CYCLE CALCULATORS

CalculatorsSpeed to Watts Calculator

Speed to Watts Calculator

On the flat, most of your watts fight air. CdA is the drag area: about 0.30–0.35 for many riders on the hoods, lower in the drops, higher sitting upright.

Inputs

Units
Rider plus bike. Gravity and rolling use the whole system.
About 0.003–0.005 on a good road tyre.

Results

Power
227 W

The Result is indicative and educational. It does not constitute a design, a technical opinion or medical advice and does not replace calculations or verification by a person with the appropriate qualifications and authorisations. Before purchase, installation or use in practice – in particular in electrical installations – the Result should be verified by a qualified specialist. Details: Terms of Service, point 4.

How the number is made

Power = ½ × CdA × 1.225 × v³ + Crr × mass × g × v + mass × g × v × (gradient % ÷ 100), with v in m/s (km/h ÷ 3.6). Air density is fixed at 1.225 kg/m³ (sea level, about 15°C). Example: 36 km/h = 10 m/s, CdA 0.32, Crr 0.004 and 80 kg on the flat → 196 W of air + 31 W of rolling = 227 W.

Mass is rider plus bike. No drafting and no drivetrain loss: this is power at the wheel, so a crank or pedal power meter reads about 2–3% more. Wind is not modelled; with a tailwind, ground speed makes this number too high, and with a headwind too low. Air density stays 1.225 kg/m³, so altitude and heat are not applied. A result below zero is shown as 0.

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Questions

What CdA should I enter?

0.32 is a reasonable hoods start. Time-trial positions can approach 0.20. An upright city bike can exceed 0.40.

Why does a descent show few watts?

Gravity is doing the work. When it covers all the drag, the sum drops below zero, and the calculator shows 0 rather than the braking you would need.

Which mass belongs in the speed-to-watts tool?

System weight: rider plus bike. Air density stays at 1.225, about sea level at 15 degrees C. A result below zero is shown as zero.

Example products

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