Multipliers: 95% for a 20-minute test, 90% for 8 minutes, 75% of one-minute peak power for a ramp test. A 60-minute test needs no correction - that is the definition.
Measure it instead of estimating it WattLog.PRO runs a guided ramp test that controls your trainer, watches your cadence for the failure point and writes the new FTP straight into your profile.
Open WattLog.PROHow the calculation works
Functional threshold power is the highest average power you could hold for roughly an hour. Almost nobody wants to ride an hour flat out to find out, so every practical protocol rides something shorter and applies a correction factor. A 20-minute all-out effort is multiplied by 0.95, an 8-minute effort by 0.90, and a ramp test takes your best one-minute power and multiplies it by 0.75.
Those factors are population averages, not laws of physics. A rider with a big anaerobic contribution will blow the doors off a short test and end up with an FTP estimate they cannot actually sustain; a diesel-engine time triallist often finds the opposite. The longer the test, the smaller the correction and the smaller the error - which is why the 20-minute protocol remains the most trusted compromise between accuracy and suffering.
Using the number
FTP is not a trophy, it is a scaling factor. Its real job is to set the boundaries of your training zones so that a prescribed interval lands at the physiological intensity it was designed for. If your FTP is 20 watts too high, every sweet-spot session you ride is secretly a threshold session, and the fatigue arrives weeks before the adaptation does.
Retest every six to eight weeks during a build, and whenever your rides start telling you something has changed - intervals that used to hurt feeling easy, or a familiar climb suddenly taking longer. Between tests, watch your perceived effort at a fixed power. That drift is usually a better early warning than the calendar.
Frequently asked questions
Which FTP test is most accurate?
The 60-minute test, because it needs no correction factor at all. In practice the 20-minute protocol is the best trade-off: the 0.95 multiplier is well validated and the effort is repeatable. Ramp tests are the least punishing but vary most between riders, because one-minute power depends heavily on anaerobic capacity.
Why is my indoor FTP lower than outdoors?
Cooling is the usual culprit. Without airflow your core temperature climbs, heart rate drifts and sustainable power falls, often by 5 to 10 percent. Position and motivation contribute too. Keep a separate indoor FTP rather than arguing with the number, and make sure you have a real fan.
How often should I retest?
Every six to eight weeks during a training block is plenty. Testing more often costs you a hard session each time and the change is usually inside the noise. Test after a rest period rather than at the end of a heavy week, or you are measuring fatigue instead of fitness.