Wheel Horsepower Calculator
Enter your crank horsepower (or wheel HP) and select your drivetrain type to see exactly how much power reaches the wheels after drivetrain losses. You can also calculate horsepower from torque and RPM, or flip the calculation to find crank HP from a dyno-measured WHP figure. Switch the power unit between HP and kW, or enter a custom loss percentage for your specific setup.
Formula
Worked example
A 300 HP RWD car with 15% drivetrain loss: WHP = 300 x (1 - 0.15) = 300 x 0.85 = 255 WHP. The drivetrain absorbs 45 HP. Converting to kW: 255 x 0.7457 = 190.2 kW at the wheels.
What is wheel horsepower and why does it differ from crank HP?
Wheel horsepower (WHP) is the power that actually reaches the driven wheels and pushes the car forward. Crank horsepower, also called brake horsepower (BHP) or engine horsepower, is measured directly at the crankshaft output before any losses occur. Every component between the engine and the tyres - the flywheel, gearbox, prop shaft, differential, axle shafts, and wheel bearings - consumes a fraction of that power through friction and rotating mass. What is left over is WHP. This is the number a chassis dyno measures: the machine spins the drive wheels and works backward from wheel force and speed to report power at the contact patch.
How to use this calculator
Pick a calculation mode from the dropdown. "Crank HP to Wheel HP" is the most common: you enter the rated engine output and your drivetrain type, and the calculator shows how much reaches the wheels. "Wheel HP to Crank HP" reverses this - useful when you have a dyno sheet and want to estimate what the engine is making at the crank. "Torque + RPM to HP and WHP" uses the standard horsepower formula (HP = torque x RPM / 5252) to first compute crank HP, then applies the drivetrain loss. Switch the power unit toggle between HP and kW at any time, and all inputs and outputs update together. If you have a measured drivetrain efficiency figure - for example from back-to-back engine and chassis dyno runs on the same vehicle - choose "Custom loss %" and enter it directly.
Drivetrain loss by layout: FWD, RWD, AWD and 4WD
Front-wheel drive (FWD) cars combine the gearbox and differential into one transaxle unit close to the engine, giving the shortest power path. Typical loss is 8-12%, averaging about 10%. Rear-wheel drive (RWD) cars run power along a propshaft to a separate rear differential, adding roughly 12-18% loss (average 15%). All-wheel drive (AWD) splits torque between axles through a centre differential or multi-plate clutch, raising the loss to 18-22%. Part-time and full-time 4WD systems, which add a transfer case and often a low-range gear set, sit at 22-28%. These figures are rule-of-thumb estimates; a well-maintained car with quality synthetic lubricants, properly inflated tyres and no worn bearings will be at the lower end of its band.
Torque, RPM and the 5252 constant
The relationship between torque and horsepower is fixed by physics: HP = (torque in lb-ft x RPM) / 5252. The constant 5252 comes from converting the angular work formula to the historical definition of one horsepower (33,000 ft-lb per minute). At exactly 5252 RPM, a torque and horsepower curve always cross on a dyno chart - a fact that is sometimes cited as a quick sanity check. If your source reports torque in newton-metres (Nm) instead, convert first: 1 lb-ft = 1.35582 Nm. Wheel torque can be estimated the same way, using WHP and RPM, and is typically plotted alongside WHP on a chassis dyno printout.
Typical drivetrain loss by layout
| Drivetrain layout | Typical loss (%) | Loss factor | Example: 300 HP crank |
|---|---|---|---|
| Front-wheel drive (FWD) | 8-12% (avg 10%) | 0.90 | 270 WHP |
| Rear-wheel drive (RWD) | 12-18% (avg 15%) | 0.85 | 255 WHP |
| All-wheel drive (AWD) | 18-22% (avg 20%) | 0.80 | 240 WHP |
| 4-wheel drive (4WD) | 22-28% (avg 25%) | 0.75 | 225 WHP |
These are industry-accepted estimates. Actual losses vary with transmission type, fluid viscosity, tyre drag and mechanical condition.
Frequently asked questions
What is the difference between wheel HP and crank HP?
Crank HP is the power your engine produces at its crankshaft, measured on an engine dynamometer or claimed by the manufacturer. Wheel HP is what actually gets to the tyres after friction losses in the transmission, driveshaft and differential. For a typical rear-wheel drive car, WHP is about 85% of crank HP.
Why is AWD drivetrain loss higher than RWD?
An AWD system adds a centre differential or electronically controlled clutch pack that routes torque to a second axle, along with extra shafts and gears. Each additional rotating component adds friction. A simple RWD car has fewer spinning parts between the engine and the wheels, so less energy is wasted.
What is the 5252 constant in the horsepower formula?
One horsepower equals 33,000 foot-pounds of work per minute. When you convert angular velocity (RPM) to linear work and simplify the units, the constant that appears is 33,000 / (2 x pi) = 5252.11. At 5252 RPM, torque in lb-ft numerically equals horsepower - which is why the two curves always cross at that RPM on a dyno chart.
Can I reduce drivetrain loss on my car?
Yes, modestly. Using low-viscosity synthetic gear oil and differential fluid, eliminating worn bearings, fitting lightweight wheel and brake components, and reducing tyre rolling resistance can each shave a percentage point or two. Racing cars use narrower tolerances and purpose-built lubricants to push FWD losses below 8% and RWD losses below 12%, but these gains are difficult to achieve on a road car.
Why do chassis dyno numbers vary between runs?
Tyre temperature, ambient temperature and humidity, correction factor (SAE J1349 vs EU 80/1269 vs uncorrected), dyno calibration and operator technique all influence the reading. The same car on the same day can show a 3-5% spread across runs. Always average at least two consecutive pulls and note the correction standard used before comparing figures from different sources.
How do I convert HP to kW?
Multiply HP by 0.7457 to get kW, or divide kW by 0.7457 to get HP. For example, 300 HP x 0.7457 = 223.7 kW. This calculator does the conversion automatically when you switch the power unit selector.