How torque, RPM and power relate
Torque is the twisting force an engine produces; engine speed (RPM) is how fast it spins. Power is the rate at which work gets done, and for a rotating shaft that is simply torque multiplied by rotational speed. Horsepower is just a particular unit for that product.
Working in pound-feet of torque and revolutions per minute, the unit conversions collapse into the constant 5252, giving hp = torque × RPM / 5252. Switch the mode and the same relationship runs backwards to recover torque from a known horsepower and RPM.
Why every dyno chart crosses at 5252 RPM
The number 5252 is not arbitrary — it is the RPM at which the horsepower and lb-ft torque curves are always numerically equal, because that is where torque × RPM / 5252 equals torque. Below it, torque reads higher than horsepower; above it, horsepower pulls ahead. If two dyno curves cross anywhere other than 5252, one of them is mislabelled.
Using the two modes
Pick "horsepower" when you have measured torque at a given engine speed and want the power it represents. Pick "torque" when a spec sheet lists peak horsepower at a stated RPM and you want to back out the torque.
- Peak torque and peak power almost never occur at the same RPM, so quote the speed alongside either figure.
- A high-revving engine can make big horsepower from modest torque simply by spinning faster.
- Gearing trades the two: lower gears multiply torque at the wheels at the cost of road speed.
Units and caveats
This calculator uses mechanical horsepower with lb-ft and RPM; metric (PS) and electrical horsepower differ slightly, and the kilowatt figure shown uses the standard 0.746 kW per hp. The result is the power at whatever point your torque was measured — crank, wheels or shaft — so be clear about where the reading came from before comparing engines.
Formula
hp = torque·rpm/5252; torque = hp·5252/rpmFrequently asked questions
- Why 5252?
- It is the RPM at which horsepower and lb-ft torque are always equal, derived from converting torque·angular-speed into horsepower units.