Convert torque and rotational speed into mechanical horsepower for automotive and machinery analysis.
Overview
Horsepower summarizes how much work an engine can do per unit time. In imperial units, a common dyno relationship ties torque and RPM: HP = (Torque × RPM) / 5252. The constant comes from unit conversions between foot-pounds per minute and horsepower.
Torque tells you twisting force; RPM tells you how fast the crank spins. The same torque at higher RPM yields more power because work rate increases. That is why peak horsepower and peak torque occur at different engine speeds.
Dynamometer charts plot both curves across RPM. When they cross at 5252 RPM in lb-ft and HP units, the numeric values match—a handy sanity check. Metric systems use kilowatts with different constants.
Use calculated horsepower to compare tunes, estimate towing capability with other constraints, or translate shop measurements into spec sheet language. Always note whether torque is wheel torque or engine torque.
Steps
Use lb-ft or N·m per tool settings.
Provide engine speed at the measurement point.
Apply HP = T × RPM / 5252 for imperial lb-ft.
Repeat across the rev range for a curve picture.
Switch between kW and HP when comparing global specs.
Label wheel HP versus crank HP context.
Math
HP = (Torque × RPM) / 5252
Derived from 1 HP = 33,000 ft·lb/min divided by 2π revolutions per minute conversion when torque is in lb-ft.
Worked cases
(320 × 4500) / 5252
≈ 274.2 HP.
At 5252 RPM, HP numerically equals torque in lb-ft
300 HP at that point.
(400 × 2500) / 5252
≈ 190.4 HP despite high torque.
Why use this
Questions