Free tool
Critical Power and W′ Calculator.
Estimate your Critical Power in watts and W′ in kilojoules from the duration and average power of two maximal cycling efforts.
Run your test results through the calculator.
Put your result to work.
Check the Critical Power testing guide before choosing your inputs. Then read CP versus FTP and how to interpret your training zones. A calculated range is a starting estimate; use the work periods, heart rate and perceived effort to understand the session.
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A worked example: 350 W for 3 minutes, 275 W for 12.
Enter 180 seconds and 350 watts for Test 1, then 720 seconds and 275 watts for Test 2. These illustrative efforts give CP = 250 W and W′ = 18.0 kJ. At a body weight of 70 kg, CP is 3.57 W/kg.
How the calculation works
The model is P = CP + W′/t, with time in seconds and W′ in joules. With two efforts, CP = (P₂t₂ − P₁t₁)/(t₂ − t₁), and W′ = (P₁ − CP)t₁. The calculator divides joules by 1,000 to display kilojoules.
With three efforts, it fits a straight line to power against inverse duration. CP is the intercept and W′ is the slope. Different fitting methods can produce different estimates from the same three efforts.
What a good fit can—and cannot—tell you
Two points always determine a line, so a perfect fit cannot establish accuracy. With three efforts, R² describes how closely those points follow this model. It cannot verify that the efforts were maximal or well paced. Similar durations, fatigue, inconsistent equipment and non-maximal inputs can distort an estimate.
This model is for suitable maximal efforts lasting several minutes. It should not be extrapolated to very short sprints or used to promise an all-day sustainable power. See the testing guide for choosing efforts.
Physiology reference: Poole et al. (2016), Critical Power: An Important Fatigue Threshold in Exercise Physiology.
What it tells you.
What is Critical Power?
An estimate of the boundary between exercise intensities where a metabolic steady state may be possible and those where fatigue-related changes keep progressing. It is not a power you can hold indefinitely.
What is W′?
The model’s estimate of work available above CP, expressed in kilojoules. The battery analogy is useful, but W′ is not a direct measurement of a single energy store and varies with testing and modelling choices.
Why it matters.
CP and W′ can inform pacing and interval training. Interpret them alongside the test protocol, your recent training and how you respond on the bike.
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