Engine torque is derived, not measured, when only a power figure is known; the RPM To Torque Calculator computes crankshaft torque from that figure and its associated engine speed.
Calculate Crankshaft Torque from Engine Horsepower and RPM
This calculator converts a rated power figure and an engine speed into the twisting force at the crankshaft, then reports the same result in alternate torque units, work per revolution, and angular speed. Engine builders, dyno operators, and tuners use it to place a single power figure back onto the torque axis when only one of the two is published.
Entering Power, Engine Speed, and Drivetrain Loss
Select Imperial (HP and lb-ft) or Metric (kW and Nm); the unit system you pick governs both the formula constant and the hero output. Enter engine power output, the engine speed that power figure was measured at, and a drivetrain parasitic loss percentage. The hero value is crank torque. The four cards below it show drivetrain-adjusted output, unit conversions, work per revolution, and rotational speed.
The 5252 Constant and How Crank Torque Is Derived
Torque is not measured by this tool — it is recovered algebraically from the definition of power as torque times angular velocity, $P = T\omega$. In imperial units the calculator solves:
$$T_{\text{lb-ft}} = \frac{\mathrm{HP} \times 5252}{\mathrm{RPM}}$$
The constant is not arbitrary: $5252 = 33{,}000 / 2\pi$, following from the definition of one mechanical horsepower as 33,000 ft·lb per minute and one revolution as $2\pi$ radians. That same 550 ft·lb/s power definition underlies the net crankshaft power ratings published under SAE J1349, which is the number most factory HP figures represent. In metric mode the tool solves the identical physics with the SI constant:
$$T_{\text{Nm}} = \frac{\mathrm{kW} \times 9549}{\mathrm{RPM}}$$
Here $9549 = 60{,}000 / 2\pi$. The remaining conversions are fixed unit definitions, not estimates: 1 lb-ft = 1.355818 Nm, 1 kgf·m = 9.80665 Nm, and 1 PS (metric horsepower) = 75 kgf·m/s, giving 1 HP = 1.013869 PS and 1 kW = 1.359621 PS. Work per crank revolution is $2\pi T$, which is why that card’s value does not change when you change RPM — only torque moves it.
The non-obvious consequence of the constant sits in the unit selector. Horsepower and lb-ft cross at exactly 5252 RPM on a dyno chart, which is a unit artifact and not an engine characteristic. In kW and Nm the same engine’s curves cross at 9549 RPM instead — above most petrol redlines, so metric charts simply never show the crossover that imperial readers treat as a landmark.
Switching this tool between systems moves that crossover point, and comparing a metric chart to an imperial one on that basis will mislead you every time. One common input mistake here: feeding the tool a chassis-dyno wheel horsepower figure and then also applying a drivetrain loss percentage, which subtracts the same losses twice.
The drivetrain loss field is a convention, not a standard. No SAE, ISO, or DIN document defines a fixed percentage — the 10–25% figures in circulation come from workshop and chassis-dyno practice, and this calculator does not endorse a number.
It is also a crude model: some driveline losses scale with load and behave like a percentage, while oil churning and rotating inertia scale with speed, so the same drivetrain does not consume a constant fraction across the whole rev range.
Note also that the drivetrain output card multiplies crank torque by the efficiency factor; it is not tire torque, because it excludes gearbox and final-drive multiplication. A 408 lb-ft engine showing 347 lb-ft here can still put several thousand lb-ft through the axles in first gear.
The tool accepts power greater than zero, engine speed of at least 100 RPM, and loss between 0% and 50%; outside those bounds it halts and clears the outputs rather than printing a result. The RPM floor exists because torque is inversely proportional to speed, so the result runs to infinity as RPM approaches zero.
Even inside the valid range the math will happily return nonsense: 350 HP at 100 RPM computes to over 18,000 lb-ft, arithmetically correct but physically impossible, because no engine produces rated power at cranking speed. This is a single-point conversion, not a curve — the RPM you enter must be the speed at which that exact power figure was recorded.
Why Work Per Revolution Ignores Engine Speed
Input Mistakes That Produce Misleading Torque Figures
Pairing peak horsepower with the RPM at which peak torque occurs mixes two different points on the curve and returns a torque value the engine never made. Entering a kilowatt figure while the selector still reads Imperial understates torque by roughly a factor of 2.7, since 1 kW is only 1.34 HP. Typing the loss as a decimal, such as 0.15 instead of 15, applies a fifteen-hundredths of a percent reduction and makes the drivetrain card look almost identical to the crank figure.
Questions About Converting RPM and Horsepower to Torque
Why do the horsepower and torque values match at 5252 RPM?
Because 5252 is the conversion constant itself. When RPM equals 5252, the formula reduces to torque equals horsepower. It is a property of the imperial units, holds for every engine, and says nothing about that engine’s design or performance.
Does this give torque at the wheels?
No. The hero value is crankshaft torque, and the drivetrain card only subtracts parasitic losses from it. Actual torque at the tire is crank torque multiplied by the selected gear ratio and the final drive, which this tool does not model.
Which drivetrain loss percentage should I enter?
There is no standardised figure to cite. Manual, automatic, and all-wheel-drive layouts all differ, and the loss varies with fluid temperature and dyno type. If you have a chassis dyno sheet and a certified crank rating for the same car, calculate your own percentage from those two numbers.
Can I use a manufacturer’s advertised horsepower figure?
Yes, provided you also use the RPM that rating was quoted at, which is usually printed alongside it. Advertised figures are net crankshaft power, so do not deduct drivetrain loss unless you specifically want an estimated wheel figure.
Why does work per revolution stay the same when I change RPM?
Work per revolution is $2\pi T$, and torque is the only variable in it. Raising RPM at fixed power lowers torque, which lowers work per revolution; raising RPM at fixed torque leaves it unchanged and raises power instead.