# Reverse FTP measurement

Often it is argued that an 8 or a 20 minute effort can be sufficient to estimate Functional Threshhold Power (FTP), an indicator of how many watts a rider can put out over the course of an hour (or 40 minutes, depending on who you ask).

For a 20 minute effort, it is often argued that a 5% reduction over the 20 minute wattage gives an accurate estimate of the maximum wattage for the 40-60 minute effort.

For the 8 minute effort, an additional 5% (for a total of 10%) is taken off the total wattage over 8 minutes.

My question is as follows: does the reverse also hold? Suppose I do an 80 minute test, or a 160 minute test, will a respective 5 or 10% increase in the wattage over that period be my FTP?

• Probably pretty close, but FTP is an estimate, and you're estimating an estimate, which will have a margin of error. You'd have a hard time putting out your FTP for such long times - you want a quiet and non-stop, flat road for perhaps 40 km. That's hard to find, hence the shorter heuristics.
– Criggie
Jul 11 '17 at 0:02
• I wouldn't put much faith in any of those numbers. Jul 11 '17 at 3:33
• Worth noting that the 8min protocol isn't simply 90% of 8min wattage, but 90% of the average of 2 8min efforts in the same session. Additionally, you'll have a tough time convincing someone to do a 80 or 160min test to estimate the power they can hold for 40-60min. Better to just do a 60min test and forget the "estimation" portion. Jan 14 '20 at 20:50