What is the difference between BHP and HP?

Horsepower (HP) is a general unit of power, while Brake Horsepower (BHP) specifically refers to power measured at the crankshaft output using a dynamometer (historically a friction brake). In practice, most automotive power figures quoted in the UK are BHP. In the US, SAE net horsepower (hp) is measured at the flywheel after accounting for standard accessories; the figures are numerically similar but not identical to a UK BHP dyno reading.

What is the BHP formula from torque and RPM?

BHP = Torque (lb·ft) × RPM / 5252. In SI units: BHP = Torque (N·m) × RPM × 2π / (60 × 745.70). The constant 5252 comes from 33000 / (2π), where 33000 is the foot-pound-per-minute definition of one horsepower.

How does the Prony brake method work?

A Prony brake clamps friction pads against a flywheel or drum. You measure the resistive force F at a known arm radius r (the torque arm). Torque equals F times r. BHP then follows from the Torque and RPM formula above. This was the original dynamometer method before electronic load cells replaced mechanical brakes.

What is BMEP and why does it predict BHP?

Brake Mean Effective Pressure (BMEP) is the average pressure that would need to act on each piston throughout its power stroke to produce the measured brake output. BHP equals BMEP times swept displacement times RPM divided by the stroke factor (2 for a 4-stroke engine, 1 for a 2-stroke). Typical naturally aspirated petrol engines produce 150-180 psi BMEP; forced-induction engines reach 200-300 psi; diesel engines 150-250 psi.

Is BHP the same as PS or kW?

No, they differ slightly. 1 mechanical BHP (SAE) = 745.70 W = 0.7457 kW. 1 metric horsepower (PS, used in Germany and much of Europe) = 735.50 W. So 200 BHP equals about 202.8 PS or 149.1 kW. The result box shows all three conversions automatically when you calculate BHP.

Why does my dyno reading differ from the manufacturer figure?

Manufacturer BHP figures are usually measured at the crankshaft (flywheel) in controlled laboratory conditions. A rolling-road dyno measures power at the wheels (wheel horsepower, WHP), which is 10-20% lower after drivetrain losses in the gearbox, driveshafts and differentials. Some manufacturers also quote figures under favourable ambient conditions. Always clarify whether a figure is flywheel BHP or wheel BHP.

Brake Horsepower Calculator

Brake Horsepower (BHP) is the standard measure of an engine’s usable power output, measured at the crankshaft using a dynamometer — historically a mechanical friction “brake,” now usually an electronic load cell. BHP is distinct from indicated horsepower (which ignores friction losses inside the engine) and from wheel horsepower (which is measured after drivetrain losses). This calculator supports three independent methods so you can compute BHP from whatever data you have to hand.

How it works

Method 1 — Torque and RPM (dyno)

The most common method uses the fundamental power equation:

P (watts) = T (N·m) × ω (rad/s)

where angular velocity ω = RPM × 2π / 60. Converting watts to brake horsepower (1 BHP = 745.70 W) gives the practical formula:

BHP = T (lb·ft) × RPM / 5252

The constant 5252 is 33 000 ÷ (2π), rooted in the original Watt definition (one horsepower lifts 33 000 lb one foot per minute). You can also solve backwards for torque or RPM if BHP is already known.

Method 2 — Prony / brake dyno (force and arm)

When a mechanical brake is fitted, you measure the resistive force F at radius r from the crankshaft centreline. Torque = F × r, and the rest follows from Method 1. This was the original 19th-century method for testing steam and petrol engines.

BHP = F (lbf) × r (ft) × RPM / 5252

Method 3 — BMEP method

Brake Mean Effective Pressure treats the whole power stroke as if a single constant pressure acted on the piston. For a 4-stroke engine:

P (W) = BMEP (Pa) × Vd (m³) × N (RPM) / (2 × 60)

The factor of 2 in the denominator accounts for the 4-stroke cycle firing once every two crankshaft revolutions (omit it for a 2-stroke). This method is useful in engine design and tuning to compare engines of different displacement on a common pressure scale.

Worked example

A tuned 2.0-litre four-cylinder engine produces 250 N·m at 5 500 RPM:

ω = 5 500 × 2π / 60 = 575.96 rad/s
P = 250 × 575.96 = 143 990 W
BHP = 143 990 / 745.70 = 193.1 BHP (≈ 196.1 PS, 144.0 kW)

The same result via the imperial shortcut: 250 N·m = 184.4 lb·ft → 184.4 × 5 500 / 5 252 = 193.1 BHP. Both routes agree to four significant figures.

Engine	Torque	RPM	BHP
1.0 EcoBoost	125 lb·ft	6 000	143
BMW 320d	295 lb·ft	4 000	225
Mustang GT V8	400 lb·ft	7 500	571
Bugatti Chiron	1 180 lb·ft	6 700	1 505

Formula note

All three methods are exact consequences of the same physics: P = T × ω. The “5252 constant” is not an approximation — it is the exact rational derivation of one mechanical horsepower (550 ft·lbf/s) expressed at crankshaft RPM. The small numerical difference from 5 250 or 5 255 that sometimes appears in older literature is simply due to rounding the value of π. This calculator uses the full-precision conversion 1 hp = 745.69987 W throughout.