Find your Critical Power (CP) and W′ from two all-out efforts, using the classic two-parameter Monod & Scherrer model.
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Both efforts should be 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 boundary between riding intensities that your body can sustain in a relative steady state and those where fatigue keeps building. W′ (W-prime) is the finite amount of work you can do above that boundary before you have to back off. Together they describe your power-duration curve in two numbers: CP sets the floor, W′ sets the size of the tank for surges, attacks and sprints. You cannot hold it forever, and it does not replace FTP.
The calculator fits the work-time form of the two-parameter model (Monod & Scherrer, 1965). Total work done in an effort is W′ plus CP times its duration:
Take a rider who averages 320 W for 3 minutes and 260 W for 12 minutes. The calculator returns a CP of 240 W and a W′ of 14.4 kJ, which is 3.33 W/kg of CP at 72 kg. Enter your own two results above to see yours.
Two points always define a line, so a two-effort fit can never tell you whether your efforts were consistent. Three or more efforts give a best-fit line you can check. This rider's 3, 6 and 12 minute results (330, 290 and 265 W) give:
| Fit | CP | W′ |
|---|---|---|
| Two efforts (3 and 12 min) | 243 W | 15.6 kJ |
| Three efforts (best-fit line) | 243 W | 16.2 kJ |
The difference between the rows shows how much a single effort can move the estimate, and why a wider, well-paced set of tests is worth the trouble. The tool above takes two efforts.
Use maximal efforts of clearly different durations inside the 2 to 15 minute window, each after a full warm-up and with enough recovery that the second is not blunted by the first. Common combinations:
| Efforts | Why |
|---|---|
| 3 min + 12 min | The popular default. Far enough apart for a stable fit, and both efforts are easy to pace. |
| 5 min + 20 min | Reuses your FTP-style 20-minute test, but the 5-minute effort is demanding on a tired body. |
| 3 + 6 + 12 min | Three efforts let you see how well the points line up. The calculator on this page takes two; the three-effort fit is shown below. |
The result is only as good as how maximal and well paced each effort was. Submaximal inputs produce a CP that is too low.
FTP is usually a single-test estimate with a fixed multiplier, which is what the FTP calculator uses. CP comes from a fitted model and adds W′, which FTP cannot give you. The two are related but not interchangeable, so do not expect an exact match. Either can anchor your training: take the training zones from your FTP or adapt them around your CP.
CP describes what you can sustain, while a route demands a specific power. To find the power needed to hold a speed on a given stretch or climb, use the watts calculator and compare the answer with your CP. If the route needs more than CP, you are drawing on W′ and the effort has a limited lifespan; if it needs less, you can hold it.
The model is best validated for maximal efforts of about 2 to 15 minutes. Shorter efforts lean on anaerobic power it does not capture, and much longer ones drift toward an aerobic steady state where the idea of power sustainable "indefinitely" breaks down in practice. Fitting is by ordinary least squares on the linear work-time form; for two efforts it reduces to the line through both points. Sources:
