CdA calculator and bike split calculator
Two flat-road calculators: the first turns power into speed and time, the second works back to your CdA from a steady ride. No login needed, and it recalculates as soon as you change a number. To work out a real course (climbs, wind, weather), use the full race plan below.
Speed and time from power
Flat road, no wind, the same power the whole way.
CdA from a ride
Enter the average power and average speed from your head unit for a stretch that was flat, low-wind, not drafting and ridden in one position.
One ride is only a rough estimate: a slight grade or a little wind can throw the number off by 10% or more. For a stable number, combine many rides.
In a real race, the flat-road formula is not enough
Both calculators above assume the road is flat, there is no wind, and your power stays the same from start to finish. A real course has climbs and descents, the wind direction changes from section to section, and you should not ride the same power the whole way either. A full race plan takes all of this into account: pick a course (211 triathlon courses worldwide, or upload your own GPX), bring in that day's weather, set how many watts to ride on every section, and predict your whole bike split.
Once you link Garmin, Wahoo, COROS or Hammerhead, every outdoor ride with power gets a CdA calculated automatically, and combining rides makes it more and more accurate, with nothing to enter yourself each time.
How it works
The power you put out, minus drivetrain losses, gets used up pushing air and overcoming tire friction (on a flat road at a steady speed there is no climbing or acceleration):
P × η = ( ½ · ρ · CdA · v² + Crr · m · g ) · v
- P is power, η is drivetrain efficiency (0.977).
- ρ is air density, worked out from temperature and elevation. The hotter it is and the higher you are, the thinner the air, and the faster you go at the same power.
- CdA is drag area, v is speed, Crr is the rolling resistance coefficient, m is the combined mass of rider and bike, and g is gravitational acceleration.
The first calculator solves for speed from a known CdA, and the second works back to CdA from a known speed. The full race plan uses the same equation, just calculated once for every section of road, with grade, wind and turns added.
FAQ
I don't know my CdA. What should I enter?
Start with the riding position closest to yours: a Tri/TT bike is about 0.25, a road bike with clip-on aero bars about 0.285, a road bike in the drops about 0.32, and on the hoods about 0.40 (based on a rider of 175 cm and 72 kg; the bigger you are, the higher the number). A lookup table is off by about ±15%. For your own number, see How to calculate CdA without a wind tunnel.
How accurate is the predicted time?
To test the physics model itself, we replayed 64 real ride power files with each day's actual power and weather: the median bike split error was 2.1%, with 86% within ±5% (how we tested it). The flat-road calculator adds the assumptions of the same power the whole way and no wind, so a real race will differ.
My CdA comes out very different from other people's. Is that normal?
The usual reasons are that the road actually had a slight grade, there was wind or drafting, or your head unit's average speed includes time spent stopped. A number from a single ride is only a reference. Ride the same stretch out and back a few times and average them, and it gets much steadier.
Does it cost anything?
Both calculators on this page are free and need no login. Trying out a full race plan is free too; only exporting files to your head unit is a paid feature. See Pricing.