Put a price in watts on air resistance: enter a CdA, a speed, the wind and your altitude, and see the power drag takes.
Last updated:
Riding-position CdA values are broad category defaults. Know your own measured CdA instead? Use the CdA Calculator to estimate it from a ride, then enter it here directly.
Aero watts are the slice of your power spent purely on pushing through the air. Here is what that costs in still air at sea level, using the standard road-position CdA values and standard wheels:
| Position (CdA) | 25 km/h | 30 km/h | 35 km/h | 40 km/h | 45 km/h |
|---|---|---|---|---|---|
| Tops (0.408) | 84 W | 145 W | 230 W | 343 W | 488 W |
| Hoods (0.324) | 66 W | 115 W | 182 W | 272 W | 388 W |
| Drops (0.307) | 63 W | 109 W | 173 W | 258 W | 367 W |
| Aerobars (0.291) | 60 W | 103 W | 164 W | 245 W | 349 W |
The gap between the tops and the aerobars is small at 25 km/h and large at 45. If you would rather derive your own CdA than use a preset, the CdA calculator works it out from a ride.
The calculator uses exactly the aerodynamic term from the site's power model, not a separate equation:
Speed and wind are in metres per second, and wind is positive for a headwind. Because speed appears twice, once in the force and once as a multiplier, aero watts rise with the cube of speed.
At 40 km/h with your hands on the hoods, aero watts across a range of headwinds (positive) and tailwinds (negative):
| Wind along your path | Aero watts |
|---|---|
| 5 m/s tailwind | 82 W |
| 2 m/s tailwind | 183 W |
| Still air | 272 W |
| 2 m/s headwind | 379 W |
| 5 m/s headwind | 572 W |
The cost of a headwind is larger than the saving from an equal tailwind, because drag depends on the square of the airspeed. A ride into the wind costs more than the ride home gives back.
Air density falls with elevation, and drag falls with it. At 40 km/h on the hoods in still air:
| Elevation | Air density | Aero watts |
|---|---|---|
| 0 m | 1.225 kg/m³ | 272 W |
| 500 m | 1.155 kg/m³ | 257 W |
| 1000 m | 1.089 kg/m³ | 242 W |
| 1600 m | 1.015 kg/m³ | 226 W |
| 2500 m | 0.913 kg/m³ | 203 W |
Less drag at altitude is not free speed, because the same thin air also reduces how much power most riders can make.
For a 75 kg rider on an 8 kg bike on the flat, hands on the hoods, the share of wheel power spent on air is:
| Speed | Share of wheel power that is aero |
|---|---|
| 25 km/h | 79% |
| 30 km/h | 84% |
| 35 km/h | 88% |
| 40 km/h | 90% |
| 45 km/h | 92% |
The rest is tire rolling resistance and, on hills, gravity. To see the complete watts breakdown for your own ride, open the homepage calculator; for the tire share alone, use the rolling resistance calculator.
This page reuses the classical-mechanics drag term documented for the cycling watts calculator and the CdA calculator, the same approach used by Analytic Cycling and similar bike-power tools. No new physics is introduced. It assumes a single scalar wind speed, no yaw-angle modeling and no drafting.
