HP to WHP Calculator
You know the crank rating. This tells you what the rolling road should actually show — as a realistic range, and with an honest read on when a low number means something is wrong.
Booking a dyno session with a factory horsepower figure in your head is how most people end up disappointed by a perfectly healthy car. The rolling road measures what reaches the tires, and that's always meaningfully less than the number on the brochure. Knowing the target beforehand turns a worrying result into an expected one.
Multiply Going This Way — Don't Divide
Crank to wheels and wheels to crank are inverse operations, and swapping them is the most common arithmetic error in this whole subject.
Going the other way it is divide: 340 ÷ 0.85 = 400. Never multiply by 0.85 in both directions.
Sanity check yourself with the round trip. Convert crank → wheels, then run the answer back through WHP to HP. If you don't land on the number you started with, one of the two steps used the wrong operation.
HP to WHP Chart by Drivetrain
Expected wheel horsepower from a crank rating, manual transmission. Add roughly four points of loss for a torque-converter automatic.
| Crank HP | FWD (11%) | RWD (15%) | AWD (23%) |
|---|---|---|---|
| 150 | 134 | 128 | 116 |
| 200 | 178 | 170 | 154 |
| 250 | 223 | 213 | 193 |
| 300 | 267 | 255 | 231 |
| 350 | 312 | 298 | 270 |
| 400 | 356 | 340 | 308 |
| 450 | 401 | 383 | 347 |
| 500 | 445 | 425 | 385 |
| 600 | 534 | 510 | 462 |
| 700 | 623 | 595 | 539 |
| 800 | 712 | 680 | 616 |
| 1000 | 890 | 850 | 770 |
The bottom rows are deliberately conservative. Because most drivetrain loss is a near-constant drag torque rather than a fixed share of output, a genuinely 800 hp car usually loses closer to 9–11% than 15% — so it'll typically beat the figure in this table.
"What Should My Car Make on a Dyno?"
This is the question behind almost every hp-to-whp search, and the answer has two parts: the arithmetic above, and a tolerance band that nobody puts on a brochure.
Manufacturer ratings are not promises
A published figure comes from an engine dynamometer, running a fresh and often hand-assembled sample, on reference fuel, at controlled intake temperature, corrected to standard atmospheric conditions. Your car is none of those things. Production tolerance alone spreads output by a few percent between two identical cars off the same line.
Layer on the realities of a dyno day and the gap widens legitimately:
- Fuel — pump octane below what the calibration assumes pulls timing and costs power
- Intake temperature — a heat-soaked engine bay after three back-to-back pulls is measurably down
- Correction factor — SAE J1349, DIN, EEC and uncorrected can differ by several percent on the same run
- Dyno calibration — different machines and different operators simply read differently
- Mileage and condition — worn plugs, a tired air filter or a partially blocked catalyst each take their cut
Normal Shortfall vs a Real Fault
Use the expected figure as the center of a band, not a pass mark. This is how to read a result:
| Result vs expected | Verdict | What to do |
|---|---|---|
| Within ±5% | Healthy | Nothing. This is a well-matched car and dyno. |
| 5–10% under | Normal spread | Check fuel grade, intake temp and whether it was the first pull of the day. |
| 10–15% under | Worth a look | Service items first — plugs, filter, boost leaks. Compare the shape of the curve, not just the peak. |
| Over 15% under | Investigate | Look for timing being pulled, a boost leak, a failing sensor, a restricted exhaust or a slipping clutch. |
| Above expected | Check the sheet | Confirm it is a wheel figure and not a coastdown-corrected "engine" estimate. Optimistic dyno calibration is also common. |
Choosing the Right Loss Percentage
The dropdown defaults are the standard working assumptions used across this site, and they're right for a near-stock car:
| Layout | Typical loss | Why |
|---|---|---|
| Front-wheel drive | ~11% | Shortest path — engine, gearbox and driveshafts, no propshaft or rear diff |
| Rear-wheel drive | ~15% | Adds a propshaft, two universal joints and a hypoid final drive |
| All-wheel drive | ~23% | Transfer case, second differential and four driven hubs |
| Torque-converter auto | +3–5 points | Converter slip, though modern units lock up under load |
If you already have a measured pair of numbers for your own car, stop guessing — the drivetrain loss calculator works out your actual percentage, and that figure will beat any table.
Convert the Rating First If It Isn't in HP
Feed this calculator horsepower, not PS or kilowatts. A car badged 400 PS is about 395 bhp, so entering 400 overstates the expected wheel figure by roughly 1.4% — about 5 whp on a 340 whp car, which is enough to make a good result look mediocre.
Convert first with bhp to PS if you're working from a European badge, or kW to bhp if you've the registration-document figure. Australian and New Zealand dyno sheets report in rear-wheel kilowatts instead — see rwkw to hp.
Related Tools
For the reverse direction use WHP to HP or the fuller WHP to HP chart. Wheel power in metric is handled by whp to kW, and the theory behind the whole gap sits in wheel HP vs crank HP.
Frequently Asked Questions
Multiply by (1 − loss). At 15%: 400 × 0.85 = 340 whp. Going the other way you divide — never multiply in both directions.
About 267 whp FWD, 255 whp RWD or 231 whp AWD. Within ±5% of that's a healthy car.
Ordinary reasons stack: pump fuel, a warm intake, dyno calibration, correction factor, and an engine with miles on it. 5–10% under is common and usually not a fault.
Beyond about 15% under, especially alongside a flat spot, boost leak, misfire or pulled timing. Under 10% with a smooth curve is measurement spread.
It is the inverse. Crank→wheels multiplies by (1 − loss); wheels→crank divides by it. Using the wrong one introduces an error that grows with power.
Generally a few points more, from converter slip. Modern units lock up under load, so the gap is much smaller than on older designs.
About 595 whp RWD at 15%. In practice expect more — at that output the real loss is nearer 10%, so 610–630 whp is common.
Yes. A 400 PS badge is ~395 bhp; entering 400 overstates the target by about 5 whp. Convert first, then apply the loss.