What is Critical Power (CP)?

Critical Power is the highest power output you can sustain in a physiological steady state. Above CP, fatigue accumulates progressively and exercise tolerance is finite. It's derived from the mathematical relationship between power and duration across multiple maximal efforts.

What is W' (W-prime)?

W' is the finite amount of work (in joules or kilojoules) you can perform above Critical Power before exhaustion. Think of it as your anaerobic 'battery.' A typical trained cyclist has a W' of 15–25 kJ.

How many test efforts do I need?

A minimum of 2 maximal efforts at different durations is required. Three efforts improve accuracy. Common durations are 3 minutes and 12 minutes for a 2-test protocol, or 1, 5, and 20 minutes for a 3-test protocol.

How does CP relate to FTP?

CP and FTP are conceptually related but not identical. FTP is typically estimated as 95% of 20-minute power, while CP is mathematically derived from the power-duration relationship. CP is often within 3–5% of FTP, sometimes slightly lower.

Cycling Watts Calculator — Critical Power & W' Estimator

Name: Cycling Watts Calculator
Author: Manish Kumar

Key Takeaways

Critical Power (CP) represents the highest power output sustainable without progressive fatigue — the boundary between steady-state and non-steady-state exercise.^[1]
W' (W-prime) is the finite anaerobic work capacity above CP, measured in kilojoules. Once depleted, power must drop to or below CP.^[2]
The 2-parameter model (P = W'/t + CP) requires at least 2 maximal efforts of different durations, ideally 3–12 minutes apart.^[1]
CP typically corresponds closely to FTP, though they are not identical. CP is a physiological boundary; FTP is a practical training anchor.

CP vs FTP — What's the Difference?

FTP (Functional Threshold Power) is commonly estimated from a 20-minute test (95% of 20-min power). CP is derived from the mathematical power-duration relationship using multiple efforts. In practice, CP and FTP are often within 3–5% of each other, but they measure different things.^[3]

For FTP-based training zones, use our FTP Calculator. To generate Coggan 7-zone training zones, see the Cycling Power Zones Calculator.

How to Test CP and W'

Perform 2–3 all-out time trials of different durations on separate days (or with full recovery between). Common protocols include:

3-minute and 12-minute all-out efforts (most common 2-test protocol)
1-minute, 5-minute, and 20-minute efforts (3-test protocol for higher accuracy)

Each effort must be truly maximal — pacing conservatively will underestimate CP. The R² value indicates model fit; aim for R² > 0.95 for reliable results.

Frequently Asked Questions

Sources & References

The Critical Power Concept: Applications to Sports Performance. Poole DC, Burnley M, Vanhatalo A et al.. Progress in Cardiovascular Diseases (2016)
Critical Power: An Important Fatigue Threshold in Exercise Physiology. Jones AM, Vanhatalo A, Burnley M et al.. Medicine & Science in Sports & Exercise (2019)
The Physiological Basis of the Power-Duration Relationship. Monod H, Scherrer J. European Journal of Applied Physiology (1965)

Manish Kumar

Certified Personal Trainer & Sports Nutritionist

NASM-CPTCertified Sports Nutritionist10+ Years Experience500+ Clients Coached

NASM-certified fitness and nutrition coach with over 10 years of hands-on experience helping people build strength, lose fat, and live healthier lives. Specializing in gym-based workouts with a strong focus on lifting technique, biomechanics, and practical exercise science. Through FitLifeRegime, sharing the tools, tips, and insights that have worked for hundreds of clients — helping you start your own fitness journey with confidence and clarity.

Learn more about Manish

NASM Certified

Certified Personal Trainer & Sports Nutritionist from the National Academy of Sports Medicine

This tool is for informational and journaling purposes only.

I am NOT a doctor.

Cycling Watts Calculator — CP & W'