What is Critical Power (CP) and W′?

Critical Power (CP) is a modelled power output, similar in concept to FTP, that marks the boundary between efforts your body can hold in a steady aerobic state and efforts that progressively drain a limited energy reserve. In practice it's sustainable for roughly 30–40 minutes, not indefinitely — it's derived directly from the mathematical relationship between power and time to exhaustion, rather than from a single test. W′ ("W-prime") is that finite anaerobic energy reserve above CP, measured in kilojoules — the size of the "battery" you can dip into for surges, hills, and hard efforts before it runs out. Both come from the two-test critical power model, using two maximal efforts of different durations.

How to Use This Calculator:

• Ride two maximal, all-out constant efforts of different durations (e.g. 3 minutes and 12 minutes). You can do both in the same session with a 30-40 minute recovery between them.
• Record your average power for each effort.
• Enter both durations (in minutes) and their average power below.