Calculator
Critical power calculator
Enter two or three maximal efforts — how long each lasted and its average power — to find your critical power (CP), your W′, and the power you can hold for any duration in between.
260 W
critical power
- W′ (work above CP)
- 18.0 kJ
- W′ lasts at CP + 50 W
- 6:00
| Duration | Power | % of CP |
|---|---|---|
| 1:00* | 560 W | 215% |
| 3:00 | 360 W | 138% |
| 5:00 | 320 W | 123% |
| 10:00 | 290 W | 112% |
| 20:00 | 275 W | 106% |
| 30:00* | 270 W | 104% |
| 1:00:00* | 265 W | 102% |
* Outside the 2–20 minute range the model fits best: it overestimates very short and very long efforts.
What is critical power?
Critical power is the highest power you can sustain from your aerobic system without steadily tiring — the boundary between efforts you can hold for a long time and efforts that run you down at a predictable rate. Below CP, you fatigue slowly. Above it, you're spending a limited reserve, and the further above, the faster it goes.
What is W′?
W′ ("W prime") is that reserve: a fixed amount of work, in kilojoules, you can do above critical power before you have to slow down. A rider with a CP of 260 W and a W′ of 18 kJ can hold 310 W for 18,000 ÷ 50 = 360 seconds, six minutes. W′ refills when you ride below CP, which is why it matters for attacks, climbs, and intervals.
How it's calculated
The two-parameter model says the work you can do in an all-out effort grows in a straight line with its duration: work = CP × time + W′. Plot each effort's work (average power × seconds) against its length, and the line's slope is CP and its intercept is W′. Two efforts fix the line exactly; with three, the calculator fits the best line through them and reports how well they agree (R²). Rearranged, the power you can hold for any duration is CP + W′ ÷ time — the curve in the chart.
How to test for critical power
- Ride two or three maximal, evenly paced efforts of different lengths between about 2 and 20 minutes — for example 3 and 12 minutes, or 3, 7, and 12.
- Do them fresh: on separate days, or in one session with at least 30 minutes of easy riding between them.
- Use average power for the whole effort, and the same power meter every time.
- Recent race files work too, as long as each was a genuine all-out effort for its length.
Critical power vs FTP
They describe the same boundary from different directions. FTP is usually estimated from a single test and scaled, while CP comes from the shape of your power across durations. For most riders CP sits close to FTP, often slightly above it. Either works for setting training zones; use the same one consistently. To estimate FTP from a standard test, use the FTP calculator.