CdA calculator (aerodynamic drag estimate, beta)

What it does

CdA (Coefficient of drag times Area) is how aerodynamic your position on the bike is. Lower CdA means you go faster for the same watts. The CdA calculator estimates your CdA from either a structured field test or a FIT file upload, with environmental corrections so the number is comparable across conditions.

URL: /calculators/ then open CdA. Direct path: /static/calculators/cda_calculator.html.

It feeds step 3 of the race planning wizard and is used by the bike power plan in step 4.

Note: this calculator is currently in beta. The basic flow works; some edge cases (very short test runs, missing wind data) return approximate results.

Inputs the form asks for

Setup

Unit: metric or imperial.
Power meter location: where the power data was captured (e.g. crank, pedal, hub). Affects drivetrain loss assumption.

Environment

Altitude (m).
Temperature (degrees C).
Relative humidity (%).
Air pressure override (hPa): leave blank to auto-calc from altitude.

Rider and bike

Rider weight.
Clothes and gear (kg).

Test data

Either:

A field test (out and back on a known straight road at constant power, both directions to average out wind), with average power and average speed entered.
A FIT file upload that the calculator parses for power, speed, and GPS.

Outputs

CdA in m squared (typical road values: 0.18 to 0.40).
The saved CalculatorResult outputs cda (and cdaResult for backwards compatibility).

How to run a clean field test

Pick a flat, straight road with low traffic, and a calm day.

Warm up.
Pick a steady target power well below threshold (e.g. 200 W).
Ride 5 km out at that power, holding speed steady.
Turn around and ride 5 km back at the same target power.
Note both segments' average speed.
The out-and-back averages cancel wind so the CdA estimate reflects your actual position, not the wind on the day.

Power meter location

The Crank / pedals vs Rear hub / smart-trainer selector matters: crank/pedal power is measured before the chain, so the drivetrain efficiency % is subtracted to get wheel power; hub and smart-trainer power is already at the wheel, so selecting hub forces efficiency to 100 % (and locks the field). Picking the wrong one shifts the CdA estimate by roughly 2 %. The form shows a colour-coded hint under the selector spelling out which mode is active and what it does, so a PowerTap-hub or trainer rider doesn't accidentally leave it on crank.

Speed bands and the race-speed read

The FIT analyzer (and the race report's aero detail) breaks flat-road CdA into speed bands: ≥24 / 30 / 35 / 40 / 45 km/h. The fastest band with enough samples is the most trustworthy read — at race speed the aero term dominates the power balance, so errors in Crr, grade or wind barely move the estimate, while at low speed they land almost entirely on CdA and read high. The race report's headline CdA (est) is therefore anchored to the fastest sufficiently-sampled band (the all-speeds average is still shown for transparency).

Selecting laps in the FIT analyzer

The lap table lists every lap in the file, in the head unit's own numbering. Laps with no usable samples in their time window appear greyed-out for completeness — they can't be selected and don't affect any result. In Race / whole file mode all usable laps are included automatically. In Lap-based test mode nothing is pre-ticked: tick exactly the laps that were test efforts and leave the rest out.

Limitations

CdA changes with position. The estimate is for the position you held during the test, not for any other position you ride in.
The FIT-file flow is still beta; expect occasional reprocessing.
Aero changes from new equipment (helmet, skinsuit, wheels) shift CdA by 0.005 to 0.020 m^2, which is a meaningful chunk; retest after equipment changes if you want the bike plan to reflect them.