "international" horsepower adopted in 1956 formalised this same value. The only reason it's sometimes called "British" is to distinguish it from the metric horsepower (PS) used in continental Europe, which is 1.4% smaller.

Watt's own improved steam engines: 10–20 hp. Brunel's SS Great Eastern ship engines: 8,000 hp. The Rolls-Royce Merlin (WW2 Spitfire): 1,030–1,760 hp depending on variant. Concorde's Olympus 593 engines: 38,000 hp each (with reheat). The Rolls-Royce Trent XWB (A350 engine): about 97,000 hp. In 240 years, British engines went from 20 hp to 97,000 hp — a 5,000-fold increase.

Almost. "bhp" stands for "brake horsepower" — power measured at the engine output shaft using a dynamometer (historically a brake). "hp" can technically mean the gross figure including power consumed by accessories. Since 2005, European regulations require "net" power (engine with all standard accessories), so bhp and hp are effectively identical for modern cars. The "b" in bhp is mostly a British tradition to emphasize that the number is a real dynamometer measurement, not a theoretical calculation.

From 1910 to 1947, Britain taxed cars by "RAC horsepower" — a formula based on cylinder bore and number of cylinders, not actual power. This incentivised narrow-bore, long-stroke engines with terrible performance. A car rated at "10 RAC hp" might actually produce 30–40 real hp. The tax warped British car design for decades, producing underpowered engines that only made sense as tax dodges. The system was scrapped in 1947, but its legacy shaped British car culture for years after.

James Watt calculated that a mill horse could turn a mill wheel 144 times per hour, doing 32,572 ft·lbf of work per minute — he rounded up to 33,000 ft·lbf/min (550 ft·lbf/s) for marketing purposes. He wanted to sell steam engines by comparing them to horses, so he likely overestimated the horse to make his engines look like better value. A real horse sustains closer to 350–500 ft·lbf/s, so Watt's "1 HP" is actually more than one horse.

Power (ft·lbf/s) = Torque (ft·lbf) × RPM × 2π / 60. This is the workhorse formula (pun intended) of US mechanical engineering. For example, an engine producing 200 ft·lbf of torque at 3,000 RPM delivers 200 × 3,000 × 6.2832 / 60 = 62,832 ft·lbf/s ≈ 114 hp. The formula works because angular velocity in rad/s times torque in ft·lbf gives power directly in ft·lbf/s.

Pushing with 1 pound of force at 1 foot per second — roughly the effort of slowly sliding a light book across a table against friction. Lifting a 1-pound weight 1 foot in 1 second. Turning a doorknob with a very light touch. It's about 1.36 watts — enough to dimly light an LED. In human terms, it's almost effortless: casual walking produces about 50–80 ft·lbf/s of mechanical power, and you don't even notice.

Yes, particularly in ballistics (muzzle energy rates), mechanical testing (dynamometer output), agricultural machinery specs, and industrial equipment designed for the US market. However, even in the US, many engineering firms are switching to SI units for international compatibility. The automotive industry increasingly quotes power in both hp and kW. Aerospace has been mostly metric since the 1990s. Ft·lbf/s survives mainly in traditional mechanical and manufacturing industries.

Bullets are rated in ft·lbf of muzzle energy (not per second), but the power of a firearm is the muzzle energy divided by barrel time. A .308 rifle bullet exits with about 2,600 ft·lbf of energy over a barrel transit time of ~0.001 seconds, meaning the instantaneous power is roughly 2,600,000 ft·lbf/s (about 3,500 hp). That's why rifle recoil feels punchy — for a millisecond, you're absorbing the reaction force of a truck engine.