Plug three maximal efforts at different durations, ideally ~2-3 min, ~5-7 min, ~12-15 min. Out comes critical power (the asymptote of your power-duration curve), W′ (your finite anaerobic capacity above CP), AeT band, and a full set of Yousuli training zones. Better than the 0.95×20-min FTP shortcut for most athletes.
Tip: use durations roughly around ~2-3 min, ~5-7 min, and ~12-15 min for the most stable CP/W′ fit. Same day is OK with full recovery between; spread across 2-3 days is better. All efforts must be true maximal, pacing kills the model.
The model in three pieces.
P = W′ / t + CP
CP is the asymptote of the power-duration curve, the highest sustainable power. W′ is a finite work capacity above CP; spend it and you crack. Linear regression of P vs 1/t from your three efforts gives both numbers.
For most athletes CP and FTP are within 5%. CP’s advantage: built from multiple efforts at different durations, so it’s less sensitive to a bad 20-min effort or anaerobic outliers that wreck the 0.95× shortcut. Better signal-to-noise.
Big W′ (25+ kJ) = sprinter / puncheur. Small W′ (15 kJ) = diesel / steady-state engine. Same CP, totally different way to race. Drives breakaway timing, finish-line tactics, and the kind of power profile to develop.
The math also assumes constant-power steady efforts, not surges. Use a smart trainer or a flat road with no traffic. And keep your 5-min effort somewhere near 5 min, the calculator uses the closest-to-5-min effort to anchor the VO2max zone, so a 3-min "5-min" effort biases the top zone.
For the lab-grade version, our cycling protocol pairs CP testing with lactate sampling and gas analysis, you walk out with measured CP, MLSS, fat oxidation crossover, and VO2max in one session.
What CP, FTP, AeT, and W′ are, and aren’t.
Functional threshold power, the highest power you can sustain ~60 minutes. Field shortcut: 0.95×20-min mean max, not universal. Anaerobicly strong riders over-estimate; very aerobic riders under-estimate. CP is usually a cleaner number.
Power-duration curve flattens to CP. Above CP, you spend W′. Below CP, in theory you can ride indefinitely. Practically, ~30-60 min hold time depending on glycogen, heat, and prior fatigue.
Aerobic threshold, the upper bound of comfortable aerobic work. Lactate near baseline; conversation easy. Roughly 60-76% of CP for most athletes, but varies with training history.
Maximal lactate steady state, lactate levels off rather than climbing. Conceptually close to CP for most, but not identical. The line between "can hold" and "cracking soon".
Finite anaerobic work capacity in kJ. Spend it above CP, recover it below. Recovery rate depends on how far below CP you are, faster as you ride easier.
Once you know CP, see how fast you can ride that power on your bike + course.
Open → Tire pressure guideLower Crr means more of your CP arrives at the wheel.
Open → All calculatorsBack to the full catalog, 30+ endurance calculators across performance, fueling, swim, bike, run.
Browse →The lab’s cycling protocol gives you CP via a calibrated step test, plus lactate-sampled MLSS, fat-oxidation crossover, and VO2max in one session. Walk out with the four numbers that actually drive your training plan.
Spotted a bug or want a feature? Email info@yousuli.co.
