What is the difference between CP and FTP?

Critical Power (CP) is derived mathematically from multiple efforts and represents the power at which the body can maintain a physiological steady state. FTP (Functional Threshold Power) is typically estimated from a single 20-minute or 60-minute test. FTP is commonly approximated as 95% of CP.

How many efforts do I need?

A minimum of 3 efforts is required, with durations spread across different time ranges (e.g., 3 min, 5 min, 12 min). Four efforts provide better accuracy. The model works best when efforts span from 2 to 20 minutes.

What does W' (W-prime) tell me?

W' is your total anaerobic work capacity above CP, measured in kilojoules. A larger W' means you can sustain harder surges above your threshold for longer. It's relevant for climbing, sprinting, and race tactics where above-threshold efforts are needed.

What is a good R² value?

R² above 0.95 indicates excellent model fit — your data is consistent and the CP/W' values are reliable. R² between 0.85-0.95 is acceptable. Below 0.85, consider retesting with better-paced efforts.

Is this a clinical tool?

No. This is an informational training calculator for cycling performance analysis. It estimates CP and W' from self-reported effort data using published mathematical models. It does not assess health status.

Cycling Critical Power Calculator

Name: Cycling Critical Power Calculator
Author: Manish Kumar

Key Takeaways

Critical Power (CP) is the highest power output you can sustain without progressive fatigue — your true aerobic ceiling for cycling.^[1]
W' (W-prime) is your finite anaerobic energy reserve above CP — the “matches” you can burn for surges and attacks.^[2]
Uses linear regression from 3–6 maximal efforts to derive both values with an R² quality indicator.
Power-duration curve and time-to-exhaustion predictions help with race pacing and interval design.

What Is Critical Power?

Critical Power (CP) represents the boundary between sustainable and unsustainable exercise intensity. Below CP, you can ride for extended periods (the physiological steady state). Above CP, the finite W' reserve is progressively depleted, and exhaustion occurs when W' reaches zero.^[1]

CP is closely related to FTP (Functional Threshold Power) but is mathematically derived rather than estimated from a single test. FTP is typically approximated as 95% of CP. For complementary power analysis, see our Cycling Power Zones Calculator and Cycling Power-Speed Calculator.

The CP / W' Model

The two-parameter CP model (Monod & Scherrer 1965, refined by Morton 2006) describes the power-duration relationship as: Power = W'/time + CP. This simple linear relationship is remarkably accurate for efforts between 2–20 minutes.^[2]

CP (Critical Power)

The y-intercept of the regression line. Represents the power you can theoretically sustain indefinitely — your aerobic power ceiling. Typically close to your 40–60 minute best effort.

W' (W-Prime)

The slope of the regression line. Represents the total amount of work (in joules) you can perform above CP before exhaustion. Think of it as a battery — each second above CP drains it, and riding below CP recharges it.

How to Collect Test Efforts

For reliable CP/W' estimates, perform 3–4 maximal efforts of different durations on separate days (or with full recovery between them):

Short effort: 3 minutes — go as hard as you can sustain for the full 3 minutes.
Medium effort: 5–7 minutes — maximum sustainable effort.
Long effort: 12–20 minutes — time trial effort.
Optional: A 1-minute all-out effort adds data for the W' estimate.

Each effort must be a true maximal, evenly-paced attempt. Poorly paced efforts (starting too hard and fading) will produce inaccurate results reflected in a low R² value.

How to Use This Calculator

Enter 3–6 maximal efforts — Duration in minutes/seconds and average power in watts.
Check R² — Values above 0.95 indicate excellent model fit. Below 0.85 suggests retesting.
Review your CP and W' — CP is your sustainable ceiling; W' is your anaerobic reserve.
Use the power-duration curve — Plan race pacing for any duration from 1–60 minutes.
Set training zones — The 6-zone system from CP provides precise power targets for structured training.

Frequently Asked Questions

Sources & References

Critical power: An important fatigue threshold in exercise physiology. Jones AM, Vanhatalo A, Burnley M, et al.. Medicine & Science in Sports & Exercise (2019)
The critical power concept and its application to cycling. Poole DC, Burnley M, Vanhatalo A, et al.. European Journal of Applied Physiology (2021)

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

Disclaimer: This tool is for informational and personal journaling purposes only. Results are estimates based on published formulas and may vary based on individual factors such as genetics, training history, and measurement technique.