Triathlon Bike Split from Power Calculator
Predict your triathlon bike-leg time from a target average power, using a physics model that solves for your steady speed on a flat course.
How it works
Solve P = (Crr·m·g + ½·ρ·CdA·v²)·v for v · Split = distance ÷ (v in km/h)
Power translates to speed through the same three resistances a cyclist always fights: rolling drag, air drag and (on a flat course) nothing from gravity. Because air drag grows with the cube of speed in the power equation, the relationship cannot be rearranged neatly, so the calculator solves it numerically to find the steady speed your target watts produce. Aerodynamics dominate at triathlon speeds, which is why a lower CdA buys more free time than almost anything else.
Worked example
At 200 W, 82 kg, CdA 0.28 and Crr 0.005 on the flat, the model settles at about 35 km/h. Over a 40 km leg that is roughly 1:08 — and dropping CdA to 0.24 would nudge the same watts a little over 1 km/h faster.
Frequently asked questions
Does this account for hills or wind?
No — it models a flat, windless course at steady power. Hills, headwinds and surging all change the real split, though over a long rolling course the ups and downs partly cancel, so a flat-course estimate is a reasonable planning baseline.
Why does CdA matter more than weight here?
On the flat, gravity plays no role, so extra weight only slightly raises rolling resistance. Aerodynamic drag, by contrast, is the biggest force at 35+ km/h, which is why an aero position and tight kit save far more time than shedding a kilo.
What CdA should I use?
A rider on the hoods is roughly 0.35–0.40 m²; a good time-trial or aero-bar position is 0.24–0.30; elite tested positions dip below 0.22. If you have never measured yours, 0.28–0.30 is a fair estimate for a triathlete on aero bars.
Related calculators
This calculator is for educational purposes. Double-check important results before acting on them.