Critical Power Calculator

Critical power is the highest power output you can sustain for a long time without fatiguing further, measured in watts. On the power-duration curve it is the asymptote that very long efforts approach. It marks the boundary between the heavy and severe intensity domains, so any effort above CP relies on your finite W' reserve and ends in exhaustion.

It fits the two-parameter work-time model. For each effort, total work equals average power times the duration in seconds, and that work follows a straight line: work equals W' plus critical power multiplied by time. Your two efforts are two points on that line, so the slope is critical power and the vertical intercept is W'. With two points the solution is exact, so no curve fitting is needed.

W' is your anaerobic work capacity, a fixed amount of work in joules that you can do above critical power before reaching exhaustion. Think of it as a battery that drains when you ride above CP and recharges when you drop below it. A typical trained cyclist has a W' of roughly 15,000 to 25,000 joules (15 to 25 kJ), though it varies widely between athletes.

Use two all-out efforts of different lengths, both hard enough to reach exhaustion, such as 3 minutes and 12 minutes. The classic recommendation is to keep both efforts in the severe-intensity domain, roughly 2 to 15 minutes, and to space them far enough apart in duration for a stable fit. Enter the shorter effort at its higher power and the longer effort at its lower power, and rest fully between tests.

The model is most accurate within the range it was built from, roughly 2 to 20 minutes. For much longer durations it tends to overestimate sustainable power, because fatigue mechanisms the two-parameter model ignores start to matter. Treat the predictions as training estimates from two field tests, not a substitute for a lab measurement or a fresh maximal effort.

They are closely related but defined differently. FTP is the power you can hold for about an hour, usually estimated from a 20-minute test, while critical power comes from the two-parameter model fit to two or more maximal efforts. CP often lands a few percent above FTP, so the two are similar but not interchangeable. Use whichever metric your training plan is built around and stay consistent.