Critical Power & W' Calculator
Your Critical Power (CP) is the highest power output you can sustain without accumulating fatigue — the physiological boundary between heavy and severe exercise. Unlike a simple 20-minute FTP test, the CP model uses multiple efforts to build a complete picture of your power-duration relationship.
Enter your best power data for 2 or more durations below. This calculator fits the peer-reviewed 2-parameter CP model to estimate your Critical Power, W' (anaerobic work capacity), and predicted power at any duration.
Your Power Data
Enter your best average power for at least 2 different durations. Use data from Strava, Garmin, Zwift, or a structured test.
What is Critical Power?
Critical Power (CP) is defined as the asymptote of the power-duration relationship — the highest metabolic rate that can be sustained by wholly oxidative (aerobic) metabolism. Below CP, exercise can theoretically continue indefinitely. Above CP, a finite amount of work (W') is available before exhaustion.
The concept was first described by Monod and Scherrer in 1965 and later formalized for cycling by Morton (1996) and Jones et al. (2010). It is now considered the gold standard in exercise physiology for defining the heavy-to-severe exercise intensity boundary.
The mathematical model is elegantly simple: P(t) = CP + W'/t, where P is power at duration t, CP is Critical Power, and W' is the curvature constant representing anaerobic capacity.
CP vs FTP — What's the Difference?
FTP (Functional Threshold Power) is typically estimated as 95% of a 20-minute all-out effort. While practical, this single-test approach has limitations: it doesn't account for anaerobic contribution during the 20-minute test, and it gives no information about your capacity above threshold.
| Aspect | FTP (20-min test) | Critical Power |
|---|---|---|
| Data required | 1 effort (20 min) | 2+ efforts (different durations) |
| Scientific basis | Empirical (Coggan, 2003) | Physiological model (Monod & Scherrer, 1965) |
| Anaerobic info | None | W' (joules above CP) |
| Accuracy | Overestimates for anaerobically strong riders | More robust across rider profiles |
| Typical relationship | CP is usually 3–8% lower than FTP for most cyclists | |
In practice, CP and FTP are closely related — for zone-based training, they are often used interchangeably. The real advantage of CP is that it gives you W', which helps predict performance in events that mix steady and hard efforts (criteriums, hilly road races, mountain bike races).
Understanding W' (W-prime)
W' represents the total amount of work (in joules or kilojoules) you can perform above your Critical Power before exhaustion. Think of it as a rechargeable battery:
- •When you ride above CP, W' depletes at a rate of (Power − CP) joules per second
- •When you ride below CP, W' reconstitutes (recharges)
- •When W' reaches zero, you must reduce power to or below CP
Typical W' values range from 10–25 kJ for trained cyclists. Sprinters and track riders tend to have higher W' values, while time trialists may have relatively lower W' but higher CP.
| W' Range | Typical Rider Profile |
|---|---|
| 5–12 kJ | Endurance-focused, lower anaerobic capacity |
| 12–20 kJ | Well-rounded rider, good at mixed terrain |
| 20–30 kJ | Strong anaerobic profile, good sprinter |
| 30+ kJ | Elite track / sprint specialist |
How to Get Your Power Data
For the most accurate results, perform dedicated efforts on separate days (or with full recovery between efforts). Here are common approaches:
From Strava / Garmin / Wahoo
Check your "Power Curve" or "Best Efforts" for peak power at 1 min, 5 min, and 20 min. These don't need to be from dedicated tests — your best efforts from recent rides work.
From Zwift / TrainerRoad
Indoor structured tests give the most controlled data. A ramp test gives 1-minute peak; add a 5-minute or 20-minute all-out effort on a different day.
Dedicated 3-trial protocol
The traditional lab approach: perform 3 all-out efforts at 1 min, 5 min, and 12–20 min on separate days. This gives the most scientifically valid CP and W' values.
How to Use Your Results
Once you know your CP and W', you can:
- •Set accurate training zones — CP is a more physiologically grounded threshold than a 20-min FTP test. See our power zones guide for how to structure your training.
- •Predict race performance — use the predicted power table to estimate what wattage you can hold for any race duration.
- •Pace efforts strategically — knowing your W' tells you how much above-threshold work you can do in a race.
- •Track progress — repeat the test every 6–8 weeks to see how CP and W' change with training. Learn more about reaching 4 watts per kilogram.
Model Limitations
The 2-parameter CP model is well-validated for durations between approximately 2 and 30 minutes. Be aware of these limitations:
- •Very short durations (<30s): The model overpredicts power because it doesn't account for neuromuscular peak power limits.
- •Very long durations (>30 min): The model slightly overpredicts because real-world fatigue, fueling, and thermoregulation reduce sustainable power.
- •Data quality: The model is only as good as your inputs. Stale data, poorly paced efforts, or different environmental conditions (heat, altitude) will reduce accuracy.
For most training purposes, these limitations are acceptable. If you need higher accuracy at short durations, the 3-parameter model (adding Pmax) is an option — but it requires more data points and a non-linear solver.
Scientific References
- Monod, H., & Scherrer, J. (1965). The work capacity of a synergic muscular group. Ergonomics, 8(3), 329–338.
- Morton, R.H. (1996). A 3-parameter critical power model. Ergonomics, 39(4), 611–619.
- Jones, A.M., Vanhatalo, A., Burnley, M., Morton, R.H., & Poole, D.C. (2010). Critical power: implications for determination of VO2max and exercise tolerance. Medicine & Science in Sports & Exercise, 42(10), 1876–1890.
- Coggan, A. (2003). Training and racing using a power meter. In Training and Racing with a Power Meter (Hunter Allen & Andrew Coggan).
- Skiba, P.F., Chidnok, W., Vanhatalo, A., & Jones, A.M. (2012). Modeling the expenditure and reconstitution of work capacity above critical power. Medicine & Science in Sports & Exercise, 44(8), 1526–1532.