Cycling Power Calculator (Watts from Speed)
Enter speed, weight and gradient to estimate the watts you need to ride.
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Cycling Power Calculator (Watts from Speed) by TheCalcTools
How the Cycling Power Calculator (Watts from Speed) works
At a steady speed, the rider’s power balances three forces: gravity on a slope, rolling resistance of the tyres, and air drag. Each force times speed is a power, and a 97.6% drivetrain efficiency is assumed.
Air drag grows with the square of air speed, so it dominates on the flat above about 25 km/h. On steep climbs gravity dominates and weight matters most. CdA and Crr defaults are typical road-bike values; your own may differ.
The formula
Worked example
Using the default values shown in the calculator:
| Speed | 30 km/h |
|---|---|
| Rider weight | 75 |
| Bike and kit weight | 9 |
| Gradient | 0 % |
| Riding position (CdA, m²) | Hoods, elbows bent (0.32) |
| Rolling resistance coefficient | 0.004 |
| Air density | 1.225 kg/m³ |
| Headwind (negative for tailwind) | 0 km/h |
- At 30 km/h on the flat with 84 kg total mass: rolling needs 28 W and air drag 116 W.
- Total power ≈ 144 W, or 1.92 W/kg.
Result:
| Power needed | 144 |
|---|---|
| Watts per kg (rider) | 1.92 |
| Power against air | 116 |
| Power against rolling | 28 |
| Power for climbing | 0 |
Frequently asked questions
How accurate is this?
It is a physics model. Real power depends on your exact CdA, tyres, road surface and wind. A power meter is the only way to measure it.
Why does speed cost so much power?
Drag force rises with speed squared and power is force times speed, so the power to beat drag rises with speed cubed. Going from 30 to 35 km/h needs about 60% more aero power.
Sources and further reading
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This calculator gives estimates for information only. Check important results independently before you rely on them. See our disclaimer.