FTP vs Critical Power (CP): Which is Better for Pacing?
Understand Critical Power and W′, try a constant-power example and review the limits of model-based pacing.
🎯 Quick Answer
FTP is a practical estimate of the maximum power you can sustain for about an hour. It is a single-dimension metric.
Critical Power (CP) is a parameter of the power–duration model. W′ represents modelled work capacity above CP. Together they add information about hard efforts, but predictions depend on the test data and the model; they cannot tell you the exact moment you will fatigue.
Breaking Down the Metrics
Functional Threshold Power (FTP)
The "Simplicity" Metric
FTP is defined as the highest power a cyclist can maintain in a quasi-steady state without fatiguing. In practice, it's typically derived by taking 95% of a 20-minute all-out effort.
- Strengths: Extremely easy to test (a single 20-minute test), widely understood, and perfectly fine for setting basic aerobic training zones.
- Weaknesses: Tells you absolutely nothing about what happens when you ride above your FTP. If two riders have an FTP of 250W, one might be able to hold 400W for 1 minute, while the other can hold it for 3 minutes. FTP cannot distinguish between them.
Critical Power & W-Prime (CP/W')
The "Dynamic" Metric
The CP model calculates two distinct numbers based on 3 to 5 maximal efforts of varying durations (from 3 minutes to 20 minutes).
- CP: Estimated from efforts at different durations. Its relationship to FTP varies by athlete and test protocol; there is no universal 5–10 W conversion.
- W' (W-Prime): A measurement of work (in kilojoules) you can perform above CP before complete exhaustion. Think of it as a battery that drains when you go above CP and recharges when you drop below it.
Why W' (W-Prime) Changes Everything
Imagine you are riding in a breakaway or racing a mountain bike course. You are constantly surging over threshold to cover attacks or clear steep technical sections. With FTP, you only know that going "into the red" hurts and will eventually cause you to blow up. You don't know when.
In a simplified constant-power example, W′ = 20 kJ and power 50 W above CP give 20,000 / 50 = 400 seconds (6 min 40 s). This is an estimate within the model, not a guarantee. Recovery during variable-power riding requires further modelling.
🚴 The Real-World Application
Review the power profile and training load in Bike Analytics alongside the session context. A modelled W′ balance can help interpret repeated surges, but it cannot establish exactly why a rider slowed or when they should attack.
📚 Scientific Foundation
Poole et al. (2016) explain CP as a boundary between exercise intensity domains and W′ as work capacity above CP. The model has estimation error, and effort at or below CP cannot be maintained indefinitely.
Power-based analysis needs usable power data. Do not substitute FTP for CP using a fixed offset, or read a modelled W′ value as a direct measurement of energy stores.
Which Should You Use?
When to use FTP:
- You primarily do long, steady-state efforts (Ironman pacing, Gran Fondos).
- You want a simple, one-test metric to set your 7-power training zones.
- You only use basic head units or free tools that don't calculate W'.
When to use Critical Power (CP + W'):
- Mountain biking and cyclocross: Examine repeated surges alongside FTP-based zones and technical demands.
- Road racing: Explore the estimated cost of repeated attacks without treating the model as a guarantee.
- Intervals: Compare planned and recorded efforts; adjust them with recovery and session context.
Stop guessing your breaking point.
Bike Analytics automatically calculates your Critical Power, tracks your W', and visualizes your battery depletion across every ride.
Download Bike Analytics for iOSLearn More
Deep Dive into Critical Power
Read our complete scientific breakdown of how CP and W' work mathematically.
Read CP Guide →Understanding FTP
Learn the standard 20-minute protocol to determine your Functional Threshold Power.
Read FTP Guide →Try the model, then review a recorded ride
Illustrative example, not athlete data: CP = 250 W and W′ = 20 kJ give t = 20,000 / (300 − 250) = 400 s (6 min 40 s) at a constant 300 W. This is a model estimate, not an exact countdown to exhaustion.
- Use a ride with recorded power and check that your threshold settings reflect a recent assessment.
- In Bike Analytics, compare the power profile, time in zones and training load of two similar rides.
- Use the CP/W′ model to understand surges, then check the result against recovery, conditions and how the session felt.
Power-based analysis needs usable power data. Do not substitute FTP for CP using a fixed offset, or read a modelled W′ value as a direct measurement of energy stores.
FTP vs Critical Power: Better for W'bal, Surges, and Pacing?
Compare FTP and Critical Power for racing, repeated surges, W'bal, and pacing decisions. Learn when CP/W' gives a better model than FTP alone.
- 2026-09-28
- FTP vs Critical Power · W'bal pacing · W prime cycling · anaerobic work capacity · CP model
- Bibliography