Critical power calculator

How the critical power calculator works

Critical power (CP) is the highest power you can sustain without steadily draining your reserve. W′ (say "W prime") is the size of that reserve: a fixed amount of work you can do above CP before you have to slow down. Together they describe how long you can hold any power above CP. This calculator is one of the free LeCoach cycling calculators.

The calculator uses the classic two-parameter model:

Work done = CP × time + W′

Each all-out effort gives one point: its duration and the total work you did (average power × seconds). Plot two or three of these points and they fall on a straight line. The slope of that line is your CP; where it crosses zero time is your W′. With three efforts the calculator fits the best line through all of them.

How long can you hold it?

Once you know CP and W′, time to exhaustion at any power above CP is W′ ÷ (power − CP). For the example rider:

The curve gets very steep close to CP: 10 W less turns 12 minutes into 17. Predictions are most reliable inside the range you tested, roughly 2 to 20 minutes, so treat long predictions as rough.

At or below CP, the model says you could go on for a long time. In practice most riders can hold CP for about 30 to 60 minutes.

W′ explained simply

Think of W′ as a battery. Ride above CP and you drain it; the harder you go, the faster it empties. Ride below CP and it slowly recharges, faster the further below CP you go.

The same 24 kJ can be spent in many ways. You can use it all in one 3-minute effort at 400 W. You can spend a piece of it on a 10-second surge at 600 W, which costs about 3.3 kJ, roughly 14 % of the battery. Or you can spend it across a series of attacks with short recoveries between them. When the battery is empty, you drop back to CP or below, however hard you try.

This is why two riders with the same CP can race very differently. A big W′ gives you more matches to burn in attacks and sprints; a high CP with a small W′ suits steady efforts like time trials.

Critical power versus FTP

Both describe the boundary between efforts you can sustain and efforts you cannot. The difference is how you get them.

• FTP is about one hour of power, usually estimated from a single test such as 95 % of a 20-minute effort. The FTP calculator covers the common tests.
• CP comes from two or three maximal efforts of different lengths and a model, and it gives you W′ as well.

CP is usually a few percent above FTP. For setting training zones the difference matters little, as long as you use one method consistently. CP and W′ add something FTP cannot: a prediction of how long you can hold any power above threshold, which is useful for pacing climbs, time trials and the long intervals in VO2max training.

How to test for critical power

1. Choose two or three durations between 2 and 20 minutes, well apart, for example 3 and 12 minutes. A third effort around 6 to 8 minutes improves the fit.
2. Do each effort fully rested, on separate days or with complete recovery between them. A tired second effort gives a CP that is too low.
3. Warm up well, the same way each time.
4. Pace each effort evenly and go all-out. The model assumes you could not have gone any harder. A half-hearted effort spoils the result.
5. Use average power from the same power meter, in similar conditions.
6. Enter the numbers. The longer effort must have the lower average power.

Read more about testing in our guide to FTP tests for cyclists.

Common mistakes

• Efforts too close together in length, such as 4 and 5 minutes. Small errors then swing CP and W′ a lot.
• Efforts that were not maximal. If you held back on one of them, both numbers come out wrong.
• Using efforts from different days and conditions without full recovery, heat or a different bike.
• Treating W′ as fixed for the day. After hard efforts it does not fully refill until you have recovered.

When the numbers change

• With training, CP rises when your aerobic fitness improves; W′ responds more to high-intensity work.
• In heat, at altitude and late in a long ride, you can hold less than your fresh test values suggest, so efforts above CP end sooner.
• For comparing riders, divide by body weight. The watts per kg calculator shows where you sit.

LeCoach tracks your power profile from every ride you sync, so you see how your short and long efforts develop over the season.