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Critical power (CP and W′) calculator

Short answer

Plot work done against time for two all-out efforts and the slope is your critical power, the intercept your W′. A 3-minute effort at 400 W and a 12-minute effort at 280 W give a CP of 240 W and a W′ of 28.8 kJ. Above CP, W′ runs down until you have to ease off.

Last reviewed 28 October 20264 published sourcesNot medical advice

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Critical power calculator

Use two all-out efforts of different lengths between 2 and 20 minutes, for example 3 and 12 minutes, ridden on separate days or with full rest.

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Enter the average power and duration of both efforts.

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Time to exhaustion above CP (CP 234 W, W′ 25.6 kJ)
PowerTime to exhaustion
254 W (CP + 20)21:20
284 W (CP + 50)8:32
334 W (CP + 100)4:16
384 W (CP + 150)2:51
434 W (CP + 200)2:08
W′ ÷ (power − CP), using the example rider in Jones et al. (2019).

The two-parameter model

Monod and Scherrer described the work a muscle group can do as a straight line against time (Monod & Scherrer, 1965). Plotting total work against time for several all-out efforts gives a line whose slope is critical power and whose intercept is W′ (Jones & Vanhatalo, 2017).

With two efforts, CP = (work₂ − work₁) ÷ (time₂ − time₁) and W′ = (power₁ − CP) × time₁. A 3- and a 12-minute trial gave the same CP (283 vs 282 W) and W′ (18.7 vs 18.3 kJ) as three trials in Simpson and Kordi's cyclists (Simpson & Kordi, 2017).

The same relationship as a curve is hyperbolic: time to exhaustion = W′ ÷ (power − CP). It holds for efforts of roughly 2 to 15–20 minutes, and even exercise at CP is sustainable for no more than about 20–30 minutes (Jones et al., 2019). Jones et al.'s example rider has a CP of 234 W and a W′ of 25.6 kJ (Jones et al., 2019).

Related tools

For a single 20-minute test, use the FTP calculator. Runners can use the same model in the critical speed calculator, and swimmers in the critical swim speed calculator.

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Common questions

How do I test my critical power?

Ride two all-out efforts of different lengths between about 2 and 20 minutes, for example 3 and 12 minutes, with full recovery between them, and note the average power of each. Simpson and Kordi found CP and W′ from a 3- and a 12-minute trial matched a three-trial test.

What is W′?

W′ (W prime) is a fixed amount of work you can do above critical power, measured in kilojoules. Riding above CP spends it; riding below CP lets it recover. Typical values in studies are around 15–26 kJ.

How is critical power different from FTP?

Critical power comes from a model fitted to several all-out efforts and marks the boundary of steady-state exercise. FTP is usually estimated as 95% of 20-minute power. They are often close but are not the same measure.

Sources

Figures on this page were checked against these publications on 28 October 2026.

  1. Monod H, Scherrer J. The work capacity of a synergic muscular group. Ergonomics. 1965;8(3):329–338. doi.org/10.1080/00140136508930810
  2. Jones AM, Vanhatalo A. The 'Critical Power' concept: applications to sports performance with a focus on intermittent high-intensity exercise. Sports Medicine. 2017;47(Suppl 1):65–78. doi.org/10.1007/s40279-017-0688-0
  3. Jones AM, Burnley M, Black MI, Poole DC, Vanhatalo A. The maximal metabolic steady state: redefining the 'gold standard'. Physiological Reports. 2019;7(10):e14098. doi.org/10.14814/phy2.14098
  4. Simpson LP, Kordi M. Comparison of critical power and W′ derived from 2 or 3 maximal tests. International Journal of Sports Physiology and Performance. 2017;12(6):825–830. doi.org/10.1123/ijspp.2016-0371

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