Critical Power & W′ Calculator
Critical power and W′ (anaerobic work capacity) from two or three maximal efforts of different durations — the hyperbolic power-duration model — with predicted power for any duration and time to exhaustion.
How it works
Work = CP × t + W′ (fit to 2–3 all-out efforts of 2–20 min); power at duration t = CP + W′ ÷ t; time to exhaustion above CP = W′ ÷ (P − CP)
The critical power model describes the sharp boundary between efforts you can sustain and those that end in exhaustion: above a certain power — the asymptote of your power-duration curve — you draw down a finite reserve, W′, at a rate proportional to how far above CP you are, and when it is gone you stop. Two or three maximal efforts between roughly two and twenty minutes are enough to fit the two parameters, because total work rises linearly with time at CP with an intercept of W′. CP sits a few percent above FTP and marks the heavy-to-severe intensity boundary; W′ (10–25 kJ) is the sprinter’s currency. Race software now tracks “W′ balance” in real time so riders know how many matches they have left.
Worked example
3 minutes at 380 W and 12 minutes at 305 W: CP = 280 W, W′ = 18.0 kJ. Predicted 20-minute power 295 W; riding at 308 W (+10%) would last about 10.7 minutes.
Frequently asked questions
Which durations should I test?
Two efforts at least 5 minutes apart in duration, both between 2 and 20 minutes — e.g. 3 and 12 minutes — on separate days or well rested. Efforts under 2 minutes overestimate W′.
Is CP the same as FTP?
Close but not identical: CP is a physiological asymptote usually 2–5% above FTP (a 1-hour practical power). Many coaches set FTP ≈ 0.96 × CP.
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General fitness estimate, not medical advice. Calorie and performance figures vary widely between individuals — use them for planning and comparison, and check with a doctor before starting intense training.