WHP to HP Calculator

A chassis dyno measures wheel horsepower (WHP), but engine specs are quoted in crank horsepower. This calculator converts your dyno's wheel figure back to crank power by adding the drivetrain loss.

WHP to HP Formula

How to Use This Calculator

1. Enter wheel horsepower from your dyno sheet.
2. Set drivetrain loss % for your layout.
3. Read estimated crank horsepower.

Worked Example

For the opposite direction, see the wheel horsepower calculator, and read wheel HP vs crank HP for the full explanation.

Why You Divide Instead of Adding the Loss Back

If a drivetrain loses 15%, the wheels receive 85% of crank power — so to recover crank HP you divide WHP by 0.85, not multiply by 1.15. Dividing 300 WHP by 0.85 gives 353 crank HP, whereas wrongly adding 15% gives only 345. The gap widens as losses rise, so using the correct operation matters for an honest power claim.

Drivetrain Loss by Layout

Loss grows with the number of components between engine and tires. Front-wheel drive is the most direct (~10–12%), rear-wheel drive sits in the middle (~15–17%), and all-wheel drive loses the most (~20–25%) because it drives two axles. Manual gearboxes are generally a touch more efficient than automatics. These are conventions, not exact constants — the same car can read differently on different dynos.

Crank HP from 300 WHP by Loss

The figure is the same whether your dyno sheet calls it hp, bhp or crank horsepower — those labels all describe mechanical horsepower at the crankshaft. Australian and New Zealand sheets report the same measurement in kilowatts, handled by rwkw to hp.

Going the other way — starting from a factory crank rating and working out what the rolling road should show — is handled by HP to WHP, which returns a realistic range rather than a single figure.

WHP to Crank HP Conversion Chart

Quick lookup using typical drivetrain losses — 12% FWD, 15% RWD, 20% AWD. Divide wheel HP by (1 − loss):

These are planning figures — real loss varies with transmission type, tire pressure and dyno type. Manual gearboxes lose less than the old torque-converter automatics that spawned the 15–20% folklore.

Why a Fixed Percentage Breaks Down on a Modified Car

Almost every WHP-to-crank conversion online treats drivetrain loss as a fixed share of output. It is not. Most of the loss is a roughly constant drag torque — bearing friction, gear mesh, oil churn, differential preload and tire deformation — and that drag barely changes when you add boost.

Follow that through:

• A stock rear-drive car makes 235 crank hp and shows 200 whp. Loss = 35 hp, or 15%.
• Tuned to 470 crank hp, the drivetrain still absorbs roughly 40–45 hp, not 70. It now shows about 427 whp.
• The percentage has quietly fallen from 15% to around 9%.

So applying a flat 15% to a heavily modified car overstates its crank power, sometimes by a wide margin. The chart above is right for a near-stock vehicle; if your engine is making close to double its factory output, expect the true loss percentage to be materially lower. The drivetrain loss calculator derives your actual figure when you have both numbers.

Why Two Dynos Disagree About the Same Car

Before arguing about the conversion factor, check whether the input number is even stable — it usually is not.

The practical rule: a dyno is a comparison instrument, not a certification lab. Same dyno, same day, same operator, before and after a modification — that comparison is reliable. One shop's number against another's, or against a brochure figure, is not.

Check Whether Your Sheet Is Already Corrected

Many dyno programs run a coastdown after the pull, measure how quickly the drivetrain slows the rollers, and print an estimated engine or flywheel figure alongside the wheel figure.

WHP, RWHP, Crank HP — Reading the Terminology

All the crank-side labels describe the same unit — mechanical horsepower of 745.6999 W. The prefix only tells you where the reading was taken, which is exactly what this conversion adjusts for.