How the formula works
Functional Threshold Power is defined as the highest power output you could sustain for approximately one hour in a true time trial. Riding all-out for a full hour is logistically and mentally brutal, so Hunter Allen and Andrew Coggan introduced the 20-minute proxy in Training and Racing with a Power Meter (2010): a 20-minute maximal effort, multiplied by 0.95, approximates 1-hour power for most trained cyclists.
The 0.95 factor exists because a 20-minute effort recruits a small but non-zero share of anaerobic capacity (your "W'" reserve, in critical-power language) that you cannot tap continuously for an hour. Coggan's calibration data show this anaerobic contribution averages roughly 5% of 20-min power across trained riders.
Coggan power zones (% of FTP)
- Z1 — Active recovery: <55% FTP
- Z2 — Endurance: 56–75% FTP
- Z3 — Tempo: 76–90% FTP
- Z4 — Threshold: 91–105% FTP
- Z5 — VO₂max: 106–120% FTP
- Z6 — Anaerobic capacity: 121–150% FTP
- Z7 — Neuromuscular power: >150% FTP (durations <30 s)
What it's good for
- Anchoring all your training prescriptions. Once FTP is set, every workout in polarized or pyramidal programming has a specific power target.
- Computing TSS for any ride — see the TSS calculator.
- Tracking fitness across a season. A 10–15 W FTP gain over a 12-week build is the canonical sign that base + threshold work is paying off.
- W/kg comparisons. Watts per kilogram dictates climbing performance; flat-time-trial performance is closer to absolute watts (drag is the dominant resistance term).
Limitations
- Pacing dependency. A 20-min test that goes out at 110% of true threshold and fades will produce a lower 20-min average than a properly paced effort. Use a power target from your previous best 20-min effort plus 5–10 W as your starting pace.
- Anaerobic outliers. Sprinters with high W' often beat 0.95 × P20 in a 1-hour test (their anaerobic reserve is bigger). Steady-state diesel riders sometimes can't quite hold 0.95 × P20. The factor is a population mean.
- Power meter calibration. A 1% offset between meters means a 3 W FTP error at 300 W. Calibrate (zero-offset) every ride and use the same meter for testing and training.
- FTP is not MLSS. They're close but not identical. For more precision, a multi-day Maximal Lactate Steady State protocol or a critical-power 3+12 min test gives a tighter estimate (Zone 2 vs LT1).
FTP in your AthleteOS dashboard
AthleteOS pulls every cycling workout from Garmin, Strava or Wahoo, runs an automatic critical-power model on your last 90 days of efforts, and updates your FTP continuously — no formal retest required. We also compute TSS and CTL/ATL/TSB for every ride, plus issue an alert when your power-duration curve shifts in a way that suggests fitness gain or fatigue accumulation. Generate your free AI plan and your bike workouts will be prescribed in W from your live FTP.
Related reading: Polarized vs pyramidal · Aerobic decoupling · TSS vs TRIMP · Brick workouts for triathlon.
Citation
Allen H, Coggan AR (2010). Training and Racing with a Power Meter, 2nd ed. VeloPress. ISBN 978-1934030554. Updated zone definitions in 3rd ed. (2019) ISBN 978-1937715939.