What is Critical Power (CP)?

CP is the highest power output you can sustain aerobically for an extended time. Efforts above CP deplete W' and cannot be held indefinitely; efforts at or below CP can theoretically be sustained without fatigue.

What is W' (W-prime)?

W' (pronounced W-prime) is your finite anaerobic work capacity — the total extra work you can do above CP before exhaustion. It is measured in joules (J) or kilojoules (kJ). A typical trained cyclist has a W' of 15–30 kJ.

How should I collect the test efforts?

Perform all-out maximal efforts at different durations (e.g. 3 min and 12 min) on separate days with full recovery. Each effort must be a true maximum — not paced. Longer durations (3–20 min) give better CP estimates.

How many data points do I need?

You need at least two points to solve for CP and W'. Three or more points give a least-squares fit and a residual error to assess model quality. Widely spaced durations (e.g. 3 min and 20 min) improve accuracy.

Why does my W' seem negative or very small?

If your two test durations are very close together, small power measurement errors cause large swings in W'. Use efforts at least 3× apart in duration (e.g. 2 min and 10 min) for stable results.

Is CP the same as FTP?

CP and FTP (Functional Threshold Power) are related but not identical. FTP is typically estimated as 95% of 20-minute power and is a practical training metric. CP is a physiological parameter that is usually 5–10% higher than FTP for most cyclists.

Cycling W' (W-Prime) Calculator

Enter two or more maximal power–duration efforts and instantly get your Critical Power (CP) and W' (anaerobic work capacity).

Maximal Effort Data Points

Add at least 2 all-out efforts. More points = better fit.

Enter your efforts on the left and click Calculate.

Critical Power

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watts

W' (W-Prime)

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kilojoules

Model Fit

Data Points

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RMSE

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watts

R²

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Effort Breakdown

Duration	Actual (W)	Predicted (W)	Error (W)

Time to Exhaustion at Power Above CP

How long you can sustain each wattage above CP before W' runs out.

Power	Time to Exhaustion

Summary

The Critical Power model describes cycling performance using two parameters: Critical Power (CP), the aerobic power ceiling you can hold indefinitely, and W' (W-prime), the finite anaerobic energy reserve above CP. Enter two or more maximal power–duration efforts to fit the model and calculate both values. More data points improve accuracy and the tool reports the root-mean-square residual so you can judge model quality.

How it works

Enter at least two maximal power–duration efforts (e.g. a 3-minute and a 12-minute all-out test).
The tool fits the linear Critical Power model: Work = CP × Duration + W'.
Critical Power (CP) is the asymptotic sustainable power — your aerobic ceiling.
W' is the finite anaerobic energy reserve above CP, measured in kilojoules.
Add more data points to improve the fit accuracy and review the residual error.
Use the results to set training zones and pace race efforts above CP.

Use cases

Determine your Critical Power without a lab test.
Calculate W' to know how much hard-effort energy you can store and recover.
Plan pacing for breakaways or climbs where you must exceed CP.
Track anaerobic capacity changes across training blocks.
Set evidence-based training zones from real race or test data.
Compare CP models with different duration pairs to check consistency.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu