Cycling: What’s My W’- how do I calculate it? and should I care?

critical power ftp w' prime bal cycling
Source: Poole et al,

Cycling: What’s My W’- how do I calculate it? and should I care?

After a post earlier in the week on dfa a1, it seemed only fitting to finish the week with another sports sciencey abbreviation that no one’s ever heard of…W’ (pronounced double-you-prime). It’s an important attribute for cyclists but perhaps most of us don’t need to worry about it too much. Well, I say it’s for cyclists but it’s a metric for anyone who trains with power.


Definition: W‘ is your anaerobic work capacity. It’s your potential for work above your critical power and could also be written “W Prime“. It’s measured in kilojoules Kj (kilojoules = watts x hours/1000, watts = Torque x Cadence).

Definition: W’bal is the balance of what you have left at any given moment.

Definition: CP is your critical power or forever power. It might equate to your hour power, CP60, CP120 or longer but it’s almost certainly a bit lower than your FTP which YOU and I probably cannot maintain for an hour.

At the start of your ride, you have a certain amount of W’. When you go up the first hill, sprint or take your turn on the front you will undoubtedly use some of the W’ and you will use it quickly. To some degree, the subsequent time spent below your CP will give your body a chance to replenish your W’ stores.

Garmin’s new stamina metric uses this principle in the areas highlighted below that indicate a rapid fall and slower recovery to a baseline that declines over time.


Cyclists have a usefully illustrative term called ‘matches’, but I couldn’t find a decent definition. Matches are efforts that significantly eat into your W’, more so than the diagram above, but you only have so many of them. You might have more than me and mine might be longer than yours (I mean, obviously 😉 ). The exact number and length will probably be determined by W’ and your rates of fatigue and recovery. W’bal thus gives you a partial insight into what you have left for your next match.

How Do I Calculate My W’?

Use an online calculator like this one: or this one

You will need 3 different duration efforts from above your CP. So your last FTP test, a 5- or 6-minute best from your CP curve and a 1-minute best from your CP curve. You just need a spread of durations from recent efforts to get a valid result. Perhaps your CP is 95% of your FTP.

As a sense check, probably everyone reading this article will get a W Prime (W’)  value between 10,000 and 25,000. From memory, Golden Cheetah initially defaults to 14,000 and for comparison, I think the average FTP of all Garmin Cheetah users is 250w.

Link: Garmin Connect

How Do I see my W’bal

Whilst you should hope to improve W’ over time, knowing the value doesn’t help you that much when riding. W’bal is more useful at any given moment in the ride.

You can assume that other platforms like Xert use a variation of it as will Garmin Stamina, which also probably periodically updates its parameters with each completed ride.

However, if you want a Garmin Connect IQ field to explicitly show W’bal then Inigo has one listed although you need to pay for it. There are probably others (please feel free to post links in the comments). You will need your FTP/CP and the W’ value from the calculator in the previous section.

Take Out – should I Care?

You should care in the sense that this is an important metric that reflects the physiology that will definitely determine how well you perform over any distance. The shorter the distances you take on, the more important it becomes. Similarly if your riding requires hard effort for hills or sprints then, again, it’s important.

I cast an occasional glance at my W’ value on the CP chart in Golden Cheetah. However, a properly structured training program will increase it for you.




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