Three maximal efforts. Two numbers that define your power profile.
Critical power (CP) is the threshold above which your finite energy reserve begins to deplete. It marks the boundary between intensity you can sustain and intensity that is always costing you. W' (pronounced "W prime") is the finite energy reserve you draw on above that ceiling. Together they predict what you can hold, for how long, and what it will cost you.
Enter the average power (mean maximal power) for three all-out efforts at different durations. The recommended combination is 3, 5, and 12 minutes, spread enough to resolve the model accurately.
Each test must be a true maximal effort for the full duration, not a hard tempo or a paced threshold attempt. The model only works if the efforts are genuine all-out bests.
You can also use your personal best average power figures from recorded rides, provided they were genuine maximal efforts at those durations.
Model fit: —. A value above 97% indicates the three efforts are consistent with the CP model. Estimated FTP is CP × 0.96.
Compared with test from —
The shaded area between the curve and the CP line represents W', which is the finite energy reserve available above critical power. Each point on the curve is the maximum average power the model predicts for that duration. Your three test efforts are plotted as points.
CP is the power you can sustain indefinitely. W' is what you spend the moment you go above it.
Think of W' as a battery. Every second you ride above CP drains it at a rate proportional to how far above CP you are. Every second below CP recharges it. Once depleted, you cannot sustain power above CP until the battery recovers.
For ultra-distance riding, CP is the ceiling rather than the target. Any extended time at or above CP depletes glycogen at a rate you cannot replenish on the move, and the fatigue it generates compounds over distance. How far below CP to target depends on how long you will be on the bike. For half-day and all-day events most riders aim for 70 to 80% of CP. For true ultra-distance riding lasting many hours or multiple days, 55 to 70% is more appropriate, and sometimes lower on harder terrain. W' determines how much headroom you have for the surges you cannot avoid, such as short steep ramps, sudden changes in wind, or the cost of getting back up to pace, before you are forced below CP to recover.
Your W' of — kJ is the total above-CP energy available before you are forced back below critical power.
These are theoretical maxima assuming W' starts fully charged. In practice, repeated surges accumulate fatigue, so recovery below CP between efforts is essential.
Model: The critical power model represents the power–duration relationship as P = CP + W'∕t, where P is power (W), t is duration (s), CP is critical power (W), and W' is the finite work capacity above CP (J). This is equivalent to a linear relationship between power and the reciprocal of duration.
Regression: CP and W' are estimated using ordinary least squares regression on the three (1∕t, P) data points. The slope of the regression line is W' (in joules) and the intercept is CP (in watts). Three points are the minimum required for this two-parameter model.
Model fit (R²): The Pearson correlation coefficient r is computed across the three points, then squared to give R². A value above 97% indicates the test data are consistent with the two-parameter CP model. Values below 97% suggest the efforts were not all maximal, or the durations were too narrowly spaced to resolve the model accurately.
Estimated FTP: Functional threshold power is defined operationally as mean maximal 60-minute power. CP is the theoretical asymptote of the power–duration curve and is consistently around 3–5% above a rider's 60-minute power in practice (Poole et al., 2016; Jones et al., 2010). The 0.96 multiplier reflects this empirical relationship.
Your CP of — W and W' of — kJ. Here is how to put those numbers to work.
Zones are calculated from your estimated FTP of — W. Choose the model that matches how you train; all three use the same underlying data.
| Zone | Name | Power range | Training purpose |
|---|---|---|---|
| Z1 | Recovery | — | Easy spinning. Active recovery between hard sessions. |
| Z2 | Endurance | — | The aerobic foundation. Most ultra-distance riding targets here. |
| Z3 | Tempo | — | Productive but sustainable. Longer intervals of 20–60 min. |
| Z4 | Threshold | — | Around FTP and CP. Classic 2×20 and sweet spot work. |
| Z5 | VO2 max | — | Short, hard intervals (3–8 min). Draws heavily on W'. |
| Z6 | Anaerobic | — | Above 120% FTP. Pure W' work: short, intense efforts. |
| Zone | Name | Power range | Training purpose |
|---|---|---|---|
| Z1 | Easy | — | Below the first lactate threshold. Builds aerobic base without accumulating fatigue. The majority of training volume lives here. |
| Z2 | Moderate | — | Between the lactate thresholds, broadly equivalent to tempo or sweet spot in Coggan-based systems. Harder than Zone 1 but produces less adaptation per unit of fatigue than either Zone 1 volume or Zone 3 intervals. In a polarized approach, used sparingly as a deliberate target. |
| Z3 | Hard | — | Above CP, drawing directly on W'. High adaptation stimulus. Short, focused intervals (2–8 min) with full recovery between efforts. |
Zone 3 starts at your CP of — W, not an arbitrary FTP percentage. This is the precise physiological boundary your test data defines.
In the polarized model, most training volume (typically 75–80% of sessions) is in Zone 1 (easy, below LT1), with a small proportion of high-quality Zone 3 work (hard, above CP). Zone 2 in this model (tempo or sweet spot in Coggan terms) arises naturally on long rides and events but is not typically the deliberate target of a training session. The fatigue it generates is better invested in Zone 1 volume or Zone 3 quality.
| Zone | Name | Power range | Training purpose |
|---|---|---|---|
| Z1 | Active Recovery | — | Easy spinning. Recovery days and between hard efforts. |
| Z2 | Endurance | — | Aerobic base. Long steady riding. The bulk of ultra-distance training. |
| Z3 | Tempo | — | Sustained moderate-high effort. Longer steady-state intervals. |
| Z4 | Lactate Threshold | — | Classic threshold work. 2×20, sweet spot. Raises FTP over time. |
| Z5 | VO2 max | — | Hard short intervals (3–8 min). High cardiovascular demand, significant W' use. |
| Z6 | Anaerobic Capacity | — | Very short, very hard (30 s–2 min). Primarily W'-dependent. |
| Z7 | Neuromuscular Power | — | Maximal sprints (≤ 15 s). Peak power and neuromuscular recruitment. |
Use your CP model to predict what you can hold for a given duration, or how long you can sustain a target power. The model is calibrated against 3–20 minute efforts, so predictions are most reliable within this range. Both assume full W' charge at the start of the effort. For applying CP to ultra-distance pacing, see the note below.
Enter a duration to find the maximum average power predicted by your CP model.
Enter a target power to find how long you can sustain it before W' is depleted.
The predictor is built around W' depletion and is designed for efforts of 3–20 minutes. It does not extend to brevet, audax, or multi-hour pacing, but your CP is still the most important number for those events.
CP is your aerobic ceiling. Any power above it draws on W', and while W' does recover when you drop back below CP, it does so slowly. The further above CP you ride, and the more often you do it, the larger the cost becomes. How far below CP to target depends on how long you plan to be on the bike. For sustained efforts of an hour or two, 80–90% of CP is achievable for most trained riders. For half-day or all-day events, 70–80% is a more realistic ceiling. For true ultra-distance riding lasting many hours or multiple days, most experienced riders average somewhere between 55 and 70% of CP, and sometimes lower on harder terrain. Riding close to or repeatedly above CP on a long event accumulates a cost that compounds as the hours pass and becomes progressively harder to manage.
W' governs your surges. On a long ride, every climb, headwind, or acceleration above CP spends W'. Recovery below CP recharges it, but only slowly. A high W' gives you more margin to respond to the terrain and conditions before you are forced to back off. A lower W' means shorter, fewer surges with longer recovery needed between them.
CP and W' are trainable. As your fitness develops, the model should be updated, typically every 6–10 weeks during a structured training block.
Knowing your CP is only useful if your training is structured around it.
In a free consultation we'll look at how your current CP and W' compare to the demands of your target events, discuss which training zones to prioritise, and work through how to structure work above and below CP to drive the adaptations you need. You'll leave with a clearer picture of what to focus on, even if we never work together.