Power Meter Guide: The Watts, Whys and Hows of Functional Threshold Power (FTP)


, by Nikalas Cook

Photography by: Have a nice day Photo / Shutterstock

Will an understanding of your Functional Threshold Power (FTP) really make a difference to your performance? In short, yes. Knowing your FTP will allow you to pace your next ride to perfection, as Nikalas Cook explains.

In my last article, I hopefully convinced you to put off buying that fancy new wheelset and instead to really invest in your cycling future and get yourself a power meter. The key points were hopefully dispelling the myth that power meters are only for “serious” / pro riders, that they’re actually pretty affordable now, and that they don’t have to be overwhelmingly complicated to use and understand.

In this article, we look at arguably the most important metric associated with using a power meter - Functional Threshold Power (FTP).

What is FTP?

Back in the day, FTP was defined as the maximum power that you could hold for an hour, but a more enlightened way to view it is as your maximum sustainable power. The reason for this is that, at one end of the spectrum, a complete novice may not even be able to pedal consistently for an hour - does that mean they don’t have an FTP? At the other end, a highly conditioned rider may be able to hold their FTP for 70 minutes - does this mean they sandbagged their FTP test?

The “power for an hour” definition just doesn’t work. Think of FTP as more like your cycling “red-line” that, if you push above it for too long, you’ll run out of gas - for an untrained novice cyclist, this may be 30 minutes or less.

Why is it important?

Well, it’s not so that you can play FTP Top Trumps with your cycling buddies!

RELATED: Feel the Power: Why a Power Meter Should Be Your First Cycling Upgrade

Knowing your FTP is important as it allows you to set accurate and personalised training / pacing zones.

We’ll get into the nuts and bolts of training / pacing zones in the next article but, if you know your “red-line”, you can then easily work out the power ranges that’ll allow you to perfectly pace everything from a long training ride to an epic Alpine climb, a 40km time trial or even a 1-minute interval.

It allows other key metrics such as Weighted Average Power, Intensity and Training Load to be calculated - we’ll dig into these metrics too in future articles.

If you know your “red-line”, you can then easily work out the power ranges that will allow you to perfectly pace everything from an epic Alpine climb to a 1-minute interval.

Also, if you’re looking to race on Zwift or take part in some group rides, the category bands on the platform are based on your w/kg at FTP. To calculate this figure, all you have to do is take your FTP and divide it by your weight (in kgs).

The w/kg category bands are:

  • A = 4.0 w/kg >

  • B = 3.2 - 3.9 w/kg

  • C = 2.5 - 3.1 w/kg

  • D = < 2.4 w/kg

For example, my FTP is 370w, and my weight is 80kg. So, this means my w/kg at FTP is 4.6 w/kg, putting me in the A cat. 

For reference, a World Tour Pro would 6 w/kg at FTP or more!

It’s also important on Zwift that if you’re taking part in workout sessions - group or individual, and using Erg Mode - that your FTP is up to date, accurate, and inputted into your settings as the power targets for given intervals will be calculated from this figure.

FTP tests can be conducted both indoors and outdoors. Photography by: (L) GP PIXSTOCK / Shutterstock (R) TORWAISTUDIO / Shutterstock

How do I test for my FTP?

Here’s the good news: you don’t have to ride as hard as you can for an hour!

The most commonly used test protocols will use a 20-minute effort and then calculate your FTP based on 95% of the average power that you maintained. Some protocols will use a 30-minute effort and take the final 20 minutes, and others will have a “tank emptying” 5-minute effort before the main one, but they’re all variations on a theme and, give or take a few watts, a solid 20-minute effort after a decent warm-up and taking 95% of that will give you a perfectly workable number.

I’m lucky enough to have a perfect “test hill” nearby that has a steady 2-3% gradient, no junctions, and takes about 20 minutes to climb. If you have a similar climb or a mostly flat loop without junctions etc. - set up a segment on it, and you’re good to go.

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For many riders, though, using an indoor trainer is the easiest and most convenient method for getting an FTP test done. Although there can be a small difference between indoor and outdoor FTP, for getting going training with power, don’t stress about it. Do try to ensure that the power meter you measure from is the same for inside and out. So, even if you usually use your trainer as your power source when riding indoors, select your on-bike power meter (that you’ll be riding with outside) as your power source when you test if at all possible. It’s not the end of the world if you can’t, and you’ll still get a number that’s usable for setting zones from.

If you use Zwift, both their FTP Test and FTP Test (shorter) in Custom Workouts are really good protocols. I’d steer clear of the Ramp Test protocols as, in my experience, these can tend to give over-inflated results.

Regardless of the exact test protocol you use, it’s a tough session so ensure that you’re well rested beforehand as, if you’re fatigued, this will negatively effect your result.

Looking for explanations? Check out the Strava Training Glossary for Cycling

You’ll generally be looking to re-test your FTP every 4-8 weeks, as hopefully, with consistent riding, you’ll see it going up!

If you’ve been riding with a power meter for a while and have a few months of rides with power uploaded onto Strava, you can go to your Power Curve, look at 20 minutes, take 95% of this value, and get a decent estimate of FTP to get going with but I really would urge you to bite the bullet and take a test.

I now know my FTP - what do I do with it?

We’ll talk about setting zones and how to use them in my next article but, for now, make sure you enter that number into Settings > My Performance > Functional Threshold Power on Strava (for more information, click here).

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