What Is Wheel Horsepower?
Wheel horsepower (WHP) is the amount of engine power that actually reaches the driven wheels after passing through the drivetrain. Manufacturers usually quote crank (or brake) horsepower measured at the engine's flywheel, but the transmission, driveshaft, differential, and axles each consume some of that power through friction and rotational inertia. The result is that the number you read on a chassis dyno (WHP) is always lower than the advertised crank figure.
How to Use This Calculator
Enter your engine's crank horsepower and the estimated drivetrain loss as a percentage. Typical losses are about 10–15% for front- and rear-wheel-drive cars and 15–25% for all-wheel-drive vehicles, since AWD systems have more rotating components. The calculator returns your estimated wheel horsepower along with the total horsepower lost to the drivetrain.
The Formula Explained
The math is straightforward:
$$\text{WHP} = \text{Crank HP} \times \left(1 - \frac{\text{Loss \%}}{100}\right)$$
If you know your dyno WHP and want to estimate crank HP instead, rearrange to: \(\text{Crank HP} = \text{WHP} \div \left(1 - \frac{\text{Loss \%}}{100}\right)\). The loss percentage is an estimate — the only way to know your true number is a back-to-back dyno test.
Worked Example
Suppose a rear-wheel-drive sports car is rated at 300 crank horsepower with an estimated 15% drivetrain loss. Then $$\text{WHP} = 300 \times \left(1 - \frac{15}{100}\right) = 300 \times 0.85 = 255 \text{ WHP},$$ with 45 HP lost to the drivetrain.
FAQ
Is drivetrain loss always 15%? No. It's a rule of thumb. Manual transmissions tend to lose less than automatics, and AWD loses more than RWD/FWD.
Does loss scale with power? Drivetrain loss is closer to a percentage than a fixed number, so higher-output engines lose more horsepower in absolute terms.
Why is my dyno number different? Dyno type, temperature, tire size, gear used, and correction factors all affect readings. Treat this as an estimate.
