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CRITICAL POWER CALCULATOR

Calculate your Critical Power and W'?

You can use the calculator below to calculate your Critical Power and W’ based on your best recent average power numbers from 3-minute, 5-minute and 12-minute time trials. We have also included an FTP calculator based on your results.

For the numbers to properly represent your current fitness, ensure they are recent (i.e. within the last week or so) and that you are fresh when you perform each time trial. In our current testing protocol for our athletes, we perform the 3 and 12-minute time trials in the same session after a long warm-up and with a lot of recovery in-between time trials (20-minutes at around 55% of FTP). We then have our athletes perform the 5-minute time trial on a different day.

To learn more about what Critical Power and W’ are, explanations can be found below the calculator.

Critical Power, W' & FTP Calculator

Critical Power, W' & FTP Calculator




What is Critical Power?

Critical Power (CP) is a physiological concept that represents the highest power a cyclist can maintain over an extended period without experiencing rapid fatigue. It is based on the balance between aerobic (sustainable) and anaerobic (short-term, high-intensity) energy systems. Critical power essentially means that above a certain wattage, your power output follows a well-defined ‘hyperbolic’ power-duration curve, which is simply the power you can sustain over different time frames.

CP is typically the power you will trend towards when riding at a high intensity for around 30-40minutes. It is essentially your maximum sustainable power and in our opinion, has more practical use than FTP, which represents your 60-minute power. This is because unless you are doing a 25-mile TT, riders rarely do anything close to a 60-minute maximal effort on a regular basis.

CP is typically determined through a series of maximal efforts, such as 3-minute, 5-minute, and 12-minute time trials. 

When riding above CP, anaerobic energy stores (W’) are used, leading to rapid fatigue.

What is W'?

W’ (often pronounced “W-prime”) represents the finite amount of energy available above Critical Power for short, intense efforts. Think of it as a battery that depletes when you ride above CP and recharges when riding below CP.

If you perform a hard sprint or a sustained attack above CP, you are using up W’. Once W’ is completely depleted, you must drop below CP to allow it to recharge. The rate of W’ recovery depends on how far below CP you ride—if you just ease off slightly, recovery is slow; if you coast or ride at a low intensity, it recovers faster.

W’ is measured in kilojoules (KJ) and a high W’ is important for explosive events such as crit racing, cyclocross, mountain biking etc.

Why use CP and W'?

The ratio between CP and W’ can give a good sense of a rider’s physiological balance i.e. someone with a team sports background often has a naturally high W’, whereas rowers for example, tend to have naturally higher CP due to their. This reflects the differences in the nature of their sports. Once involves repeated sprints, the other is largely steady state for minutes at a time.

The main application though is that once you know your CP and W’, you can begin to estimate the specific power a rider can hold over a given duration and build a power duration curve, which you may have seen in platforms such as WKO or Golden Cheetah.

For example, if your CP is 300w and your W’ is 20KJ, you could sustain 400w for 200s.

This is because one watt is equal to on joule per second:
400w-300w => 20000/100 = 200s

What is a good CP and W'?

As FTP is such a well known concept, an easy way to gauge how well developed your CP is to think about it relative to your FTP, which will remain a valid metric. Both represent your aerobic fitness, but your CP should be about 5% higher.

For W’ though, it can be difficult to appreciate a metric measured in KJ means. As a massive generalisation however, below is a rough gauge:

Moderately trained male riding 8-10 hours = something around 16-20KJ is good

A very well trained male cyclist = low to mid 20s is good

Moderately trained female = 11-15KJ

Well-trained female = 14-18KJ

Key differences between CP, W' and FTP

MetricDefinitionWhat It Represents
CPMax sustainable power before fatigue rapidly increases (circa 30-4-0-minute power)A physiological threshold separating aerobic and anaerobic effort
W’Energy reserve for efforts above CPA battery that depletes and recharges based on intensity
FTPEstimated max sustainable power for ~60 minutesA training metric for setting zones

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