What is CdA in cycling?

By Olivier Demichel, former CNRS researcher, triathlete and founder of AeroX.

CdA is the most important metric for quantifying a cyclist's aerodynamic efficiency. Understanding CdA explains why some riders are faster than others at the same power output.

Physical definition of CdA

CdA stands for Coefficient of Drag × Area. It is the product of two quantities:

Cd: the drag coefficient, a dimensionless number that depends on body shape and the airflow around it. A more streamlined profile has a lower Cd.
A: the frontal area, measured in square meters (m²), representing the cross-sectional area the cyclist presents to the airflow.

Cd the shape × A the area (m²) = CdA your aerodynamic drag (m²)

AeroX measures A in real time with a webcam: it is the part of your drag you control directly through your position. Cd is measured in a wind tunnel or on a track: the CdA shown by AeroX combines your measured frontal area with an estimated Cd. You see live the effect of each adjustment (bar height, aero bar width, torso angle, head position) on your frontal area.

Factors that influence CdA

1
Body position ≈ 80% of losses
Head tucked, torso flat, elbows close: your body creates most of the drag, so that is where the biggest gains are.
2
Equipment
- Aero helmet −5 to −10 W
- Fitted clothing −3 to −8 W
- Overshoes −1 to −3 W
3
Bike
Aero frame, deep wheels, aero bars: a real impact, but secondary to body position.
4
Accessories
Bottle, saddle bag, race number: every item exposed to the wind adds drag.

Equipment gains: orders of magnitude at around 40 km/h.

Typical CdA values by position

Front Position CdA (m²) A (m²)

1. Hands on the hoods, arms straight, head up
CdA (m²) 0.40 – 0.50
A (m²) 0.52
2. Hands in the drops, arms slightly bent
CdA (m²) 0.30 – 0.35
A (m²) 0.44
3. Hands on the hoods, forearms horizontal, head tucked, torso almost flat
CdA (m²) 0.27 – 0.32
A (m²) 0.42
4. On the aero bars, head up
CdA (m²) 0.20 – 0.25
A (m²) 0.36
5. Optimised TT position: head tucked, elbows close
CdA (m²) 0.18 – 0.22
A (m²) 0.33
6. Everything optimised: clothing, bike, helmet
CdA (m²) 0.17 – 0.20
A (m²) ≈ 0.33

A: frontal areas measured with AeroX on the same rider (1.78 m, 72 kg), O. Demichel, “S’entraîner à l’aérodynamisme” (2026), table 2.1. CdA: typical ranges, all body sizes. Row 6: same position as row 5; the gain comes from Cd (clothing, bike, helmet), not from the area.

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Measured performance impact

On the flat with no wind, air already accounts for about 78% of total resistance at 25 km/h, and over 90% around 45 km/h. Reducing your CdA is therefore the most powerful performance lever, far more than adding watts.

The power needed to overcome the air is P_air = ½ × ρ × CdA × v³. It is proportional to your CdA: 10% less CdA means 10% fewer watts against the air, at the same speed. And it grows with the cube of speed: riding 10% faster takes about 33% more watts against the air.

Moving from an upright position to an optimized compact position reduces frontal area by about 20%; with aerobars, by about 37% (measurements from the book S'entraîner à l'aérodynamisme, download here). At the same power, that means 2 to 3 km/h faster, and 4 to 5 km/h on aerobars. Body position offers the biggest levers, not equipment.

What each position costs, in watts, compared with the most aerodynamic one:

Watts lost compared with position 6 at 30 km/h at 40 km/h

1. Hands on the hoods, arms straight, head up
+94 W
+223 W
2. Hands in the drops, arms slightly bent
+50 W
+118 W
3. Hands on the hoods, forearms horizontal, head tucked, torso almost flat
+39 W
+92 W
4. On the aero bars, head up
+14 W
+34 W
5. Optimised TT position: head tucked, elbows close
+5 W
+13 W
6. Everything optimised: clothing, bike, helmet
reference
reference

Extra power needed to ride at the same speed as in position 6 (everything optimised). Air resistance only, P = ½ × ρ × CdA × v³, using the middle of each CdA range (air at 1.225 kg/m³, no wind).

And you, how many watts does your position cost you?
To find out, use our calculator →
or optimise your position with AeroX →

How to reduce your frontal area

1
Bend your forearms
Shoulders and torso drop: less area facing the air.
2
Tuck your head into your shoulders
Your head stays in line with your shoulders instead of sticking out above them.
3
Use aero bars
Forearms rest in front of the torso; arms no longer spread out.
4
Bring your elbows closer
Arms stay in front of the torso instead of widening your silhouette.
5
Lower your torso to horizontal
Your back lowers and flattens: the torso shows less height to the air.
6
Reduce the space between your hands and your helmet
Less air rushes towards your torso.

Adapt these tips to your body, your flexibility and your riding. They change as you train: it is up to you to progress on each of these levers to optimise your position. As a rule, favour a position you can hold over one that is only optimal for aero.

Which of these levers works for you? It can be measured: the AeroX Diagnostic compares your positions live. Discover the Diagnostic →

How to measure your CdA

Wind tunnel, track testing, the Chung method on the road, on-bike sensors, calculation from a ride or camera-based frontal area measurement: each method has its strengths and constraints. Compare CdA measurement methods →

Calculate my CdA with the calculator →

Measure your frontal area, position by position

The AeroX Diagnostic: up to 10 positions compared live, a hold test on the best one, then your recommended position with your gains in time and km/h. One-off payment.

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Last updated: October 2026. Data based on 300+ AeroX sessions and published cycling aerodynamics research.