Critical Power calculator (CP and W′)

What it does

The Critical Power calculator estimates your CP (the asymptote of your power-duration curve, in watts) and W' (W-prime, your anaerobic work capacity above CP, in kJ). CP and W' together are the inputs for the Grepiac bike zones, and they drive the bike split projection in the race planning wizard step 2.

URL: /calculators/ then open Critical power. Direct path: /static/calculators/critical_power_calculator.html.

Inputs the calculator accepts

The page is built around CP and W' so you can enter them directly if you already have them, or compute them from a test.

If you already know your numbers

CP (W): your critical power.
W' (kJ): your W-prime.
AeT (W): your aerobic threshold in watts (optional).

Time-to-target predictions

Once CP and W' are set, the calculator can answer two prediction questions:

Target Power (W) + Time @ Target Power (s): how long you can hold a given target power above CP, based on W' usage rate.
Target Time (s) + Power @ Target Time (W): what power you can sustain for a given target duration, based on CP and W'.

Outputs

CP in watts.
W' in kJ.
Predicted time at a target power or predicted power for a target time, depending on which input pair you used.

When run from the wizard, the saved CalculatorResult outputs include cpw (CP in watts) and wkj (W' in kJ); those values become the step 2 key display on the wizard dashboard.

Fit sanity warning

The model fit is only as good as the test efforts behind it. The calculator now flags an unstable fit when the solved W' falls outside the plausible 10–30 kJ range (warning below 5 or above 35 kJ) or when the solved CP comes out at or above your best test power — both are signs of noisy data, efforts that weren't truly maximal, or durations entered against the wrong powers. The numbers still display so you can see what went wrong; re-test with well-spread durations (roughly 2, 8 and 15 minutes) rather than training on a flagged fit.

How to run a CP test

The most common protocol that produces both CP and W' is the 3 minute and 12 minute test:

30 minute structured warm-up with two 1 minute hard efforts.
3 minute all-out as a maximal sustained effort.
10 minute easy recovery.
12 minute all-out at the highest power you can hold for 12 minutes.

Take the average power from each effort and fit the CP/W' model. The calculator does the fit for you when you enter both durations and powers in the matching fields.

A simpler alternative is the 20 minute test: ride 20 minutes all-out and take 95 percent of the average as your FTP. FTP is an approximation of CP for most riders.

When to retest

Every 4 to 8 weeks during a build phase.
Less often in base or recovery blocks.
After a structural change (new bike fit, new training stimulus).