The formula
FTP = average power over 20 minutes × 0.95
Where it comes from
FTP is the power you can hold for an hour, and measuring it properly means an hour flat out, which hardly anyone is willing to do. The twenty-minute test is the practical alternative: take 95% of the average power over those twenty minutes. The 5% taken off is the estimated gap between what you can hold for twenty minutes and what you can hold for sixty.
How to work it out by hand
- Warm up properly and ride twenty minutes at the highest power you can sustain
- Note the average power over those twenty minutes
- Multiply it by 0.95
- Apply each zone's percentage to that number
What is worth knowing
Watts per kilo say more than bare watts as soon as there is a hill involved: on the flat absolute power rules, because what you are beating is the air, and uphill the ratio to weight rules, because what you are beating is gravity. A fit amateur sits around 3 to 3.5 W/kg of FTP; professionals go beyond 5.5. And it is worth repeating the test every six to eight weeks: training on an old FTP means training in the wrong zone, almost always too easy.