Work out your aerodynamic drag area from a real ride's power and speed, then put it to use in the main watts calculator.
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This is a field estimate from your power and speed, not a wind-tunnel or velodrome aero-testing measurement. Once you have a CdA estimate, use it directly in the Aero Watts Calculator or the watts calculator's advanced drag mode.
CdA is your drag coefficient multiplied by your frontal area, in square metres, for rider and bike together. The commonly used road positions from Jeukendrup's High-Performance Cycling are below, with the small saving from aero wheels shown alongside. Lower is faster:
| Position | Standard wheels | Aero wheels | Deep aero wheels |
|---|---|---|---|
| Hands on the tops | 0.408 m² | 0.396 m² | 0.388 m² |
| Hands on the hoods | 0.324 m² | 0.314 m² | 0.308 m² |
| Hands in the drops | 0.307 m² | 0.298 m² | 0.292 m² |
| Aerobars | 0.291 m² | 0.283 m² | 0.277 m² |
A committed time-trial position with a skinsuit can go well below these, into the low 0.2s. Your own value depends on your body, bike, clothing and helmet, which is why measuring it beats guessing.
The table shows the power spent on air alone, in still air at sea level, for a range of CdA values. Notice how fast it climbs with speed:
| CdA | 30 km/h | 40 km/h | 50 km/h |
|---|---|---|---|
| 0.2 m² | 71 W | 168 W | 328 W |
| 0.25 m² | 89 W | 210 W | 410 W |
| 0.3 m² | 106 W | 252 W | 492 W |
| 0.35 m² | 124 W | 294 W | 574 W |
| 0.4 m² | 142 W | 336 W | 656 W |
At any speed, every 0.01 m² of CdA is worth a fixed number of watts, and that number grows with the cube of speed. For a given CdA at a specific speed and altitude, the aero watts calculator gives the exact figure.
Advanced methods analyse whole rides with a virtual-elevation technique, and some services estimate CdA from FIT or GPX files. This calculator is the simple steady-state version.
The calculator runs the standard power model in reverse. It first works out the gravity and rolling-resistance power, subtracts them from the power you measured, and what remains is the aerodynamic part:
Worked as a round trip: a 72 kg rider on an 8.5 kg bike in the drops with aero wheels, at 35 km/h on the flat at 100 m altitude, has a true CdA of 0.298 m² and needs 199 W. Feed that power back in and the estimator returns 0.298 m². That is a consistency check of the model against itself, not a comparison with an outside measurement.
A field CdA is only as good as the assumptions behind it. For the same ride as above, here is how much a single wrong assumption shifts the answer:
| Mistake | Estimated CdA | Error |
|---|---|---|
| Assuming winter tires instead of race tires | 0.258 m² | -13.2% |
| Not noticing a 2 m/s headwind (assuming still air) | 0.433 m² | 45.4% |
| Power meter reading 2% high | 0.305 m² | 2.3% |
Wind causes the biggest error, followed by tire choice and power-meter accuracy. The estimate also assumes a steady, undrafted effort and a single scalar wind speed rather than a modeled yaw angle.
Once you have a number, type it into the advanced mode of the main Cycling Watts Calculator to replace the position presets, and every speed-and-power answer there will reflect your real aerodynamics. To see the tire side of the equation, use the rolling resistance calculator.
