Brake Horsepower Calculator converts measured torque and engine speed into BHP. Formula: BHP = torque(lb-ft) × RPM ÷ 5252.113, or torque(N·m) × RPM ÷ 7120.8.
What Manufacturers Publish vs. What a Dyno Actually Measures
Brake horsepower is measured at the engine’s output shaft or flywheel, before any mechanical energy is lost to the gearbox, driveshaft, differential, or accessories like the alternator and water pump. Wheel horsepower — what a chassis dyno prints — always comes in lower, typically 12–18% less on a rear-wheel-drive car, because drivetrain losses have already been subtracted. Manufacturer spec sheets almost always quote BHP. Tuner dyno sheets almost always quote WHP. This calculator works with BHP only.
Formulas Used by This Calculator
Brake Horsepower
Imperial (lb-ft input) BHP = (Torque (lb-ft) × RPM) ÷ 5252.113 Metric (N·m input) BHP = (Torque (N·m) × RPM) ÷ 7120.8
5252.113 is derived from 33,000 ÷ 2π, where 33,000 ft·lbf/min is the classical definition of one mechanical horsepower. 7120.8 is its SI equivalent: (60 × 745.699 W) ÷ 2π.
Power Unit Conversions (from BHP)
kW = BHP × 0.7457PS = BHP × 1.01387MW = kW ÷ 1000
Energy Rate Conversions (from BHP)
BTU/hr = BHP × 2544.43kcal/hr = BTU/hr × 0.251996MJ/hr = (kW × 1000 × 3600) ÷ 1,000,000
Work Rate Conversions (from BHP)
ft·lbf/min = BHP × 33,000ft·lbf/s = ft·lbf/min ÷ 60ft·lbf/hr = ft·lbf/min × 60
Torque Unit Conversions (from entered torque)
N·m = lb-ft × 1.355818lb-ft = N·m ÷ 1.355818kgf·m = N·m ÷ 9.80665lb·in = lb-ft × 12
How the Calculation Flows
Pick a torque unit system, enter measured torque and RPM, then hit Calculate Power. Both fields must be positive numbers — zero, negative, or blank inputs fail validation and produce no output.
BHP is calculated first. Every other figure derives from that single result, grouped into four output cards:
- Power Equivalents — kW, PS, and MW. These map BHP to the scientific and European automotive standards used in type-approval documents and international spec sheets.
- Energy Rate Equivalent — BTU/hr, kcal/hr, and MJ/hr. Same mechanical power expressed as a heat rate, useful when comparing engine output to cooling system or HVAC load capacity.
- Work Rate — ft·lbf/min, ft·lbf/s, and ft·lbf/hr. These trace directly back to the 33,000 ft·lbf/min origin of the horsepower unit itself.
- Torque Conversion — Input torque cross-converted to the opposite unit system, plus kgf·m and lb·in in both modes.
Inputs recalculate silently on every keystroke. Clicking Calculate also scrolls the output panel into view.
Switching Unit Modes Overwrites Your Torque Input
Change the Torque Unit dropdown and the torque field resets to its built-in default — 300 for Imperial, 400 for Metric — and RPM snaps back to 5000. Any value typed before the switch is gone.
Carrying a raw number across unit systems would silently change its meaning: 300 lb-ft is not 300 N·m. Rather than allow that confusion, the tool substitutes a representative default whenever a mode change is detected.
To convert a specific torque value, enter it in one mode, read the converted figure from the Torque Conversion card, switch modes, then type that converted number in manually.
Worked Example: 300 lb-ft at Peak Power RPM
A naturally aspirated engine produces 300 lb-ft of torque at 5000 RPM on an engine stand. Inputs: Torque Unit = Imperial, Measured Torque = 300, Engine Speed = 5000 RPM.
BHP (hero output):
(300 × 5000) ÷ 5252.113 = 1,500,000 ÷ 5252.113 = 285.60 BHP
Power Equivalents card: 212.97 kW · 289.56 PS · 0.213 MW
Energy Rate Equivalent card: 726,688 BTU/hr · 183,122 kcal/hr · 766.70 MJ/hr
Work Rate card: 9,424,778 ft·lbf/min · 157,080 ft·lbf/s · 565,486,691 ft·lbf/hr
Torque Conversion card: 406.75 N·m · 41.48 kgf·m · 3,600 lb·in
At 5000 RPM, BHP (285.60) reads lower than the torque figure (300 lb-ft). Numerically they are equal only at 5252 RPM — a direct consequence of the divisor, and exactly why every dynamometer plot shows those two curves crossing at that speed.
Frequently Asked Questions
Why 5252.113 and not the rounded 5252?
33,000 ÷ 2π evaluates to 5252.1126…, not a clean integer. Carrying more decimal places reduces rounding error, most noticeably at low RPM where a small divisor error produces a visible difference in the BHP result.
Where does 7120.8 come from?
Converting N·m and RPM to watts requires multiplying by 2π ÷ 60. Converting watts to horsepower requires dividing by 745.699. Combined: (60 × 745.699) ÷ 2π = 7120.8. It is simply the metric counterpart to 5252.
PS is always slightly above BHP — is that right?
Yes. One PS (Pferdestärke, metric horsepower) equals 735.499 W; one British mechanical horsepower equals 745.699 W. PS is a smaller unit, so any given output requires more of them. At 1 BHP = 1.01387 PS, a 300 BHP engine reads as roughly 304 PS — a gap that shows up whenever German and British spec sheets are compared for the same car.
What happens if I enter zero?
Zero fails validation, same as a negative number or blank field. All outputs revert to dashes and the alert box shows “Data Required.” Mathematically, zero torque or zero RPM both yield zero power — but strictly positive inputs are required to return any result.
Why does the Energy Rate card show BTU/hr for a petrol engine?
It is not measuring exhaust heat or cooling loss — it expresses mechanical shaft power in the units used to rate thermal machinery. 1 BHP = 2544.43 BTU/hr is a standard equivalence used when sizing engine-driven heat pumps, marine gensets with integral heat exchangers, or any application comparing shaft power directly against a heating or cooling load specification.