CP and W′: the key values for training and racing better and more efficiently
FTP is the number almost every cyclist trains on — handy, measurable and built into everything. But it's one number, and training and racing better asks for two: your Critical Power and your W′. Here's where FTP gives you a good picture, and where CP and W′ take you further.
Ask ten riders about their form and nine will quote their FTP. Understandable: it's one clear number you get from a single test and can compare against anyone. The problem isn't that you use FTP — it's that you try to read too much into it. Because the moments that decide a race happen somewhere FTP doesn't describe.
What FTP is — and why it's handy
FTP stands for Functional Threshold Power: roughly the highest power you can sustain for about an hour. In practice you estimate it from a single test — often 95% of your best 20 minutes. That makes it a fine, low-threshold measure of your endurance power: easy to measure, to repeat, and to base your zones on.
For most of your training that's enough. Easy endurance, tempo, threshold work — that happens around your FTP, and there one number is a fine compass.
Where FTP falls short
A race isn't decided around your FTP, but far above it: the attacks, closing a gap, the sprint. Those are short, hard efforts you can't hold for an hour — and about exactly that area FTP says nothing. It describes what you can do for a long time, not what happens when you go over the top.
On top of that: FTP is a practical estimate, not a physiological boundary. It's a conveniently chosen point on your power-duration curve, not the curve itself. And it's that curve — how your power relates to how long you can sustain it — that tells the real story.
- •Estimate from one test (± best hour, often 95% of 20 min).
- •Describes your endurance power — what you sustain for long.
- •One number, easy to measure and compare.
- •Says nothing about what happens above your limit.
- •Comes from the power-duration relationship itself (several durations).
- •The boundary between what you sustain and where the clock starts.
- •More physiologically grounded than FTP.
- •Comes with a second number: W′, your attack battery.
The number FTP hides: W′
As soon as you work with Critical Power, you get something FTP doesn't have: W′(pronounced “W-prime”) — the finite amount of work you can deliver above your CP before you blow up. Your attack battery. Every attack draws it down; drop below and it recharges.
That's exactly the part that decides a race, and exactly the part a single number papers over. CP is your endurance engine, W′ is your attack battery — two things, not one.
Two riders, same FTP, different race. One with a big W′ can latch on three times; one with a small W′ is on the rivet after a single attack. Equal on paper — a world apart in the finale. One number won't show you that difference.
What this means for your race
FTP isn't a bad number — it's an incomplete one. Use it as a compass for your endurance work by all means, but don't count on FTP to explain why you fade or come through in the finale. That sits in your CP and your W′.
And there's a third question even CP and W′ don't answer yet: how fast does that battery recharge? That's your personal recovery profile — your W-tau — and it's what a race full of repeated efforts really turns on.
Beyond a single number?
The CP test maps your Critical Power and W′, with my analysis by email — two numbers that tell you what FTP alone can't.