Comparing FTP Testing Methods for Accuracy

Functional Threshold Power (FTP) is the cornerstone metric for training with a power meter. It determines your training zones, measures progress, and provides objective evidence that your fitness is improving—or not. But there’s no single universally accepted way to test it. Three primary protocols dominate: the ramp test, the 20-minute test, and the 8-minute test. Each produces slightly different numbers, and understanding why matters for training accurately.

The Ramp Test: Fast, Painful, Questionable

The ramp test increases power output at regular intervals (usually every minute) until you can’t continue. Software analyzes where you failed and estimates your FTP from the highest sustained power achieved, typically around 75% of your peak one-minute power from the test.

TrainerRoad and Zwift both use variations of ramp testing as their default FTP assessment. The appeal is obvious: it’s over quickly (usually 20-25 minutes including warm-up), requires minimal pacing strategy, and produces consistent results when repeated.

The problem is that ramp tests measure your ability to produce power across a range of intensities, then derive FTP through a formula. That formula assumes a standard relationship between short-duration max power and threshold power. For most people, it’s reasonably close. For some, it’s significantly off.

Athletes with a strong anaerobic system—sprinters, track cyclists, or anyone with explosive power—tend to test artificially high on ramp tests. Their exceptional 1-minute power skews the calculation upward, resulting in an FTP estimate they can’t actually sustain for an hour. Conversely, diesel-engine endurance types with modest sprint capacity might test low because their true threshold endurance doesn’t match the formula’s assumptions.

The 20-Minute Test: The Traditional Standard

The classic 20-minute FTP test involves a proper warm-up, a 5-minute all-out effort to clear the anaerobic system, a short recovery, and then a maximal 20-minute effort. Your FTP is calculated as 95% of your average power during that 20-minute block.

This protocol comes from the work of Dr. Andrew Coggan, who developed the power-based training zones most cyclists use today. The 5% reduction accounts for the fact that most people can sustain slightly more power for 20 minutes than they can for a full hour—the difference between a long hard effort and true physiological threshold.

The 20-minute test more directly measures your sustained threshold capacity. Pacing matters: start too hard and you blow up before the end, producing an artificially low FTP. Start too conservatively and you finish with energy left over, also underestimating your actual threshold. Nailing the pacing requires experience or several practice attempts.

For athletes who race or ride at threshold frequently, the 20-minute test tends to be the most accurate. It measures exactly the physiological quality FTP represents—sustained high-intensity power output. The weakness is the same as its strength: it requires an hour of high-quality effort including warm-up and prep interval, and it’s brutal enough that most people avoid doing it more than a few times per season.

The 8-Minute Test: Twice the Pain, More Data Points

The 8-minute protocol involves two all-out 8-minute efforts separated by 10 minutes of easy spinning. FTP is calculated as 90% of the average power across both intervals. If your first interval averages 300 watts and your second averages 290 watts, you take the mean (295 watts) and multiply by 0.9, giving you an FTP of 265.5 watts.

This test attempts to capture threshold power while being short enough to avoid the pacing difficulties of a full 20-minute effort. The logic is that most athletes can hold slightly above threshold for 8 minutes, especially when motivated by the knowledge that it’ll end soon.

Two intervals instead of one also helps average out bad pacing or a single poor effort. If you start too hard on the first interval and fade, the second interval—presumably paced better—balances it out. The reverse is also true: if nerves cause a conservative first effort, you can make up ground on the second.

The weakness here is that 8 minutes dips significantly into anaerobic contribution. Athletes with strong anaerobic capacity can push much harder for 8 minutes than they can sustain for an hour, making the 90% multiplier too generous. For pure endurance types with limited anaerobic reserves, 90% might be too conservative, underestimating true FTP.

Which Test Is Most Accurate?

For most cyclists, the 20-minute test provides the most accurate FTP estimate because it most closely resembles the actual physiological state FTP describes. It’s long enough to deplete anaerobic reserves and force you to operate primarily on aerobic metabolism, but short enough to be sustainable as a regular test.

The ramp test is best for convenience and consistency. If you test every 4-6 weeks and track trends over time, small inaccuracies in the absolute number matter less than whether it’s moving up or down. Use it when you need a quick check-in or when testing frequently to monitor training response.

The 8-minute test works well for athletes who struggle with pacing or who find 20 minutes too demoralizing. It’s also useful if you’re coming back from injury or illness and don’t want to commit to a full 20-minute suffer-fest before you’re sure you’re ready.

None of these tests is wrong. They measure slightly different things and produce slightly different results. The key is choosing one protocol and sticking with it. An FTP that slowly climbs from 250 to 270 over three months using ramp tests represents real improvement, even if a 20-minute test would have given you 260 and 280 instead.

Testing Conditions Matter More Than Protocol

Whichever test you choose, keep the conditions as consistent as possible between tests. Use the same bike, trainer, or outdoor route. Test at the same time of day when possible—FTP can vary by 5-10 watts between morning and evening for some athletes. Be similarly rested (or similarly fatigued) each time.

Outdoor vs indoor testing presents another variable. Wind resistance, terrain variation, and pacing challenges make outdoor FTP tests harder to standardize, but they might better represent real-world performance. Indoor tests eliminate variables and make power targets easier to hit precisely, but they can produce slightly inflated numbers because you’re not dealing with wind, braking, or cornering.

Whatever you choose, document it. “FTP = 265 watts from an indoor 20-minute test done at 9 AM after a rest day” is much more useful than just “FTP = 265 watts” when you’re trying to compare results three months later.

When Your FTP Feels Wrong

Sometimes test results don’t match reality. Your training zones feel too hard or too easy, intervals that should be sustainable aren’t, or workouts prescribed at 90% FTP feel like Zone 2. When this happens, trust your experience over a single test result.

Retest using a different protocol. If the ramp test gave you 280 watts but threshold intervals feel impossible above 250 watts, try a 20-minute test. If that confirms the lower number, use it. The test serves the training, not the other way around.

Also consider that FTP isn’t static. Fatigue, stress, poor sleep, or training overload can temporarily suppress your numbers. A bad test doesn’t necessarily mean you lost fitness—it might mean you need a recovery week before testing again.

Beyond FTP: What the Tests Don’t Tell You

All three protocols focus on threshold power, but threshold is just one point on your power curve. Two cyclists with identical 250-watt FTPs might have wildly different 5-second sprint power, 5-minute VO2 max power, or 4-hour endurance capacity.

FTP testing tells you where to set your training zones for threshold and tempo work. It doesn’t reveal whether your weakness is sustained climbing power, short punchy accelerations, or multi-hour endurance. For that, you need a broader view of your power profile across different durations.

Use FTP as the foundation it is—an important metric that enables structured training—but don’t let it become the only metric you care about. The number matters less than what you can do with the fitness it represents.

How Often Should You Test?

Testing every 4-6 weeks during focused training blocks makes sense. More frequent than that and you’re spending valuable training time on testing rather than adapting. Less frequent and your training zones might drift out of alignment with your current fitness.

If your FTP is improving rapidly (new to power training, coming back from time off, responding well to a new program), test more often—every 3-4 weeks. If it’s stable or improving slowly (experienced athlete in maintenance phase), every 6-8 weeks is sufficient.

Skip testing during recovery weeks, immediately after illness or injury, or when life stress is high. You’ll just produce a depressed number that doesn’t reflect your actual fitness, which creates confusion about whether to adjust training zones or not.

The right answer is whichever protocol you’ll actually do consistently, under similar conditions, often enough to track meaningful changes. Perfect accuracy matters less than reproducible trends. Pick your test, stick with it, and let the data accumulate. The patterns reveal more than any single number ever could.