Functional Reserve Capacity Calculator

About this calculator

Critical power (CP) and W′ (pronounced "W prime," also called functional reserve capacity or the anaerobic work capacity) come from the two-parameter critical power model: CP is the highest power you could theoretically sustain indefinitely without fatigue accumulating, and W′ is a fixed, finite store of work — measured in kilojoules — above CP that gets depleted during hard efforts and slowly refills during easy ones. The model is fit from two maximal, all-out time-to-exhaustion tests at different durations (commonly 3 and 12 minutes), plotting power against time to solve for both parameters.

Cyclists and runners training with power meters use CP and W′ to pace efforts precisely: CP functions like a more physiologically grounded version of functional threshold power (FTP), while W′ tells you exactly how many kilojoules of "anaerobic battery" you have left for surges, attacks or a finishing sprint, and how long you can hold any power above CP before it runs out.

This calculator takes the power and duration from two maximal efforts and returns your critical power, your W′ (FRC), and the time to exhaustion you can expect at any given wattage above CP.