This tool estimates the power in watts you need to ride a bike at a given speed, using the standard cycling physics model. It adds the power to overcome three forces — rolling resistance, aerodynamic drag and gravity on a climb — and divides by drivetrain efficiency to give the watts at the pedals. It is useful for pacing climbs, setting training targets and understanding where your effort goes.
How it works
The estimator computes three components at the wheel, where v is ground speed in m/s and slope is the road angle:
- Rolling resistance: Crr × mass × g × cos(slope) × v
- Aerodynamic drag: 0.5 × air density × CdA × v³
- Gravity (climbing): mass × g × sin(slope) × v
These are summed and divided by a 0.97 drivetrain efficiency (≈3% loss) to get rider power. It uses typical road-bike defaults: CdA 0.32 m², Crr 0.005, air density 1.225 kg/m³ and g 9.80665. Because drag scales with v³, it dominates at speed on flat ground, while gravity dominates on steep climbs.
Example
An 80 kg rider-plus-bike at 30 km/h on the flat:
| Component | Power |
|---|---|
| Rolling resistance | ~33 W |
| Aerodynamic drag | ~113 W |
| Gravity | 0 W (flat) |
| Total at pedals | ~151 W (1.88 W/kg) |
Add a 5% gradient at the same speed and the gravity term jumps, raising the total to roughly 487 W — showing why climbing is so much harder.
Enter your rider + bike mass, target speed, and the road gradient. The result includes watts, watts per kilogram, and a breakdown of where the power goes — all computed in your browser.