Torque and engine speed define mechanical shaft power, and the Brake Horsepower Calculator resolves that pairing into a flywheel figure recorded before drivetrain losses reduce it.
Calculate Brake Horsepower at the Flywheel from Engine Torque and RPM
This brake horsepower calculator turns a measured torque figure and the engine speed it was recorded at into shaft power. Engine builders, dyno operators, and anyone reading a torque number off a test sheet use it to recover the matching power figure.
Entering Measured Torque and Engine Speed
Pick the torque unit: Imperial (lb-ft) or Metric (Nm). Enter the torque value, then the RPM at which it was measured. The result is brake horsepower in mechanical HP under either setting. The cards beneath convert that power into kW, PS, BTU/hr and ft·lbf/min, and convert your torque entry into Nm, kgf·m and lb·in.
How the 5252 and 7120.8 Divisors Produce Brake Horsepower
Power equals torque times angular velocity, $P = T\omega$. RPM is revolutions per minute, not radians per second. That mismatch is what the divisor absorbs. In Imperial mode the tool solves:
$$\mathrm{BHP} = \frac{T_{\text{lb-ft}} \times \mathrm{RPM}}{5252.113}$$
The divisor is $33{,}000 / 2\pi$. It follows from the definition of one mechanical horsepower as 33,000 ft·lbf per minute, or 745.6999 watts. That is the same power unit behind SAE J1349 net crankshaft ratings. Metric mode divides by 7120.8 instead:
$$\mathrm{BHP} = \frac{T_{\text{Nm}} \times \mathrm{RPM}}{7120.8}$$
That constant is $745.7 \times 60 / 2\pi$. Note what it is built from: mechanical horsepower, not metric horsepower. Enter Newton-metres and you still get imperial BHP, not PS. The PS figure is calculated separately in the power equivalents card, at 1 PS = 75 kgf·m/s. It runs about 1.4% higher numerically than the same power in HP. The remaining conversions are fixed definitions. One HP is 0.7457 kW, 2544.43 BTU/hr, and 33,000 ft·lbf/min. One lb-ft is 1.355818 Nm, and one kgf·m is 9.80665 Nm.
One mistake shows up constantly. Users assume the formula produces brake horsepower. It does not. The arithmetic is a unit conversion and nothing more. The result is brake horsepower only if the torque came off a brake dynamometer coupled to the crankshaft or flywheel. That measurement method is where the term originates, via the Prony brake. Enter chassis-dyno torque and you get wheel horsepower with the wrong name on it.
The tool needs torque above zero and RPM above zero. The field minimums are 0.1 and 1. Anything zero, blank or negative clears the outputs and shows a data-required notice. Neither field has a ceiling, and that is where the math quietly stops being useful.
Enter 300 lb-ft at 100,000 RPM and it returns over 5,700 BHP. The formula has no concept of valve float or bearing speed. It also has no way of knowing that no engine holds peak torque near that speed.
The energy-rate card carries its own catch. Those BTU/hr and kcal/hr numbers are the shaft power restated in heat units. They are not fuel burn or heat rejection. Both of those are far larger, since only a fraction of the fuel energy reaches the crankshaft.
Where the Brake in Brake Horsepower Is Measured
Input Mistakes That Distort the Horsepower Result
Entering Newton-metres while the selector still reads Imperial inflates the result by a factor of 1.36. Pairing a peak torque value with redline RPM, rather than the speed that torque was recorded at, produces a power figure the engine never made. Typing a fastener torque spec in lb-in or kgf·m straight from a service manual, without converting first, throws the result off by more than an order of magnitude.
Questions About Calculating Brake Horsepower from Torque
Is brake horsepower the same as the horsepower on a spec sheet?
The unit is identical. The test protocol differs. Modern US figures are SAE J1349 net crankshaft power with accessories fitted. Pre-1972 BHP figures were SAE J1995 gross, measured with accessories stripped off, which is why they read higher.
Why does metric mode return BHP instead of PS?
The 7120.8 divisor is built from mechanical horsepower at 745.7 watts. It returns HP whatever torque unit you enter. The PS equivalent, based on 735.5 watts, appears in the power equivalents card.
Can I enter torque from a chassis dyno?
The arithmetic works. The label does not. Drivetrain losses are already inside a chassis-dyno figure, so the output is wheel horsepower. This calculator applies no loss correction and cannot convert between the two.
Why do the HP and lb-ft figures match at 5252 RPM?
Because 5252.113 is the divisor. At that speed the formula reduces to power equalling torque. It is a property of the imperial units and holds for every engine, whatever its design.
What do the BTU per hour and kcal per hour values mean?
They restate the calculated shaft power as an energy rate, using 1 HP = 2544.43 BTU/hr. They describe mechanical output only. Fuel consumption and heat rejected to the coolant and exhaust are separate and larger.