CdA calculator (aerodynamic drag coefficient)

What it does

The CdA calculator returns your drag coefficient × frontal area (CdA, in m²) — the single most important variable for flat-course bike speed at race pace. Two paths:

• Field-test mode — you enter the measured numbers from a controlled flat-course ride.
• FIT-file mode — you upload a FIT file from a known course and the calculator extracts power, speed, and elevation per second to back-solve CdA over the ride.

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

Inputs the form asks for

Environment

• Unit (metric / imperial).
• Altitude (m) — affects air density.
• Temperature (°C) — affects air density.
• Relative Humidity (%) — small density correction.
• Air Pressure Override (optional, hPa) — bypass the altitude/temp-based density estimate if you have a barometer reading.

Rider and bike

• Power meter location (crank-based, hub-based, etc.) — used for drivetrain-loss correction.
• Rider Weight (kg).
• Clothes & Gear (kg) — added to rider weight.
• Bike Weight (kg).

Test data (field-test mode)

• Distance (m or km depending on unit toggle).
• Total Climb (m) — net elevation change over the test segment.
• Wind (+head / −tail) — headwind positive, tailwind negative. Magnitude in m/s or km/h.
• Power (Watts) — average over the test ride.
• Crr (Rolling Resistance coefficient) — typical road defaults around 0.004; faster tubeless / TT setups around 0.0028. See Crr calculator for per-tire numbers.
• Drivetrain Efficiency (%) — default ~97-98%.
• Time (HH:MM:SS) — time over the test segment. Average speed is computed from distance / time, not entered directly.

Activity picker (when opened with ?athlete_pid=)

If the calculator is opened from a coach/athlete page, a dropdown of the athlete's recent bike activities appears at the top. Pick one and click Use this ride — the FIT file loads automatically AND the temperature + altitude form fields are pre-filled from that activity's recorded values:

• Temperature = the average of the recorded ambient temperature samples (after filtering to plausible -20 to +60°C).
• Altitude = the median of recorded altitudes (median rather than mean to suppress noisy-GPS spikes).

This means the FIT-mode density calculation always uses the conditions you actually rode in, not a textbook default. The status line confirms what was pre-filled, e.g. "Pre-filled from FIT: 26°C, 4m."

If the FIT doesn't have temperature data (some indoor / older files), the temp field is left at whatever default you'd set — nothing breaks.

FIT-flow tuning (when you upload a FIT)

When you upload a FIT file the calculator exposes a set of tuning knobs that control which seconds of the ride are used for the fit:

• Analysis mode — single-pass average vs sliding-window vs per-lap split.
• Smoothing window (s) — averages power and speed over N seconds before solving. Reduces sensor noise.
• Min/Max Speed — bound the fit to a speed band so coasting and stops don't pollute the solution.
• Min/Max Power — bound the fit to a power band so out-of-the-saddle accelerations don't dominate.
• Min/Max Grade — exclude segments where elevation change per metre exceeds a threshold (steep climbs and descents invalidate the flat-course CdA model).
• Min/Max CdA — sanity bound on the solver output; values outside this range are flagged as suspect.

Outputs

• CdA (m²) — your fitted drag coefficient × frontal area.
• For FIT-flow uploads: per-segment CdA values, a histogram of solutions across the ride, and flags for segments that fell outside the trusted range.
• When saved from the race planning wizard, the CalculatorResult row's cda output drives the bike split projection in step 2.

How to run a field test

The CdA Field Test assessment documents the canonical protocol:

• Choose a flat, wind-sheltered, out-and-back 3-5 km course.
• Pick race-day power and hold position constant.
• Ride 3-5 laps at steady power.
• Record distance, total climb (should be ~0 net), time, and conditions.

Out-and-backs eliminate wind bias when averaged across the direction.

See also

• Post-race recap CdA section — the same physics auto-runs on any bike file matched to a race; no need to open the calculator separately.
• CdA Field Test assessment,
• CdA glossary,
• Crr calculator.

Common gotchas

• Course not flat enough. Total climb over the segment should be at most a few metres. If the road has even subtle rises, the climbing power masks the aero solution.
• Position not constant. CdA changes with position. If you move on the bars mid-test (sit up, scratch your nose, drop to aerobars), you get an averaged-position CdA that doesn't represent any single posture.
• Wind not captured. Even a 2 m/s wind shifts CdA estimates noticeably. Use the wind input or test on a calm day.
• FIT-flow filters too tight. If the min/max bounds exclude most of the ride, the fit runs on too few points. Loosen the filters before assuming the data is bad.