Estimate your Critical Power (CP) and W′ (anaerobic work capacity) from two maximal effort tests, using the established Monod & Scherrer model.
Both efforts should be genuinely maximal, all-out efforts for their full duration — a paced or sub-maximal effort will underestimate both CP and W'.
Want FTP instead? FTP Calculator.
Critical Power (CP) is the highest power output a rider can theoretically sustain without progressively drawing down a finite reserve of anaerobic work capacity, called W′. Above CP, you're spending down that reserve; below CP, it slowly recovers. CP and W′ together describe a rider's power-duration curve far more completely than a single FTP number can.
This calculator uses the classic two-parameter model (Monod & Scherrer, 1965), fit from two maximal efforts of different durations using the equivalent linear work-time form:
CP is the slope of the line through your two efforts' work and duration; W′ is the intercept. See "Sources & methodology" below for the model's origin, assumptions and limitations.
W′ (pronounced "W-prime") is your anaerobic work capacity in joules — a fixed amount of work available above Critical Power before you're forced to slow down. It explains why you can surge past your sustainable pace briefly, but not indefinitely, and why repeated hard surges eventually catch up with you even if your average power looks sustainable.
FTP is typically estimated from a single field test using a fixed multiplier (see the FTP Calculator). CP is fit from two or more maximal efforts using a documented mathematical model, and comes with W′ as a bonus — a number FTP alone doesn't provide. The two are related but distinct: don't expect them to match exactly, and don't treat one as a replacement for the other.
Ride two separate maximal, all-out efforts of clearly different durations — for example a 3-minute effort and a 12-minute effort, each performed with a full warm-up and enough recovery between them (a different day is common). Record the average power for each effort and enter both above.
Take a rider who averages 320 W for 3 minutes and 260 W for 12 minutes:
That's 3.21 CP W/kg for a 165 lb rider. Enter your own two efforts above to calculate your own CP and W'.
This model is best validated for maximal efforts of roughly 2-15 minutes — much shorter efforts are dominated by anaerobic power the model doesn't capture, and much longer efforts approach aerobic steady state, where CP's implied "indefinitely sustainable" power breaks down in practice. Results are only as good as how genuinely maximal your two test efforts were.
This calculator uses the classic two-parameter Critical Power model:
CP and W′ are fit by ordinary least squares on the linear work-time form (total work = W′ + CP × duration); for exactly two efforts this reduces to solving the line through both points exactly. The model is best validated for maximal efforts of roughly 2–15 minutes — see "Accuracy and limitations" above for where it breaks down outside that range.
