How accurate is this calculator?

It uses the Martin et al. (1998) road-cycling power model — validated to within ~3 W (R²=0.97) — and is accurate to within a few percent in still air. It already includes a ~2.4% drivetrain loss, so the watts you enter are pedal (crank) power. Real-world wind, gusts, road surface variation and changing position are the main remaining sources of error.

What CdA value should I use?

Typical values: hoods position 0.35-0.40, drops 0.30-0.35, aero bars 0.22-0.28. A professional TT rider may achieve 0.20.

Coefficient of rolling resistance. Typical values: racing tyres on smooth road 0.003-0.005, all-round tyres 0.005-0.007, mountain bike tyres 0.010-0.015.

Does this account for wind?

This calculator assumes still-air conditions. Use the Wind Resistance Calculator for headwind/tailwind effects.

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Power to Speed Calculator — Estimate Speed from Watts

Estimate cycling speed from power output accounting for all resistances.

Power Output

Sustained power at the pedals, e.g. 150–250 W endurance, 250–350 W threshold

Total Weight (rider + bike)

Rider + bike + kit combined, e.g. ~83 kg for a 75 kg rider on an 8 kg bike

CdA (Drag Area)

Hoods ~0.35, Drops ~0.32, TT ~0.24

Crr (Rolling Resistance)

Road tyre ~0.005

Gradient

Road slope %: positive uphill, negative downhill, 0 for flat

How We Calculate This

Speed from Power Formula

This uses the Martin et al. (1998) road-cycling power model. The power that reaches the road equals the sum of three resistance forces multiplied by speed, where θ = arctan(gradient ÷ 100):

Aerodynamic drag: P_aero = 0.5 × air density × CdA × v³
Rolling resistance: P_roll = Crr × mass × g × cos(θ) × v
Gravity: P_grav = mass × g × sin(θ) × v

Not all of your pedal power reaches the road — the chain and bearings lose roughly 2.4%, so road power = 0.976 × rider power. The calculator uses Newton-Raphson iteration to solve for the speed where P_aero + P_roll + P_grav = 0.976 × P_rider.

Frequently Asked Questions

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Watts Per Kilo Calculator

Calculate your power-to-weight ratio in watts per kilogram.

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Last updated: February 2026

All calculations are estimates. Always verify results and consult a professional bike fitter where appropriate.