Critical power and functional threshold power both try to answer a similar practical question: where does sustainable hard cycling end and rapidly fatiguing work begin? They are closely related, but they are not the same metric, they are not derived in the same way, and you should not assume one can be converted into the other with a fixed percentage.
Use FTP if you want a simple, widely supported number for training zones, workout targets and day-to-day cycling decisions. Use critical power (CP) plus W′ if you want a more detailed model of your power-duration relationship and your finite capacity for work above CP. For many cyclists, the best answer is not CP or FTP: keep FTP for practical training and use CP/W′ as an additional diagnostic model when you have reliable multi-duration maximal data.
Critical Power vs FTP at a Glance
| Question | FTP | Critical Power |
|---|---|---|
| What is it? | A practical performance threshold used for training and pacing | A parameter derived from the power-duration relationship |
| How is it usually estimated? | Commonly from a 20-minute test, ramp test or longer field effort | Usually from several severe-intensity efforts fitted to a mathematical model; validated single-session alternatives also exist |
| Second parameter? | No required companion metric | Yes: W′, the modeled finite work capacity above CP |
| Main strength | Simple, accessible and widely integrated into cycling platforms | Describes both threshold-like power and above-threshold work capacity |
| Main limitation | The estimate depends on the protocol and does not describe supra-threshold capacity | Requires suitable maximal efforts and careful modeling; estimates are protocol-sensitive |
| Best for | Training zones, workouts, broad pacing guidance | Power-duration analysis, severe-intensity performance and detailed modeling |
What Is FTP?
Functional threshold power is a practical cycling benchmark intended to represent a high, approximately steady-state power that can anchor training zones and performance tracking. In real-world cycling, FTP is usually estimated rather than measured through one universal protocol.
The familiar 20-minute method commonly takes 95% of the rider's best 20-minute average power. Other platforms use ramp tests, longer efforts or proprietary estimates. That convenience is one reason FTP became so useful: a rider with a power meter or smart trainer can obtain a workable training anchor without laboratory testing.
But FTP should not be interpreted as a magical wattage that every rider can hold for exactly 60 minutes. Your actual time to exhaustion near FTP varies with physiology, training history, fatigue, fueling, environment, pacing and the test used to estimate the number.
If your main goal is to build conventional training zones, start with our cycling power zones guide and the Cycling Power Zone Calculator.
What Is Critical Power?
Critical power is one parameter of a mathematical relationship between power and the duration for which that power can be sustained. In the classic two-parameter model, performance above CP is described using two values:
- CP: the power asymptote of the model and a physiologically meaningful boundary associated with the transition into the severe-intensity domain.
- W′: a finite amount of work, expressed in kilojoules, that can be performed above CP within the model.
The important distinction is that CP is not normally produced by taking a fixed percentage of one effort. It is estimated from the shape of your performance across multiple durations, or from a validated alternative protocol. That means CP uses more of your power-duration profile than a single-duration FTP estimate.
Researchers have shown that CP has meaningful links to exercise-intensity domains: above CP, oxygen uptake and other physiological disturbances cannot stabilize indefinitely, and tolerance becomes increasingly constrained. It is still a model, however, not a literal promise that power just below CP can be sustained forever.
What Is W′?
W′—pronounced “W prime”—is the second major reason CP can offer information that FTP alone does not.
Think of W′ as a modeled finite work capacity above critical power. If your CP were 300 W and you repeatedly rode well above 300 W, the CP model would describe those efforts as drawing from W′. The farther above CP you ride, the faster that modeled capacity is expended.
That makes W′ useful for questions such as:
- How much hard work can I perform above my threshold-like power?
- Why can two riders with similar CP respond differently to short attacks?
- How might repeated severe efforts alter what I can do later in a race or interval session?
W′ should not be treated as a perfectly measurable battery. Research on intermittent cycling shows that its modeled reconstitution below CP depends on factors such as recovery intensity and duration, and simple models can overpredict how much capacity has returned. Use W′ as a performance model, not as an exact real-time fuel gauge.
How Are Critical Power and W′ Tested?
The traditional approach uses several maximal or near-maximal severe-intensity trials that span different durations. Those performances are fitted to a power-duration model to estimate CP and W′.
One research protocol that directly compared CP with FTP used 3-, 7- and 12-minute time trials for the CP calculation. Other protocols use different duration combinations, constant-power trials, or a 3-minute all-out test.
This matters because the chosen protocol can change the estimate. Research has demonstrated that the durations selected for predictive trials influence calculated CP, while W′ can be particularly sensitive to protocol and recovery choices. A convenient CP number generated from poor or stale data is not automatically better than a well-executed FTP test.
Practical rule: if you calculate CP from field data, use genuinely hard efforts across appropriately separated durations, avoid building the model from several nearly identical efforts, and re-test under repeatable conditions.
If you instead want a straightforward FTP benchmark, see our guide to ramp tests versus 20-minute FTP tests.
Are Critical Power and FTP the Same?
No. They can be close, and they are often strongly correlated, but the evidence does not support treating them as interchangeable values.
In a 2021 study of 17 trained cyclists and triathletes, CP averaged 256 ± 50 W and 20-minute-test-derived FTP averaged 249 ± 44 W. The two measures were very strongly correlated, but the individual limits of agreement were wide enough that the researchers advised against using the values interchangeably for performance analysis.
That is the practical problem with statements such as “CP is always X% of FTP” or “add Y watts to FTP to get CP.” Even if a group average looks tidy, the individual rider can differ meaningfully.
Why Can CP and FTP Differ?
Several reasons can move the two estimates apart.
1. They use different information
A common FTP test compresses your performance into one primary effort. A conventional CP model uses performances across multiple durations.
2. Anaerobic contribution affects shorter tests
A rider with strong short-duration capacity may produce a disproportionately good 20-minute effort. Applying a universal percentage to that performance does not guarantee that the result matches CP.
3. Protocol selection matters
CP depends on the model and predictive trials chosen. FTP also depends on whether it came from a ramp test, 20-minute protocol, longer effort or software estimate.
4. Fitness is not one-dimensional
Two cyclists can share a similar threshold-like power while having very different sprint power, VO₂max-range performance and W′. That is why your full power curve can reveal information that one threshold value cannot.
Which Should You Use?
| Your goal | Best starting point | Why |
|---|---|---|
| Set conventional cycling training zones | FTP | Simple and widely supported |
| Follow workouts from common cycling apps | FTP | Most workout libraries are built around FTP percentages |
| Track a familiar threshold benchmark over time | FTP | Easy to repeat if you keep the protocol consistent |
| Model performance across several hard durations | CP + W′ | Uses the power-duration relationship rather than one threshold estimate |
| Analyze repeated supra-threshold efforts | CP + W′ | W′ adds information about modeled work above CP |
| Diagnose strengths and weaknesses in a serious power profile | Both | FTP is practical; CP/W′ adds a different layer of analysis |
For Most Amateur Cyclists, FTP Is Still the Easier Default
If you train three to six times per week, use standard training plans and mainly need reliable intensity targets, FTP remains an efficient solution. It plugs directly into zone systems, smart-trainer workouts and common cycling software.
The key is consistency. Do not switch from a 20-minute protocol to a ramp test to an estimated FTP every few weeks and then interpret every small watt change as fitness. Use the same method under similar conditions whenever possible.
You can then build sessions such as threshold workouts around that anchor while using heart rate and perceived exertion as checks on whether the target remains appropriate.
When CP and W′ Become More Valuable
CP and W′ become especially interesting when your question is no longer simply “What are my zones?” but “Why do I perform the way I do across different durations?”
Imagine two riders who both have an FTP around 280 W. Rider A has a relatively high CP but modest W′. Rider B has a similar threshold benchmark but a larger amount of modeled work available above CP. Their optimal responses to attacks, short climbs and repeated severe intervals may differ even if a standard FTP chart gives them nearly identical training zones.
That does not mean W′ should dictate every workout. It means the CP framework can expose a dimension of performance that FTP alone does not describe.
What Should Triathletes Use?
For most age-group triathletes, FTP is the more practical day-to-day anchor. Long-course triathlon bike pacing normally sits well below threshold, and execution depends on aerodynamics, durability, fueling, heat management and how much running ability you preserve—not just on whether your threshold model is called FTP or CP.
CP can still be useful for a triathlete who wants deeper performance analysis, particularly when tracking the power-duration relationship or evaluating high-intensity training. But switching from FTP to CP will not fix poor pacing or make a generic percentage automatically more accurate.
Can Critical Power Replace FTP?
Technically, you can build a training system around CP. Practically, there is no requirement to replace FTP if your current system works.
A sensible hierarchy is:
- Choose one primary training anchor. For most cyclists, FTP is the easiest.
- Use CP/W′ when it answers an additional question. For example, profiling severe-intensity capacity or analyzing repeated hard efforts.
- Do not mix percentages blindly. An FTP-based workout prescription should not automatically be applied as the same percentage of CP.
- Track trends using consistent methods. A stable protocol is more useful than constantly chasing a supposedly superior metric.
A Practical Example
Suppose a cyclist completes a 20-minute test at 300 W. Using the common 95% method gives an FTP estimate of 285 W.
The same rider then completes several valid maximal efforts across different severe-intensity durations. A fitted model estimates CP at 292 W and W′ at 18 kJ.
The wrong conclusion would be:
“CP is 7 W higher, so CP is the true FTP.”
The better interpretation is:
- 285 W is the current FTP estimate produced by that specific 20-minute protocol.
- 292 W is the CP estimate produced by the selected power-duration model and trials.
- 18 kJ adds information about modeled work capacity above CP.
- The rider should use the metric that matches the training or analysis method rather than force both values to agree.
Common Mistakes
Converting CP and FTP with one fixed percentage
Group averages do not create a reliable individual conversion formula.
Calling W′ an exact anaerobic tank
It is a useful model parameter, but depletion and reconstitution are more complex than a perfectly rechargeable battery.
Building CP from weak efforts
A model can fit bad data. If the underlying efforts are not maximal or the duration range is poorly chosen, the output can look precise while being misleading.
Using a threshold number without checking training response
Power is only part of the picture. Heart rate, RPE, interval completion, decoupling and recovery still matter.
Changing testing methods every time fitness changes
Consistency makes longitudinal comparison far more useful.
Critical Power vs FTP: The Decision
Choose FTP when you need simplicity and compatibility. Choose CP + W′ when you need a richer model of the power-duration relationship. Use both when the extra information has a clear purpose.
Do not assume a fixed conversion between them, and do not interpret a few watts of difference as proof that one model is universally more “correct.”
If your immediate goal is to turn a threshold value into usable training intensities, continue with Cycling Power Zones Explained or enter your current FTP in the Cycling Power Zone Calculator.
Frequently Asked Questions
Is critical power usually higher than FTP?
It can be, and one direct comparison study found a slightly higher group-average CP than 20-minute-test-derived FTP. But individual differences were wide enough that you should not assume CP will always be a fixed number of watts or percentage above FTP.
Can I calculate critical power from one 20-minute test?
Not with the conventional two-parameter CP approach. CP is normally estimated from multiple suitable efforts across different durations or from a validated alternative protocol. A single 20-minute effort does not provide the same information as a power-duration model.
Is W′ the same as anaerobic capacity?
Not exactly. W′ is a parameter of the critical-power model describing finite work capacity above CP. It is influenced by high-intensity energetic processes, but treating it as a direct, perfectly isolated measurement of “anaerobic capacity” is too simplistic.
Should I use CP instead of FTP for cycling zones?
Only if your training system is designed around CP. Most consumer cycling platforms and training plans still make FTP the simpler default for zone prescription. Do not simply apply an FTP percentage table to CP without checking the underlying method.
How often should I update CP or FTP?
Update when training has plausibly changed your fitness or when the current value clearly stops matching your workouts—not just because a few days have passed. Use repeatable testing conditions and the same protocol when tracking trends.
- Karsten et al. — Relationship Between the Critical Power Test and a 20-min Functional Threshold Power Test in Cycling
- Vanhatalo et al. — Determination of Critical Power Using a 3-min All-Out Cycling Test
- Skiba et al. — Modeling the Expenditure and Reconstitution of Work Capacity Above Critical Power
- Morgan et al. — Reliability and Functional Assessment of the FTP Test in Highly Trained Athletes
- Karsten et al. — Inter-trial Recovery Times for Determination of Critical Power and W′
- Bishop et al. — Predictive Trial Duration and the Critical Power Function
