CdA (Aero Drag) Calculator
CdA Calculator (Virtual Elevation Method) 🚴♂️💨
What is CdA?
CdA is your bike-and-body coefficient of drag multiplied by frontal area - the single number that determines how much power air resistance costs you at any given speed. Lower is more aerodynamic. This tool estimates it from a real ride using the "virtual elevation" method: it models what your ride 's elevation profile should look like for a given CdA, and searches for the CdA that makes that modelled profile match what your bike computer actually recorded.
How to Use This Calculator:
- Ride a loop or out-and-back course with a power meter recording the whole way.
- Export the ride as a .fit file (not GPX - GPX has no power data).
- Enter your total mass, rolling resistance, and air density below.
- Upload the file.
This is an estimate, not a lab measurement
The result is only as good as the ride you feed it - noisy GPS elevation, wind, coasting, and stop-start riding all show up as error. Use the chart below to sanity-check the fit: if the "virtual" line swings wildly more than the actual elevation line, treat the CdA number with caution.
For a good test ride:
- A focused, steady-effort ride of roughly 20-40 minutes - not a general training ride with lots of surges and easing off.
- A flat loop or out-and-back that returns close to its start elevation, ridden continuously without stopping (few or no traffic lights/junctions).
- Calm, still conditions - wind isn't modelled here.
- A power meter with good coverage the whole way, and consistent pedaling rather than lots of coasting.
You + your bike + everything you're carrying (bottles, shoes, helmet) - get this from a scale, it's the single biggest input to get right.
How much energy your tyres lose rolling - depends on tyre model, pressure, and road surface. Roughly ~0.003 for fast tyres on smooth tarmac, ~0.005 for typical road tyres (a reasonable default if unsure), ~0.006+ for gravel/touring tyres or rougher roads.
The air temperature and barometric pressure during your ride (a weather report for that day/location works) - colder, higher-pressure air is denser and costs you more drag at the same speed.
Using air density: 1.2041 kg/m³