20-minute vs 8-minute FTP test: which one to do and what the multipliers mean

The two protocols, side by side

The 20-minute test is one maximal 20-minute effort, with FTP set at 95% of the average power [Allen et al. 2019]. The CTS 8-minute test is two 8-minute all-out efforts split by 10 minutes of easy spinning, with FTP set at 90% of the higher effort's average power [Carmichael & Rutberg 2017]. Same target: roughly your 60-minute power.

The 20-minute protocol is the one most amateurs meet first, on Zwift or in TrainerRoad. After a thorough warm-up you ride one 20-minute time trial as hard as you can hold evenly, then multiply the average power by 0.95. The 5% haircut exists to approximate the drop-off between 20-minute power and the hour of power that FTP is meant to represent [Allen et al. 2019]. It is a single number from a single effort, which makes it simple and also fragile.

The 8-minute protocol comes from Chris Carmichael and Jim Rutberg's The Time-Crunched Cyclist [Carmichael & Rutberg 2017]. After warming up you ride two 8-minute efforts all-out, separated by about 10 minutes of easy recovery, and CTS sets training intensities from 90% of the average power of the higher of the two efforts. Some coaches average both efforts instead; either way the reduction is 10%. Two shorter efforts also reveal how well you recover and repeat hard work, which one long effort cannot.

Both protocols chase the same physiological quantity: the highest power a rider can sustain in a quasi-steady state for roughly an hour, the definition Andrew Coggan gave FTP [Allen et al. 2019]. Neither test lasts an hour, because a true 60-minute time trial is brutal and rarely repeated. Both are field surrogates that lean on a fixed multiplier to bridge the gap — and the multiplier is where they part ways.

Why the multiplier differs: 95% versus 90%

A shorter maximal effort produces a higher average power relative to true threshold, so it needs a bigger discount. Eight minutes sits well into the severe-intensity domain near VO2max power, drawing more on finite anaerobic work capacity — W-prime — than a 20-minute effort does. The extra 5% subtracts that anaerobic contribution [Karsten et al. 2021].

On the power-duration curve, the shorter the effort, the further it sits above your sustainable threshold and the larger the anaerobic share of the work. Karsten and colleagues [Karsten et al. 2021] measured W-prime at 16.6 kJ in trained cyclists — worth only about 4.6 watts when spread across a full hour, but a far larger slice of an 8-minute effort. They concluded the two-parameter critical power model's validity should be limited to durations under 20 minutes, precisely because anaerobic contribution distorts shorter efforts. The 90% multiplier is the correction for that distortion; the 20-minute test needs only 5%.

Field tests already read high against physiological threshold, and shorter efforts read higher still. Vinetti and colleagues [Vinetti et al. 2023] found that in junior road cyclists, even the 95%-adjusted 20-minute field FTP came in around 14% above laboratory critical power (269 versus 236 watts), on a steep outdoor climb. FTP correlated almost perfectly with aerobic power (MAP, r = 0.95) but also moderately with the anaerobic W-prime (r = 0.66). Compress the effort to 8 minutes and the anaerobic lean grows, which is why the multiplier has to work harder.

This is why the two numbers are not interchangeable. Karsten and colleagues [Karsten et al. 2021] reported limits of agreement from minus 19 to plus 33 watts between critical power and 20-minute FTP — a spread of method noise before fitness even enters. Switch from the 8-minute to the 20-minute protocol mid-season and your FTP can move several percent from the change of method alone. Pick one and stay with it, so a moving number means moving fitness.

Which test fits which rider

Diesels — riders whose threshold sits close to their peak aerobic power — pace long efforts evenly and are best served by the 20-minute test. Anaerobically-gifted riders and novices, who fade or blow up over 20 minutes, get a steadier number from the 8-minute test, though its short duration over-weights their anaerobic strength.

The 20-minute test rewards pacing discipline, which most amateurs lack. Borszcz and colleagues [Borszcz et al. 2022] showed that warm-up structure alone shifts the resulting 20-minute FTP by clinically meaningful amounts, and pacing errors do more damage than that: a rider who goes out too hard and cracks at minute 14 records a falsely low number. A diesel with an even power delivery holds the 20 minutes comfortably; a punchy rider fights it the whole way.

The 8-minute test is easier to pace and to repeat, which is why CTS recommends it for novices and age-group racers. Eight minutes is short enough that even inexperienced riders can hold a near-even effort, and two efforts give a repeatability check. The tradeoff is the one the ramp test also suffers: because 8 minutes leans anaerobic, a sprinter-type gets a flattering number that the 90% multiplier may not fully rein in, so a ramp-test FTP that reads too high tends to read high here too.

The honest caveat is that neither field test gives ground truth. Borszcz and colleagues [Borszcz et al. 2018] found that on an individual basis the 20-minute estimate and a real 60-minute effort disagreed by 40 to 60 watts — for a 250-watt rider, the gap between a productive session and a blown one. The 8-minute test stacks a second correction on top of a shorter, noisier effort. Both are starting estimates to be refined, not verdicts to be trusted.

Why you may not need either test

Both the 20-minute and 8-minute tests are protocols, not the definition of FTP — the same point the parent guide makes. If you already ride with power, the near-maximal efforts sitting in your Strava history can estimate threshold power within a few percent, without scheduling a dreaded all-out test day at all.

FTP is defined as roughly 60-minute power [Allen et al. 2019]; the 20-minute and 8-minute tests are just two field routes to that estimate, each with its own multiplier and its own error. As the broader guide on estimating FTP without a test argues, the data your power meter has already collected is usually enough to model threshold from the power you actually produced. A power-duration model fit to months of riding draws on dozens of efforts of varying length instead of one anxious afternoon.

AdaptCycling reads whatever efforts you did — a 20-minute climb, a hard 8-minute segment, a 12-minute time trial — from Strava and folds them into a Critical Power fit, rather than forcing one protocol on you. If you happen to run a clean 8-minute or 20-minute test, that data simply enters the same curve; nothing is wasted and nothing is mandatory. The estimate refines itself as new rides arrive, and the free tier turns it into a real training plan.

A field test still earns its place as a periodic cross-check. When the modeled FTP and a clean field effort disagree by more than 5%, one of them is wrong, and the field test usually breaks the tie. Do that roughly twice a year rather than every six weeks, and whichever protocol you choose — 20-minute or 8-minute — keep it constant so the number reflects your fitness and not your method.

Quick answers

Sources cited in this guide

1. 01
   Allen et al. 2019. Training and Racing with a Power Meter (3rd ed.). VeloPress.
2. 02
   Carmichael & Rutberg 2017. The Time-Crunched Cyclist: Race-Winning Fitness in 6 Hours a Week (3rd ed.). VeloPress.
3. 03
   Borszcz et al. 2018. Functional Threshold Power in Cyclists: Validity of the Concept and Physiological Responses. International Journal of Sports Medicine.
4. 04
   Borszcz et al. 2022. Functional Threshold Power Estimated from a 20-minute Time-trial Test is Warm-up-dependent. International Journal of Sports Physiology and Performance.
5. 05
   Borszcz et al. 2019. Is the Functional Threshold Power Interchangeable With the Maximal Lactate Steady State in Trained Cyclists?. International Journal of Sports Physiology and Performance.
6. 06
   Karsten et al. 2021. Relationship Between the Critical Power Test and a 20-min Functional Threshold Power Test in Cycling. Frontiers in Physiology.
7. 07
   Vinetti et al. 2023. Functional Threshold Power Field Test Exceeds Laboratory Performance in Junior Road Cyclists. Journal of Strength and Conditioning Research.

See the direct comparisons

• AdaptCycling vs TrainerRoad →

More inside FTP without a test

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2. 02

   How to estimate FTP without a power meter

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3. 03

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   Strava, Xert, Intervals.icu, and TrainingPeaks can each report a different FTP. Why the estimates diverge and which one to trust.

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4. 04

   Is your ramp test FTP too high? Why it happens

   Ramp tests overestimate FTP for anaerobically-gifted riders and underestimate it for diesels. Why the 75% rule misfires and how to correct it.

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5. 05

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   How often to re-test FTP as a self-coached cyclist — twice a year, not every six weeks — and why modeled estimates change the cadence.

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