# Calculating FTP from Strava Data



## Robert1 (Mar 27, 2012)

Can I get a decent approximation of my FTP from taking the 20 minute power value from Strava's "Best Efforts Table" for some hard rides and then subtracting 5% from that? I have no power meter.


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## Duane Gran (Feb 3, 2004)

In several discussions I've observed on this topic the only accurate sections will be those which are up hill where the gradient data is accurate. It matches up pretty well.

Aside from satisfying curiosity, knowing the number without having access to a power meter may not serve much purpose though.


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## Robert1 (Mar 27, 2012)

Just based on looking at the numbers for like 2 or 3 minute power in the flats I was moving at around 25mph just to get around 300W. There's no way at my weight of 170 I'd have to sustain 25mph for an hour just to reach an FTP of 300W, right?

I guess I was hoping to be able to track improvements through the data. Why is it not useful for that? I understand it doesn't take wind into account but if all else is the same shouldn't changes in performance show relative changes in the power data?


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## Cableguy (Jun 6, 2010)

Robert1 said:


> There's no way at my weight of 170 I'd have to sustain 25mph for an hour just to reach an FTP of 300W, right?


You've got it bass ackwards! Your speed would be the variable. And on a flat terrain with neutral wind, I think you'd actually have to do about 300w to average 25mph unless you were quite aero (i.e. aero bars). 



Robert1 said:


> I guess I was hoping to be able to track improvements through the data. Why is it not useful for that?


You're talking about using Strava's power estimation? It's not useful info because it's generally off by a *huge* margin, even on hills it's not very accurate. By yourself, not drafting anyone else, if Strava etimates you did 250w for X minutes, you could have very well actually done 200w or 300w. There are *many* variables to consider, and one of the big ones is the day to day changes in the wind... the difference between a 5mph headwind and 5mph tailwind is enormous, though you may not even be able to tell the difference.


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## RaptorTC (Jul 20, 2012)

I wouldn't recommend it as Strava's power numbers are not in the least bit reliable. According to Strava I held 564w for 10 minutes and 400w for my whole 2 hour ride a few weeks ago. I really don't think I'm that much of a hardman, haha.


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## Robert1 (Mar 27, 2012)

I understand that speed is the output based on the power. My point was that I figured 300W on the flats would not translate to speeds as high as 25mph. I'ts pretty rare that someone can hold 25mph+ for an hour yet you hear guys saying they have an ftp of 330 or 350 and unless there pretty heavy guys you'd have to have a pretty incredible speed in the flats with that kind of ftp.




Cableguy said:


> You've got it bass ackwards! Your speed would be the variable. And on a flat terrain with neutral wind, I think you'd actually have to do about 300w to average 25mph unless you were quite aero (i.e. aero bars).
> 
> 
> 
> You're talking about using Strava's power estimation? It's not useful info because it's generally off by a *huge* margin, even on hills it's not very accurate. By yourself, not drafting anyone else, if Strava etimates you did 250w for X minutes, you could have very well actually done 200w or 300w. There are *many* variables to consider, and one of the big ones is the day to day changes in the wind... the difference between a 5mph headwind and 5mph tailwind is enormous, though you may not even be able to tell the difference.


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## Robert1 (Mar 27, 2012)

Ya I''ve heard some pretty crazy results from some people on strava's power. Mine have been very consistent I just don't know how accurate the absolute values are. When I look at a stretch I did back in April all out compared to recently, my best 20 minute power was about 5W higher most recently and I set a PR so seems about right at least from a relative perspective.



RaptorTC said:


> I wouldn't recommend it as Strava's power numbers are not in the least bit reliable. According to Strava I held 564w for 10 minutes and 400w for my whole 2 hour ride a few weeks ago. I really don't think I'm that much of a hardman, haha.


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## Babb915 (Nov 1, 2012)

good to know that!


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## kamikazeDad (Sep 14, 2012)

I just started using Strava, and would agree that it is not a good measure of anything other than fun. I love the way it helps to motivate me to keep going at the level I want, but when I look at the power results they do vary too much to be useful as a guide. Best that you use some of the online calculators I have seen and find a training spot and use strava to get your times on that spot. The times you gather can then be used to estimate where you are with your fitness and FTP better than strava power. 

As for 300w to do 25 mph ... maybe, just depends on grade, wind, road, bike, weight, wheels, jersey fit ... etc  but that is not crazy.


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## Guod (Jun 9, 2011)

RaptorTC said:


> I wouldn't recommend it as Strava's power numbers are not in the least bit reliable. According to Strava I held 564w for 10 minutes and 400w for my whole 2 hour ride a few weeks ago. I really don't think I'm that much of a hardman, haha.


You sure you don't have your weight setting in kg with the number being in lbs? E.g. 170kg instead of 170lbs.

I did the same thing for my first ride on strava, not paying attention and ended up with some astronomical numbers for power while on a climb. My speed was normal though....


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## Undecided (Apr 2, 2007)

Robert1 said:


> I understand that speed is the output based on the power. My point was that I figured 300W on the flats would not translate to speeds as high as 25mph. I'ts pretty rare that someone can hold 25mph+ for an hour yet you hear guys saying they have an ftp of 330 or 350 and unless there pretty heavy guys you'd have to have a pretty incredible speed in the flats with that kind of ftp.


I'm just a few pounds lighter than that. I have a 25 mile course I've ridden a few times on my road bike in regular kit, with no special aero efforts. It's a rolling route that is a net descent over the first 15 miles and a net climb over the last 10 miles (almost all of each on very minor grades, e.g., 1%; a bit steeper on the climbing portion) and an overall net elevation loss of ~200 feet. Three rides around 300 watts: 25.6 mph at 314 watts, 25.4 mph at 305 watts and 25.4 mph at 310 watts (12 second slower than the preceding ride, to be precise). Wattage from a crank-based power meter.

So, sure, sounds about right.


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## Robert1 (Mar 27, 2012)

Thanks for that confirmation. My power data from strava has been pretty consistent. I was just questioning the absolute values. When I went to this site 

Bike Calculator

It was showing a speed more in the 22mph range for 300W on the flats. But maybe that's off. I guess in the end it doesn't really matter just as long as whatever method you use is consistent.



Undecided said:


> I'm just a few pounds lighter than that. I have a 25 mile course I've ridden a few times on my road bike in regular kit, with no special aero efforts. It's a rolling route that is a net descent over the first 15 miles and a net climb over the last 10 miles (almost all of each on very minor grades, e.g., 1%; a bit steeper on the climbing portion) and an overall net elevation loss of ~200 feet. Three rides around 300 watts: 25.6 mph at 314 watts, 25.4 mph at 305 watts and 25.4 mph at 310 watts (12 second slower than the preceding ride, to be precise). Wattage from a crank-based power meter.
> 
> So, sure, sounds about right.


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## robdamanii (Feb 13, 2006)

The most important thing to remember is that power is an output measure while HR is an input measure. Speed is too variable to be used reliably. 

If you're using HR as the basis for calculating power, it will be off depending upon sleep, hydration status, etc. 

This means that your power calculation will vary based upon extremely variable conditions. Not terribly reliable.


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## Robert1 (Mar 27, 2012)

Uh, well except strava doesn't use HR data in calculating it's power data. 

Assuming gps elevation data for a given ride is accurate, the main variable not accounted for by strava when calculating power is wind. All other things being equal, I've noticed my power data is pretty damn consistent


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## Alex_Simmons/RST (Jan 12, 2008)

Robert1 said:


> Assuming gps elevation data for a given ride is accurate, the main variable not accounted for by strava when calculating power is wind. All other things being equal, I've noticed my power data is pretty damn consistent


Just because an algorithm provides consistent output does not mean that it is right.

The _best _you can hope to estimate power output from hill climb speed is to be within 5% of actual power. 

Even when climbing, where many variables affecting the speed-power relationship are reduced, simple things like a very light wind still have a big impact on any power estimate made from hillclimb speed.

On flatter terrain all bets are off, way too many variables to reliably estimate power from speed.

So while you might get an estimate of power from climbing speed, the information is of little value other than for comparing yourself on the same hill under the same conditions (which in itself can be a helpful fitness test, but you cannot expect to reliably measure power elsewhere without a power meter). 

So you may as well just simply measure time/speed up a set climb and use that as a fitness marker.


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## robdamanii (Feb 13, 2006)

Alex_Simmons/RST said:


> Just because an algorithm provides consistent output does not mean that it is right.
> 
> The _best _you can hope to estimate power output from hill climb speed is to be within 5% of actual power.
> 
> ...


This.

Explains it quite well.


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## kmak (Sep 5, 2011)

There is a 1.1 mile closed off loop in Houston that many cyclists train on, including me (mostly for interval training). I recently bought a power meter, so my strava data has149 laps on the loop with "strava power" and actual power. Based on my data, strava power isn't very accurate - for instance, I have data on ten laps at 19.3 mph average; the "strava power" ranges from 146-153 watts, where my actual power was 176-182 watts.


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## JackDaniels (Oct 4, 2011)

Robert1 said:


> Thanks for that confirmation. My power data from strava has been pretty consistent. I was just questioning the absolute values. When I went to this site
> 
> Bike Calculator
> 
> It was showing a speed more in the 22mph range for 300W on the flats. But maybe that's off. I guess in the end it doesn't really matter just as long as whatever method you use is consistent.


Entering the numbers for the ride today is almost exact for that calculator considering I think my quarq reads about 3% higher than it should. I think 300w solo, on flats, with no wind, on a normal road bike is right about at 23mph.


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## Kerry Irons (Feb 25, 2002)

*High watts*



JackDaniels said:


> Entering the numbers for the ride today is almost exact for that calculator considering I think my quarq reads about 3% higher than it should. I think 300w solo, on flats, with no wind, on a normal road bike is right about at 23mph.


Actually 23 mph on the flats with no wind is about 250 watts. 300 watts gets you close to 25 mph.


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## jmitro (Jun 29, 2011)

Robert1 said:


> Uh, well except strava doesn't use HR data in calculating it's power data.
> 
> Assuming gps elevation data for a given ride is accurate, the main variable not accounted for by strava when calculating power is wind. All other things being equal, I've noticed my power data is pretty damn consistent


using Strava to estimate your power is about like using a quarter mile drag race time to estimate your car's horsepower.
IE it sucks.
If you want to keep fooling yourself despite what everyone is telling you on this thread, go ahead.


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