Cycling calculator
Critical power calculator
Fit the two-parameter critical power model from two maximal efforts — get CP, W′ and your whole power-duration curve. Free, instant, no sign-up.
Shorter effort
Longer effort
Your power-duration curve
| Duration | Power |
|---|---|
| 1 min | 566.7 W |
| 5 min | 300 W |
| 10 min | 266.7 W |
| 20 min | 250 W |
| 30 min | 244.4 W |
| 60 min | 238.9 W |
CP moves with threshold work and W′ with repeated hard efforts — the coach builds the mix your curve needs.
What critical power is
Critical power is the highest intensity you can hold in a steady state — below it, effort is sustainable for a long time; above it, a clock starts running. W′ (pronounced 'W prime') is the size of that clock: a fixed amount of work, measured in kilojoules, available above CP. Spend it and you stop, regardless of how far above CP you were. Together the two describe nearly the whole of your durability on a bike.
How the fit works
The hyperbolic form t = W′ ÷ (P − CP) is awkward to fit, but its linear twin is not: total work in a maximal effort is W = CP·t + W′, a straight line through your two trials. CP is the slope of that line and W′ is the intercept. Two points define it exactly, which is also its weakness — a single mispaced trial moves both numbers, with no third point to reveal the error.
CP is not FTP
They are close, and often used interchangeably, but they are different quantities. FTP is conventionally 95% of a 20-minute test, an approximation built for convenience. CP is fitted from your own efforts and sits near the power you could hold for 30 to 50 minutes — usually a little above FTP, not the same number. Your 60-minute power is shown alongside for that reason: it is asymptotically approaching CP from above, never equal to it.
Choosing the two efforts
Three and twelve minutes is the classic pair, and any two efforts from about three to twenty minutes work provided both are genuinely maximal and separated by enough time. Trials too close together put the two points almost on top of each other, and the slope through them becomes mostly noise — which is why this calculator refuses a pair less than two minutes apart. Test rested, on the same terrain or trainer, and pace each one to finish empty rather than to survive it.