Crank To Wheel Hp Calculator converts engine crank horsepower to estimated wheel horsepower, or reverses measured WHP back to BHP. Formula: WHP = crank HP × (1 − drivetrain loss %).
The Gap Between Engine Power and Road Power
The number on a manufacturer’s spec sheet is measured at the crankshaft — the raw output of the engine before it enters the gearbox, driveshafts, differentials, and hubs. Every component between the flywheel and the tire adds friction, and friction costs power. What reaches the road is always less than what leaves the engine.
That gap matters when you’re tuning, reading a dyno sheet, comparing cars with the same rated BHP, or trying to understand why swapping from RWD to AWD costs measurable performance. The calculator runs in both directions: enter crank output to find wheel power, or enter a dyno figure to estimate what the engine is actually making at the flywheel.
Formulas Used
Loss Percentages by Configuration
- FWD base drivetrain loss: 10%
- RWD base drivetrain loss: 15%
- AWD base drivetrain loss: 20%
- Manual / Dual Clutch additional loss: +0%
- Traditional Automatic additional loss: +3%
Efficiency Multiplier
- Total Loss (%) = Drivetrain Base Loss + Transmission Loss
- Efficiency Multiplier = (100 − Total Loss) ÷ 100
Crank to Wheel Mode
- WHP = BHP × Efficiency Multiplier
- HP Lost = BHP − WHP
Wheel to Crank Mode
- BHP = WHP ÷ Efficiency Multiplier
- HP Lost = BHP − WHP
Unit Conversions (applied to both input and output values)
- kW = HP × 0.745699872
- PS = HP × 1.013869
Alternative Layout Comparison — Crank to Wheel Mode
- Alt WHP = BHP × (1 − Alt Layout Total Loss%)
- WHP Spread = Higher Alternative WHP − Lower Alternative WHP
Alternative Layout Comparison — Wheel to Crank Mode
- Alt BHP Required = WHP ÷ (1 − Alt Layout Total Loss%)
- BHP Spread = Higher Required BHP − Lower Required BHP
How It Works
Select a direction first. Crank to Wheel takes engine-rated BHP as input and returns the power delivered at the wheels. Wheel to Crank reverses it — enter a dyno reading to estimate flywheel output.
Two variables build the loss percentage. Drivetrain layout sets the base: FWD routes power through fewer components than RWD, and RWD through fewer than AWD — hence the stepped loss values of 10%, 15%, and 20%. Transmission type adds on top. A traditional automatic torque converter contributes an additional 3% over a manual or dual-clutch unit, so a RWD automatic reaches a total of 18% loss before any calculation runs.
Those two values sum to a total loss percentage. Subtract from 100 and divide by 100 to get the efficiency multiplier. Multiply BHP by that multiplier for C2W; divide WHP by it for W2C. The power lost figure is always the absolute difference between the two.
The calculator also converts the final output and the power lost into kilowatts and PS using fixed constants, and runs the same engine figure through the other two drivetrain configurations. The spread value is the absolute difference between those two alternative outputs — not between your selected layout and either alternative.
Where the Fixed-Percentage Model Breaks Down
Real drivetrain loss is not a flat percentage — the code’s own disclaimer acknowledges this. Two physically distinct mechanisms are involved, and they scale differently.
Fixed mechanical friction from bearing preload, gear tooth contact, and seal drag costs roughly the same absolute power regardless of RPM. Viscous drag from gear oil and transmission fluid scales with rotational speed. At low RPM, fixed friction dominates and the loss percentage appears higher because it’s consuming a larger share of a smaller total. At high RPM, viscous drag increases and the percentage climbs again from an entirely different cause.
A diesel making peak power at 3,500 RPM and a high-revving petrol engine peaking at 7,000 RPM will show different real-world drivetrain loss percentages even with identical hardware. The values here represent reasonable mid-range averages — reliable for planning, comparison, and estimating. A chassis dyno replaces them when precision is required.
Worked Example
A turbocharged straight-six is factory-rated at 382 BHP. The car is rear-wheel drive with a traditional automatic gearbox.
Inputs: Mode → Crank to Wheel | Drivetrain → RWD | Transmission → Automatic | Engine HP → 382 BHP
Loss calculation:
Total Loss = 15% (RWD) + 3% (Automatic) = 18%
Efficiency Multiplier = (100 − 18) ÷ 100 = 0.82
Primary output:
WHP = 382 × 0.82 = 313.24 WHP
HP Lost = 382 − 313.24 = 68.76 HP
Lost in kW = 68.76 × 0.745699872 = 51.27 kW
Lost in PS = 68.76 × 1.013869 = 69.71 PS
Metric equivalents of 313.24 WHP:
313.24 × 0.745699872 = 233.58 kW
313.24 × 1.013869 = 317.58 PS
Drivetrain efficiency: 82.00% efficient | 18.00% total loss | 0.82× multiplier
Alternative layout comparison (transmission stays Automatic, +3% in all cases):
FWD WHP = 382 × (1 − 0.13) = 332.34 WHP
AWD WHP = 382 × (1 − 0.23) = 294.14 WHP
WHP Spread = 332.34 − 294.14 = 38.20 WHP
That 38.2 WHP difference across three drivetrain types comes from the same engine at the same tune — purely the mechanical cost of routing power differently to the wheels.
Frequently Asked Questions
What happens if I enter 0 or a negative number?
Any non-positive value is treated as invalid. Output fields display dashes, the insight note switches to a warning state, and no calculation runs. The minimum accepted value is any positive number above zero.
Why do my drivetrain and transmission selections reset when I change the calculation direction?
Switching between Crank to Wheel and Wheel to Crank resets the drivetrain to RWD, transmission to Manual, and the HP field to a matching default — 300 BHP for C2W, 255 WHP for W2C. This prevents a BHP figure from silently carrying into a field that expects WHP, which would produce a result that looks plausible but is wrong.
Does changing drivetrain or transmission require clicking Calculate?
No. Both dropdowns and the HP input field trigger a live recalculation on every change. The Calculate button exists only to scroll the page to the result — the math runs regardless of whether you click it.
Why is the PS output slightly higher than the HP figure for the same power?
One imperial horsepower equals approximately 1.013869 metric horsepower (PS or CV). The PS unit is fractionally smaller in magnitude, so any power value expressed in PS will be a slightly larger number. At 300 BHP that difference is about 4 PS; at 500 BHP it becomes 7 PS. The gap is proportional and grows linearly.
The alternative layout comparison only shows two configurations. Where is my selected layout?
The comparison always displays the two layouts you did not select. RWD selected → shows FWD and AWD. FWD selected → shows RWD and AWD. The spread is the absolute difference between those two displayed alternatives — not between your configuration and either of them.