Critical Power Calculator

Critical Power is the highest power you can hold without your effort drifting toward exhaustion. It sits close to FTP but comes from real test data instead of a single percentage, and it ships with a second number FTP ignores: W-prime, the work you can spend above CP. The calculator above takes two hard efforts and returns both.

What Critical Power is

The two-point model takes your two maximal efforts and separates the power you can hold all day (Critical Power, in watts) from the fixed reserve you can spend above it (W-prime, in kilojoules). Think of W-prime as a matchbook. Burn it on a climb or a sprint and you have to ride below CP to build it back.

• Power 1 over Time 1: a short maximal effort, for example 3 minutes all out.
• Power 2 over Time 2: a longer maximal effort, for example 12 minutes all out.

Both efforts feed the model. Get either one wrong and both outputs move.

Reading CP and W-prime

Your CP is the power you can defend for a long climb or a steady time trial. Ride at it and you stay in control. Ride above it and you start draining W-prime, with the clock running.

W-prime tells you how much you have for the surges. A rider with 20 kJ above CP can spend it on one long attack or three short ones. Watch your CP number against your FTP too. They usually land within a few percent of each other, so a CP far above your known FTP means a test was paced badly, not that you found free watts.

The mistake that wrecks the result

The two efforts have to be genuinely maximal and far enough apart in duration. Three and twelve minutes works. Three and five does not, because the durations sit too close and the math gets unstable.

Do both on the same day, or within a day or two, and rest hard between them. A submaximal first effort inflates W-prime and drops CP. Test fresh, pace each one honestly, and the numbers hold.

For how CP, FTP, and the energy systems behind them fit together, read the training science guide.