Functional Reserve Capacity Calculator

How it is calculated

The tool solves the two-parameter critical power model from two all-out efforts. Times are entered in minutes and converted to seconds (s1 = t1 × 60, s2 = t2 × 60). Then W′ = (P1 − P2) × s1 × s2 ÷ (s2 − s1) in joules and CP = (P2 × s2 − P1 × s1) ÷ (s2 − s1) in watts. The first effort must be the shorter and stronger one: if t2 is not longer than t1, or P1 is not above P2, there is no result. W′ is shown in kJ and J, CP and W′ are also divided by body weight (70 kg when empty). If a target power above CP is entered, time to exhaustion is W′ ÷ (target − CP) in mm:ss; otherwise a dash is shown.

Common mistakes

• Entering the efforts in the wrong order. The short, harder effort goes in P1/t1 and the longer, easier one in P2/t2, otherwise there is no result.
• Using times in seconds. The fields are minutes, so a 3 minute test is 3, not 180.
• Feeding in efforts that were not maximal. A pacing test that left something in the tank understates both CP and W′, and the two-point fit amplifies even small errors in the power values.
• Choosing two efforts that are too close in duration, such as 5 and 6 minutes: the subtraction in the formulas then magnifies noise, so CP and W′ swing wildly.
• Reading a dash for time to exhaustion as an error. It simply means the target is empty or at or below CP, a power that the model says could be held without draining W′.