HP to WHP Calculator

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.

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.

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:

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:

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.

Why the Percentage Falls as Power Rises

There's a subtlety most drivetrain-loss tables gloss over: a large share of the loss is fixed mechanical drag — bearing friction, gear mesh, oil churn — that doesn't scale with engine output. The same gearbox absorbs roughly the same power whether it's fed 200 HP or 500 HP.

That means the loss percentage shrinks as power climbs. If a transmission eats 40 HP, that's 20% of a 200 HP engine but only 8% of a 500 HP one:

The practical consequence: applying a flat 15% to a heavily modified engine understates what actually reaches the wheels. Percentage-based estimates are most reliable near the output the percentage was derived from, and drift on very high-output builds.

Different Dynos Read Differently

Two chassis dynos can read the same car nearly 10% apart, and neither is faulty — they load the engine in fundamentally different ways.

• Inertia dynos (Dynojet is the common example) calculate power from how quickly the car accelerates a drum of known mass. They typically read higher.
• Load-bearing dynos (Mustang, Dyno Dynamics) apply controlled resistance and can hold a steady speed. They typically read lower, and are better for tuning.

This is why a WHP figure only means something against another figure from the same dyno, in the same session. Comparing across shops tells you very little. Our guide to how horsepower is measured covers the dyno types and correction factors in full.