Sub-Max FTP Calculator

As I’ve said, I don’t like to test. With 20+ years of data to draw on, I can usually peg FTP by looking at what I’m doing at LTHR -30 and LTHR - 15.

I found this after listening to a Couzens podcast. His calculator matches up with a lot of the data points that I have from over the years. YMMV.


I can’t quite work the maths out.

It seems like the lower power value for 65% heart rate, the more your FTP goes up. So if I put 210 as my 75% and 180 as my 65%, it gives an FTP of 240. But if I reduce the 65% HR power to 140, my FTP goes up to 280.

Does that make sense?

That sounds a lot like there’s a linear plot between your power values, which is used to calculate FTP. Such that the greater the difference in power between the two, the steeper the slope of the line, and thus the higher the resulting FTP.

So if you set the same power for 65% and 75%, it simply reports that power as your FTP.

It probably provides a decent estimate, for data that falls in the expected range, for athletes with the type of power profile that they’re trying to estimate. Personally, I’d rather just adjust FTP based on RPE during workouts, if I didn’t want to test anymore.

If I’m understanding, it’s meant to set a baseline in the early season (base) when there haven’t and shouldn’t have been maximal efforts recorded yet.

Seems reasonably accurate using my own numbers. If someone’s doing intensity without having much real aerobic foundation probably not too useful.

I just plugged some of my numbers in from some workouts in the last couple of weeks and it gives me an “FTP” of 340W as opposed to the 265W i’m using in TR. Rather than an hour at 340W I can manage about 4 minutes! Not much use to me I’m afraid

This is what I’d expect most folks will see unless they are doing very long Z2 and longer SS rides.

It was spot on for me, but I’m riding mostly subthreshold and very high volume. I’ve held 90% FTP for 3hrs and 94% FTP for 90mins in the past week during a build block so my aerobic system is pretty well developed and my top end is lacking. Pretty flat power curve.

@Tanner1280 I might have missed this from you discussing it in another post (if so, apologies), but how do you know you haven’t just increased FTP? Are you testing? If not, what’s to say you’re not just holding tempo (or whatever) for 3hrs?

@tshortt I’ve increased it quite a bit. I was 315w on the TR ramp test in Jan and I’m at 330-340w now. This is somewhat conservatively based off of a 20min effort about 2000kj into a ride. I’ve done 311w for 60min 305w for 90 and 295w for 180min last week during a trainer ride just under 4hrs.

I’ve been more surprised that my 5 and 1min power came up too.

I was thinking it was off myself but talking to a friend who coaches pro IM athletes he said it’s in line with the volume and type of training I’m doing lately.


Finally got round to giving this a go. Reason was that I wanted to get an idea of my FTP on my TT bike without taking a ramp test in the middle of a road bike training block.

I found I had to push quite hard to get my heart rate up to the 2 levels, and then needed to back off quite a bit to keep the heart rate from going over. So when I took the average watts from the 2 10min intervals, the result was a bit high.

However, when I took the average watts from the last 5 minutes of each interval, it gave me a value about 8 watts lower than my last Trainer Road road bike ramp test. Considering that I think I might have added 5 watts in the 3 week block since then, that would put me at about 13 watts below road bike FTP which seems like a pretty realistic place to start.

Yes because the curve becomes steeper - you have increased by more power between 65 and 75 so the jump to 85 will be more. You are using hr reserve I believe or kervonian