How do you calculate FTP from a 20-minute test?
Multiply the average power of an all-out, evenly paced 20-minute effort by 0.95: hold 260 W for the test and your estimated FTP is 247 W. The 5% haircut exists because 20 minutes is short enough to lean on your anaerobic reserves; FTP is meant to be the power you could hold for roughly an hour, where those reserves barely register. That single multiplication is what every FTP calculator does — the useful part is knowing when it lies to you.
Functional Threshold Power is the highest power you can hold in a quasi-steady state — hard, but metabolically stable — for on the order of an hour. It's the anchor for training zones, TSS and interval targets, which is why one accurate number beats a flattering one: every workout you build from an inflated FTP is slightly too hard, and the wheels come off in week three, not day one.
Why 95% might be too generous
The ×0.95 factor is an average across riders — and the average hides the interesting part. A punchy rider with a big sprint and strong 5-minute power funds a meaningful slice of a 20-minute test anaerobically, so their 20-minute average overstates what they can hold for an hour. Coaches quietly use 0.93 or even 0.90 for that rider type. On a 300 W test, the choice matters:
| Factor | Estimated FTP | Who it fits |
|---|---|---|
| × 0.95 | 285 W | Steady diesel riders, well-paced test |
| × 0.93 | 279 W | Balanced riders, or a slightly hot start |
| × 0.9 | 270 W | Punchy riders with a big sprint and 5-min power |
Same 300 W test, computed with the calculator's engine. If Zone 4 intervals off your ×0.95 number always fall apart, you've met your correction factor.
The ramp test: convenient, and flattering
Trainer platforms love ramp tests — ten-ish minutes, no pacing skill needed. The estimate is 75% of your best one-minute power at the end of the ramp. But the finish of a ramp is an anaerobic event, so the same punchy riders the 20-minute test flatters get flattered harder here, while steady diesels often test low. The spread is real: a rider who finishes a ramp at 390 W gets told 293 W, holds 271 W from a 20-minute test — and can actually sustain about 260 W for an hour.
The three estimates above span 33 watts — roughly 13% — for one rider on one week of form. That's not a broken formula; it's three different physiological questions wearing the same name. Pick one protocol, note the number, and retest with the same protocol — the trend is worth more than the value.
The 8-minute test
Two all-out 8-minute efforts with recovery between; FTP is 90% of the better one. It hurts less to pace than 20 minutes and suits riders who can't face the long version — but 8 minutes leans even harder on anaerobic power, hence the deeper 10% haircut. Treat its answer the same way: a starting point to confirm in training.
Which FTP test should you use?
| You are… | Best test | Why |
|---|---|---|
| New to structured training | Ramp | No pacing skill needed; repeatable |
| A steady, diesel-style rider | 20-min (or 60-min) | Long tests read you accurately |
| Punchy — big sprint, strong 5-min | 20-min at ×0.93 | Ramps and ×0.95 both flatter you |
| Experienced and honest with pain | 60-min | It's the definition — no factor, no debate |
Whatever you pick, keep picking it: zone drift you can trust beats a bigger number you can't.
What your FTP unlocks
An FTP of 247 W puts your endurance riding at 137–185 W and threshold work at 223–259 W — the power zones calculator lays out all seven, with a printable sheet for the trainer wall. And whether your number is good is mostly a question of watts per kilogram — the W/kg calculator has the published benchmark table. Save your result above once and both tools open already knowing it.