The wattage you can hold for roughly an hour. Every training zone derives from it — estimated as 95% of your 20-minute average power.
FTP anchors the whole training structure: endurance sits below roughly 75% of it, tempo at 76–90%, threshold at 91–105%, VO2max at 106–120%. Rather than riding a full hour flat out, you take 95% of your 20-minute average — a 300-watt average gives you 285 watts of FTP. The most common mistake is going out too hard: you blow up after ten minutes and end up measuring your pacing rather than your threshold. Warm up for 15 to 20 minutes with a few short hard efforts, ride evenly, and test on fresh legs instead of at the end of a heavy week. One caveat that gets glossed over: FTP correlates well with real performance but is not the same thing as a lab-measured lactate threshold — it is a field value, not a physiological law. Repeat it every four to eight weeks under identical conditions.