About the Wheel HP to Crank HP Calculator
This wheel horsepower to crank horsepower calculator estimates how much power an engine makes at the flywheel from the number measured at the wheels on a chassis dyno. Some power is always lost through the transmission, driveshaft, differential, axles and tyres, so wheel horsepower (WHP) is lower than the manufacturer’s rated crank horsepower.
Use it to compare a dyno sheet with a factory rating, to judge whether a modification delivered what was promised, or to plug a realistic crank figure into a quarter-mile or power-to-weight calculation. Pick your drivetrain for a typical loss, or enter a custom percentage if you know your setup.
Drivetrain loss is an estimate. It varies with transmission type, gearing, dyno brand (inertia dynos read differently from load-bearing ones), tyre pressure and temperature. Treat the result as an approximate range rather than an exact engine rating.
With the default inputs, the estimated crank horsepower is 352.9 hp. Change any value above to recalculate instantly.
How to use the wheel hp to crank hp calculator
- 1Enter the wheel horsepower from your dyno sheet.
- 2Choose your drivetrain type, or pick Custom and enter a known loss percentage.
- 3Read the estimated crank horsepower and power lost.
- 4Compare against the factory rating or feed it into a quarter-mile estimate.
Formula and method
Drivetrain loss is quoted as a share of the engine’s output: if 15% of crank power is lost, the wheels receive 85% of it. So crank horsepower equals wheel horsepower divided by (1 − loss). A common shortcut multiplies WHP by (1 + loss) instead, which gives a noticeably lower number; both are shown so you can compare.
Typical figures are around 10–13% for front-wheel-drive manuals, 13–18% for rear-wheel drive and 18–25% for all-wheel drive, with automatics toward the high end. Some tuners argue losses are partly fixed rather than proportional, so very high-power builds may lose a smaller percentage.
- WHP
- Wheel horsepower measured on a chassis dyno
- loss
- Drivetrain loss as a decimal (15% = 0.15)
- Crank hp
- Estimated flywheel horsepower
Worked examples
300 whp rear-wheel-drive car
With a 15% loss the wheels see 85% of engine power, so 300 ÷ 0.85 = 352.9 crank hp. About 52.9 hp is lost in the drivetrain. The quick multiply method (300 × 1.15) gives only 345 hp.
250 whp front-wheel-drive hatchback
Front-wheel-drive transaxles are efficient, so a 12% loss is typical: 250 ÷ 0.88 = 284.1 crank hp.
450 whp all-wheel-drive build
All-wheel drive adds a transfer case and extra differentials. At 20% loss, 450 ÷ 0.80 = 562.5 crank hp, or 419.5 kW.
350 whp automatic with a 22% custom loss
Using a measured or assumed 22% loss, 350 ÷ 0.78 = 448.7 crank hp, meaning almost 99 hp never reaches the tyres.
Frequently asked questions
What is the difference between WHP and HP?+
HP (brake or crank horsepower) is measured at the engine’s flywheel, which is what manufacturers advertise. WHP is measured at the driven wheels on a chassis dyno, after losses in the transmission, differential and tyres.
How much horsepower is lost through the drivetrain?+
Rules of thumb are about 10–13% for front-wheel drive, 13–18% for rear-wheel drive and 18–25% for all-wheel drive. Automatics with torque converters usually lose more than manuals.
Should I multiply or divide to get crank horsepower?+
Divide. If 15% of crank power is lost, WHP = crank × 0.85, so crank = WHP ÷ 0.85. Multiplying WHP by 1.15 understates crank power by about 2% at a 15% loss, and multiplying by 1.20 is about 4% low at a 20% loss.
Why do different dynos give different WHP numbers?+
Inertia dynos such as Dynojet and load-bearing dynos such as Mustang measure differently and use different correction factors. The same car can read 10% or more apart, so compare results on the same dyno.
How do I convert crank horsepower to wheel horsepower?+
Multiply crank horsepower by (1 − loss). A 400 hp rear-wheel-drive car with 15% loss should make about 340 whp on the dyno.