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Calculator · Bike

CdA estimate.

Power equation · FIT file analyzer · Ghibli aero sensor

Two ways to find your CdA. Single-point, one steady-power ride, one number. FIT analyzer, upload your FIT file for lap-by-lap CdA with histograms, scatter, and time-series; pulls Ghibli aero-sensor data automatically when present. Field-test protocol included, the answer is only as good as the test.

Read the protocol ↓ Skip to calculator

Field-test protocol

CdA is the most demanding test in cycling. Get the protocol wrong and the number is fiction.

01

Conditions

  • Calm wind, under 5 km/h ideal; out-and-back loops mandatory above that
  • Flat road, gradient under 1%, smooth surface, no traffic
  • Stable air, mid-day, no thermal currents
  • Dry conditions, rain corrupts FIT speed/elevation data
02

Equipment

  • Calibrated power meter, zero-offset before ride
  • Known Crr, from a prior Crr test or default 0.0036 for road tires
  • Race kit + race position, or your CdA is for someone else
  • Ghibli aero sensor optional, FIT analyzer reads it automatically
03

Execution

  • Steady power, ±5%, no surges, no coasting
  • Identical position, never sit up, never change grip; this is the #1 protocol failure
  • 20+ min duration, longer = better signal-to-noise
  • Out-and-back if any wind, cancels asymmetric wind effects
  • Pre-weigh full kit, rider + clothes + bike + bottles, calibrated scale
Environment & rider
%
m
°C
% (optional)
hPa (optional)
Skip to use altitude-derived pressure.
Default = decent road tire. Use measured value if you have one.
+ head / − tail
kg
kg
kg
Single-point ride summary
W
km
m
: :

Result

CdA—
Speed—
Slope grade—
Air density—
Pressure—
Assumes dry air unless humidity is entered. Pressure override expects station pressure, not sea-level reduced.

FIT file analyzer

Upload a .fit file from a steady ride. Reads Ghibli aero-sensor channel automatically when present. Uses the environment + rider inputs above.

race / test rides
Wind is assumed roughly constant over the test.
noise reduction
Sample filters (advanced)
Laps in file
Tick laps to include (lap mode). In race mode the whole file is used.
#Use?Dist (km)Time (min)Avg speedAvg powerCalc CdAGhibli CdA

Where your CdA sits

Population reference for adult cyclists in race position.

> 0.36

Untrained / upright

Sitting up on hoods, hands on tops, casual ride. Most weekend riders. Easy 30-watt gain available with position work.

0.30 – 0.36

Average road

In the drops, race position, mainstream road geometry. The default for road racing.

0.24 – 0.30

Good TT / aero road

Clip-ons or a TT bike, clean position, race kit. Where most well-set-up triathletes sit. Aero helmet adds 0.005–0.015 reduction.

0.20 – 0.24

Good TT

Dialled TT position, tested kit, narrow shoulders, low arms. Pro-domestique territory at higher end.

< 0.20

Elite TT

Wind-tunnel-optimised pro position. Rare. Watt impact: each 0.01 m² off CdA = ≈4 W at 40 km/h.

Coach note

CdA is the cheapest watts you’ll ever buy, if you can hold the position.

A 0.05 m² CdA reduction is ~20 watts at 40 km/h. That beats most training blocks. Three things that move the needle:

  • Position, arm pad height, shoulder width, head position, helmet choice. Test combinations on the same day to compare.
  • Kit, tight tri-suit, race shoes, calf compression. ~10–15 W depending on baseline.
  • Bike, aero frame, deep wheels, integrated cockpit. Diminishing returns past mid-tier.

The catch: a low-CdA position you can’t hold for race duration is theatre. Real CdA = the average across all the times you sat up to drink, glanced down, looked over your shoulder. The protocol above forces an honest read.

For the gold standard, field aero testing on a closed loop with sensor data, the lab handles it as a half-day session with multiple position changes tested side-by-side. Most athletes find 15–30 watts on their first visit.

Field aero testing Coaching

Related calculators

Crr (rolling resistance)

Reverse-pair: known CdA → solve for Crr. Run both for a complete bike-physics picture.

Open → Gear × Speed × Power

Plug measured CdA + Crr into the power equation to project race speed.

Open → All calculators

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Want CdA measured side-by-side with position changes?

Field aero testing at the lab tests multiple positions, kits, and equipment in a single half-day session, same protocol used by World Tour teams. Most athletes leave with 15–30 watts found and a documented baseline to test against next year.

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Spotted a bug or want a feature? Email info@yousuli.co.