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 and power breakdown
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.
How the three forces compete at different speeds and gradients
The interplay between the three forces shifts dramatically depending on where you are riding.
On the flat at speed, aerodynamic drag is almost the entire story. At 40 km/h a rider in the hoods position spends roughly 85% of total power fighting air resistance. This is why professional time trialists adopt an extreme tuck position and choose clothing with low drag coefficient — even a modest 10% reduction in CdA saves tens of watts at race speed. Rolling resistance is nearly constant regardless of speed, so it matters more proportionally at low speeds.
On a steep climb at slow speed, the aerodynamic term almost vanishes. At 10 km/h the cube-of-speed relationship means drag is a tiny fraction of the total — gravity and rolling resistance dominate. This is why climbing performance correlates so closely with power-to-weight ratio: the rider’s job is simply to lift mass against gravity at a rate determined by available power per kilogram.
At intermediate speeds on shallow gradients, all three forces are meaningful and the balance shifts continuously. A 2% gradient at 28 km/h splits the load roughly between aero drag, gravity, and rolling resistance, which is why shallow false-flat climbs feel disproportionately hard compared to flat ground.
What the defaults model
The default CdA of 0.32 m² represents a rider on the hoods of a road bike in a moderately upright position. An aggressive TT position can drop this to 0.22 to 0.25 m²; a touring upright position may be 0.40 m² or higher. The Crr of 0.005 represents good-quality road tyres on smooth tarmac. Higher values (0.007 to 0.012) apply to wider tyres, knobby rubber, or rough surfaces. Adjust the inputs to model your actual equipment if planning a specific event.
Enter your rider plus 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.