Critical power calculator
Critical power, W’ and an FTP estimate from a 3, 5 and 12 minute effort. A guide from three maximal rides, not a lab test.
Use the best recent average power for each effort, in watts. The 3-minute effort should be the hardest, and the 12-minute effort the most evenly paced. Test the 3 and 12 minute efforts in one session, with a long warm-up and about 20 minutes easy between them. Do the 5-minute effort on another day.
How the estimate is made
Three maximal efforts
The fit uses average power from a 3, 5 and 12 minute time trial. Do them fresh, within about a week, so the three numbers describe the same fitness. A tired 12-minute effort pulls critical power down.
Critical power and W’
Power falls as the effort gets longer. A straight line through the three points gives critical power, close to what you can hold for 30 to 40 minutes, and W’, the work you can do above that power. Above it, W’ runs down. Below it, W’ refills.
The FTP estimate
FTP is critical power times 0.96, the same factor as the old calculator. Training platforms still ask for an FTP. Critical power is the figure we use for the hard, sustained effort. It should sit a little above this FTP.
What is critical power?
Critical power is the highest power you can hold for a long time before fatigue rises quickly. It sits between the aerobic work you can sustain and the anaerobic work you can only keep up briefly. Above that wattage, the power you can hold follows a curve: a harder effort lasts less time.
On the bike it is the power you trend towards in a hard ride of about 30 to 40 minutes. That is the maximum you can sustain. We find it more useful than FTP, which stands in for a 60-minute effort. Unless you are racing a 25-mile time trial, a maximal hour is rare.
It is worked out from a series of maximal efforts, such as 3, 5 and 12 minute time trials. Ride above critical power and you spend W’. Fatigue arrives quickly.
What is W’?
W’ (pronounced “W prime”) is the limited energy you can spend above critical power on short, intense efforts. Think of it as a battery. It runs down when you ride above critical power, and it refills when you ride below it.
A hard sprint or a sustained attack uses W’. Once it is empty, you have to drop below critical power before it can refill. Ease off only slightly and recovery is slow. Coast, or ride at a low intensity, and it comes back faster.
W’ is measured in kilojoules. A high W’ matters in explosive racing, such as criteriums, cyclocross and mountain biking.
Why use critical power and W’?
The balance of the two says something about the rider. Someone from a team sport often has a naturally high W’. A rower, for example, tends to have a higher critical power. One sport is repeated sprints. The other is steady work for minutes at a time.
Once you know both, you can estimate the power a rider can hold for a given time, and build the power-duration curve you see in software such as WKO or Golden Cheetah.
At 300 W critical power and 20 kJ of W’, 400 W lasts about 200 seconds. One watt is one joule per second. 400 W minus 300 W is 100 W, and 20,000 joules divided by 100 W is 200 seconds.
What is a good critical power and W’?
FTP is the familiar number, so a simple check is to set critical power next to it. Both describe aerobic fitness, and critical power should be about 5% higher. FTP stays a valid metric.
W’ in kilojoules is harder to picture. As a rough gauge:
Moderately trained man
Riding 8 to 10 hours a week, around 16 to 20 kJ is a good W’.
Very well trained man
A good W’ sits in the low to mid 20s kJ.
Moderately trained woman
A good W’ is often 11 to 15 kJ.
Well trained woman
A good W’ is often 14 to 18 kJ.
Key differences
Critical power
The highest power you can hold before fatigue rises quickly. Around a 30 to 40 minute effort.
The threshold between aerobic work and the effort that spends W’.
W’
The energy reserve for efforts above critical power.
A battery that empties and refills as the intensity changes.
FTP
An estimate of the power you could hold for about 60 minutes.
A training number for setting zones.
The calculators are a starting point. Coaching is where the number turns into the sessions you ride.
More from the library: VO2max · Event run pace · Critical swim speed · Calorie needs · Cycling coaching · Training articles