FTP and power zones: estimate, train, avoid traps
FTP is the anchor of almost every power-based number: zones, Intensity Factor, TSS. Getting it roughly right matters more than getting it precisely right, and you can estimate it from rides you already do.
What FTP is
Functional Threshold Power (FTP) is, in theory, the highest average power you could hold for about one hour, as close as it gets to the boundary between a hard-but-sustainable effort and one that quickly drains you. It is a convenient simplification, not a physiological constant: depending on your training and your day, your real one-hour power can sit a few percent either side. It still works well as a reference because it is stable enough from week to week to hang everything else on.
FTP is expressed in watts, and also relative to body mass. A rider with an FTP of 250 W and a mass of 70 kg has 3.57 W/kg. Relative power is what matters on climbs; absolute power matters on the flat.
Estimating FTP from ordinary rides
A true one-hour effort is unpleasant and rarely done. The shortcut most people use is a hard 20-minute effort and 95 % of its average power:
FTP ≈ 0.95 × best 20-minute power
Example: your best 20 minutes in a month was 263 W, which gives 0.95 × 263 = 250 W. The 5 % discount acknowledges that you can hold a bit more for 20 minutes than for an hour.
You do not have to schedule a test. GearWatts looks at the power curve of every ride and, for each month, takes the best 20-minute power you produced and applies the 0.95 factor. The result is a monthly FTP line you can read as a trend: rising, flat or falling. On a single ride, when you have not set an FTP in your profile, GearWatts uses the one stored in the file if there is one and otherwise falls back to the same 95 % estimate of that ride; if you know your value, put it in your profile so every ride uses the same yardstick.
The power curve
The power curve (or mean-maximal power curve) shows, for each duration, the best average power you produced: 1 second, 5 seconds, 1 minute, 5 minutes, 20 minutes, 60 minutes and so on. GearWatts computes the best average for durations from 1 second up to 4 hours. The curve always falls from left to right, and its shape tells you what kind of rider you are:
- High values at 5–30 s: sprinting ability.
- High values at 1–5 min: anaerobic and VO2max capacity, useful for short climbs and attacks.
- High values at 20–60 min: threshold, the number FTP is derived from.
- Values at 2 h and beyond: durability, what you can hold deep into a long ride.
Comparing a recent period against the previous one or against your best-ever curve shows where you gained or lost. A 5-minute value that rose 15 W while the 20-minute one stayed flat tells you something about your recent training that your average speed never will. It also explains why the monthly FTP can be flat even while you feel stronger: you may have been training something else. Remember that a curve point only exists if you gave an effort of that length; a missing improvement may just mean no hard effort.
The seven power zones
The classic model divides intensity into seven zones as percentages of FTP. GearWatts uses these boundaries, and the table shows them for an FTP of 250 W.
| Zone | % of FTP | Watts (FTP 250) | Typical use |
|---|---|---|---|
| Z1 Recovery | below 55 % | up to 137 | Spinning, warm-up, recovery days |
| Z2 Endurance | 55–75 % | 138–187 | Long steady rides, aerobic base |
| Z3 Tempo | 75–90 % | 188–224 | Sustained moderate-hard riding, group pace |
| Z4 Threshold | 90–105 % | 225–262 | Intervals of 8–30 min around FTP |
| Z5 VO2max | 105–120 % | 263–299 | Intervals of about 3–8 min |
| Z6 Anaerobic | 120–150 % | 300–374 | Efforts of 30 s to 3 min |
| Z7 Neuromuscular | above 150 % | 375 and up | Sprints under 30 s |
Zones 1 to 4 are the bulk of the training time for most riders: most of the volume in Z2, with quality in Z4. Zones 5 and 6 are short, demanding sessions with long recoveries; Z7 is about maximum effort and is usually described by peak power, not by time in zone, because the watts change second to second. The zone boundaries are population conventions. Your own limits will not match them to the watt, and a week of tracking how you feel in each zone is worth more than a debate over 2 %.
GearWatts shows time in each zone for every ride and for a period. To see how the load adds up over weeks, read Normalized Power, TSS and the fitness–fatigue chart.
When FTP estimates mislead
- You never gave a hard effort. The estimate is a floor, not your real number. A month of endurance rides gives a low FTP, and the zones built on it will make every ride look harder than it was. A sensible check: if the monthly value drops in a month you only rode easy, ignore the drop.
- Bad pacing. Starting a 20-minute effort too hard and fading, or leaving too much in the tank, lowers the average. Even pacing is what the 0.95 factor assumes.
- Riders with a strong sprint. The factor can overestimate FTP when a high anaerobic capacity props up the 20-minute number. Riders who are better over long durations, the opposite: 0.95 may underestimate.
- Indoor versus outdoor. Indoors you often produce less power at the same effort, because heat and the lack of airflow limit you, and the trainer's own power may differ by a few percent from your power meter. Outdoors, terrain changes and coasting alter the shape of efforts. Keep the data from the same source when you compare months.
- Power meter calibration. A meter that has not been zeroed, or drifted with temperature, moves your FTP and zones along with it. Zero before the ride and compare against another source if numbers jump.
- Short rides. A ride with less than 20 minutes of data has no 20-minute point; in that case there is no estimate at all.
Treat the estimate as a trend line and as a starting point, update your profile when a new best 20 minutes holds up over several weeks, and let the zones follow. A modest, consistent FTP you trust beats a perfect one you cannot reproduce.