Brake Horsepower Calculator
Calculate brake horsepower (BHP) and see how it differs from wheel horsepower.
Brake horsepower (BHP) is the power measured at the engine's crankshaft, before any losses through the drivetrain. It's the figure manufacturers advertise, and it's calculated the same way as any crank horsepower.
Brake Horsepower Formula
The term "brake" comes from the Prony brake, an early device that measured engine power by applying a braking load. Today BHP means crank horsepower measured with all accessories — it's effectively the same as the SAE net figure quoted in brochures.
How to Use This Calculator
- Enter torque in lb-ft and RPM.
- Read BHP, with an estimated wheel horsepower shown below.
Worked Example
To work between crank and wheel figures, use the wheel horsepower calculator, and read wheel HP vs crank HP for the full explanation.
BHP vs Wheel Horsepower
Brake horsepower is measured at the crankshaft on an engine dyno, before the transmission, driveshaft, and differential take their cut. Wheel horsepower (WHP), measured on a chassis dyno, is always lower by the drivetrain loss — typically 10–25% depending on layout. When you see a manufacturer's power figure, it's BHP (specifically SAE net); when a tuner quotes a dyno number, it's usually WHP.
Why It's Called "Brake" Horsepower
The name comes from the Prony brake, a 19th-century device that measured an engine's output by applying a friction brake to its shaft and recording the force needed to slow it. The principle — loading the engine and measuring the work it does against that load — is still how modern dynos work, so "brake horsepower" remains the term for crank-measured power.
How an Engine Dyno Actually Applies the Load
"Brake" horsepower means the engine was loaded and the load measured. Three mechanisms dominate, and each suits a different kind of testing.
| Type | How it loads the engine | Used for |
|---|---|---|
| Water brake | Pumps water against a resisting rotor; heat carried away by the water | High-power testing, cheap to run, cannot motor the engine |
| Eddy current | Induced currents in a disc create magnetic drag | Fast, precise load control; excellent for steady-state points |
| AC / DC electric | Motor-generator absorbs power and can also drive the engine | Development work — can measure friction by motoring the engine |
The electric type matters for one reason worth knowing: because it can drive the engine as well as absorb from it, manufacturers use it to measure internal friction directly. That is how published friction and accessory-drag figures are obtained.
What Is Bolted On During the Test
This is the single biggest reason two honest BHP figures for the same engine can differ. A modern net rating requires the engine to run its production equipment:
| Component | Fitted for modern net rating? | Typical draw |
|---|---|---|
| Alternator (charging) | Yes | 1–5 hp |
| Water pump | Yes | 1–4 hp |
| Cooling fan | Yes | 2–10 hp |
| Production intake and air filter | Yes | 2–8 hp |
| Full production exhaust | Yes | 3–15 hp |
| Power steering pump | Usually | 1–3 hp |
| Air conditioning compressor | No — declutched | 3–10 hp when engaged |
Certified vs Uncertified Ratings
Under SAE J1349, a manufacturer may publish a figure it measured itself, or submit to a witnessed certification process. Certified results carry an SAE seal, and the distinction is real:
- Certified — the test was observed by an approved third party, following the standard exactly. Two certified figures are directly comparable.
- Uncertified — measured in-house to the same standard, but unwitnessed. Still honest, still comparable in practice, just not audited.
A handful of manufacturers have revised published figures downward after certification, which tells you the process has teeth. Europe's equivalent is ECE R85, implementing EC Directive 80/1269 — also a net standard, and close enough to SAE net that the two are broadly comparable.
The Correction Factor Nobody Mentions
Air density changes with temperature, pressure and humidity, and an engine's output moves with it. Every credible rating is therefore corrected to standard conditions — SAE J1349 uses 29.23 inHg dry pressure and 77 °F.
Two consequences:
- A published BHP figure is a corrected number, not what the engine made in the room on the day.
- A dyno printout marked "uncorrected" is not comparable with it — and different correction standards (SAE, DIN, EEC) give different headline figures from one identical run.
Work out the correction for your own conditions with the SAE correction factor calculator, or see air density correction.
BHP, HP and PS Are Not All the Same Size
Two of those three are identical and one is not, which causes endless confusion:
- BHP and hp — the same unit, 745.6999 W. A 300 bhp engine and a 300 hp engine make identical power. See bhp to hp.
- PS — metric horsepower at 735.4988 W, about 1.4% smaller. A 200 PS badge is 197 bhp. See bhp to PS.
- kW — the SI unit and the one on EU registration documents. Convert with bhp to kW.
BHP and Estimated WHP (at 5,500 RPM)
| Torque (lb-ft) | BHP | ~WHP (15% loss) |
|---|---|---|
| 300 | 314 | 267 |
| 350 | 367 | 312 |
| 450 | 471 | 401 |
Frequently Asked Questions
Brake horsepower is the power measured at the engine's crankshaft before drivetrain losses, calculated as (torque × RPM) ÷ 5252. It's the figure manufacturers usually advertise.
In practice they're used interchangeably for crank power. BHP specifically refers to power measured at the crank with a braking load, while 'HP' is the general unit.
No. BHP is measured at the crank; wheel horsepower is lower because the drivetrain absorbs 10–25% of the power before it reaches the road.
From the Prony brake, an early instrument that measured engine output by applying a known braking force to the spinning shaft.
Multiply BHP by (1 minus the drivetrain loss). For a typical RWD car with 15% loss, multiply by 0.85.