Enter your wheel (net) horsepower and drivetrain type to estimate the gross horsepower at the flywheel, or reverse the calculation to predict wheel power from a factory crank rating.
Related Calculators
- All Automotive Calculators
- Corrected Horsepower Calculator
- Drawbar Horsepower Calculator
- Weight Loss To Horsepower Calculator
Formula
Net → Gross:
HPgross = HPnet ÷ (1 − L)
Gross → Net:
HPnet = HPgross × (1 − L)
From measured loss:
HPgross = HPnet + HPloss
where HPgross = flywheel/crank horsepower, HPnet = wheel horsepower, L = drivetrain loss as a decimal (e.g., 0.15 = 15%), HPloss = power absorbed by the drivetrain.
Interpretation
Gross horsepower is what the engine produces at the crankshaft before any drivetrain parasitic losses. Net (wheel) horsepower is what actually reaches the ground and what a chassis dyno reads. The gap between the two is the energy burned turning transmissions, driveshafts, differentials, and axles.
Typical drivetrain losses fall in these ranges:
- FWD manual: ~10%
- FWD automatic: ~12%
- RWD manual: ~15%
- RWD automatic: ~17–18%
- AWD manual: ~20–22%
- AWD automatic: ~22–25%
If your calculated loss is below 8% or above 30%, recheck the inputs — those numbers usually indicate a dyno correction factor mismatch, a misreported crank rating, or a non-standard drivetrain.
Quick Reference Table
| Net (whp) | 15% loss | 18% loss | 22% loss |
|---|---|---|---|
| 150 | 176 hp | 183 hp | 192 hp |
| 250 | 294 hp | 305 hp | 321 hp |
| 350 | 412 hp | 427 hp | 449 hp |
| 450 | 529 hp | 549 hp | 577 hp |
| 550 | 647 hp | 671 hp | 705 hp |
FAQ
Is drivetrain loss a fixed percentage or a fixed number of horsepower?
Neither is perfectly accurate. A lot of drivetrain loss is a fixed parasitic load (bearings, gear mesh, fluid churn) that grows only modestly with power, so a flat percentage overstates losses on high-output cars. For a rough estimate the percentage method is fine; for precise work, use the "From Measured Losses" mode with a coast-down dyno figure.
Why doesn't my dyno number match the factory horsepower rating minus 15%?
Factory ratings are measured on an engine dyno under controlled conditions, and dynos vary in calibration (Dynojet typically reads higher than Mustang or DynoPack). Atmospheric correction, tire slip, drivetrain temperature, and fuel quality all shift the result by several percent.
What's the difference between gross, net, and SAE certified horsepower?
Gross (pre-1972) was measured on a bare engine with no accessories. Net is measured with full accessories installed, which is closer to real-world output. SAE J1349 and J2723 are modern certified standards — those numbers are already "net" at the flywheel, not at the wheels.
Can I use kW or PS instead of hp?
Yes. Select the unit next to each input. The calculator converts internally and returns the result in the unit you entered, with hp, kW, and PS equivalents shown underneath.
