Entering winter, as well as transitioning out of the cold-weather months and into spring, are the times riders ask us most to explain the differences between testing their Functional Threshold Power (FTP) outdoors versus indoors. The truth is, there are a lot variances between the two approaches, many of which you’ll discover soon make it inappropriate to compare each test’s results to one another — I’ll touch on this later in this post. For now, let’s dive into why you should test your FTP and how to test it outdoors and indoors.

** Our default testing format to estimate an athlete’s FTP was changed to the TrainerRoad Ramp Test since this post was published

There are three commons ways — via a race file, 1×20 test or 2×8 test — to calculate your FTP from riding outdoors; and two ways — via a 1×20 test or 2×8 test performed on the trainer — to calculate it indoors. I’ll start with the most standard methods for testing your FTP outdoors and the reasons all riders who are serious about optimizing their workouts and fitness should learn what their FTP is now rather than later.

Why Knowing Your FTP Is Valuable

If you’re a TrainerRoad user, it’s likely you already know the weighty reason we advocate for knowing your FTP. Once we know your FTP, we’re able to customize every workout you do so that it’s optimized to your current fitness level. The more precise your FTP is, or perhaps better said, the more precise your FTP estimate is, the more effective your future workouts will be. That’s why, instead of going in with a previously estimated FTP, the first workout we highly encourage all our athletes to complete is one of our structured FTP tests.

If you’re not a TR user, the same reasons to know your FTP apply. Firstly, once you’ve correctly estimated your FTP, you will have a benchmark of your fitness, which will help inform you in the future on whether you’re getting faster or not. Secondly, after you know your FTP, you’re able to use that information to structure your interval workouts more effectively. So, instead of going into a workout saying, “I’m going to ride at an 8 out of 10 or 6 out of 10,” you have an objective number. This allows you to figure out exactly what your power numbers need to be — no guesswork involved.

How to Calculate Your FTP By Looking at a Power File

By taking the time to do a little post-race analysis you can quickly get a feel for what your FTP is. The approach is simple: Look at a race where you’ve had to put out a consistent, all-out effort for 8 minutes, 20 minutes or even an hour. (It’s important to note this can be tricky as races are often anything but consistent, especially when you’re riding with others.) Next, calculate your average power — not normalized power — for your 8- or 20-minute effort. Finally, reduce your average power by 10 percent if you’re using an 8-minute effort or by 5 percent if you’re using a 20-minute effort.

Why do you have to adjust your average power? FTP is analogous to your hour power — or at least it’s supposed to be. For most it’s not. This has a lot to do with the mental fortitude it takes to ride at your highest possible limit consistently for an hour-long duration. This is tough!

Most athletes mentally limit themselves before their body physically does. This is the impetus of an FTP test — it’s the reason you can do shorter intervals to decipher your hour power. When you test for shorter durations, mental strength isn’t as big of a variable which means you can get a better idea of your physical potential rather than having mental limiters come into play and mess with your power results.

The longer you ride, the smaller the percentage you have to reduce your average power. Why? More likely than not, your max 20-minute effort is going to be higher than your max 60-minute effort. Same thing goes with your max 8-minute effort — it’ll likely be even higher than your 20- and 60-minute maximum efforts. That said, reductions of 5 or 10 percent are established protocols that are proven to roughly equate most closely to your true FTP, aka your 60-minute power.

What about if you have an hour-long race file you want to analyze to find your FTP? As long as it is a well-paced and consistent effort, this is actually an excellent way to measure FTP. With this approach, you do not have to reduce the percentage of your power — the estimate is what it is.

How to Calculate Your FTP By Testing Outdoors

A lot of riders think that in order to perform their FTP test outside they need to find a hill to reduce variables and be more consistent. The issue with this kind of thinking is that you’re inserting a huge variable just by going uphill instead of on a flat. There’s less inertia when you’re going uphill, which means all the way around your pedal stroke you’re forced to apply more pressure on the pedals.

This can make a measurable difference in the way your muscles put down power than if you’re riding fast on flat ground. So right there is a big variable you’re inserting by assuming that your FTP estimate is the same uphill as it is on any other type of terrain.

Your best approach is to find a stretch of uniform road that you can put out power consistently without interruption for either 8 or 20 minutes. Depending on the type of athlete you are and your testing experience, finding either an 8- or 20-minute stretch of road will be more or less important to you. Put in another way: It’s best not to decide on the stretch of road you find for testing purely based on convenience, although that’s understandably a factor. Let me explain:

To decide if the 8-minute or 20-minute FTP test is right for you, consider your primary cycling discipline and your experience. For shorter duration athletes who are used to performing at VO2max, we commonly recommend an 8-minute FTP test. These are criterium racers, cyclocross racers and cross country mountain bikers — riders who are generally more accustomed to working at this uncomfortably high workload.

For athletes who are used to putting out longer, sustained efforts in their races, we almost always recommend a 20-minute FTP test. These are road racers, triathletes, 40 kilometer time trialists, endurance mountain bikers and gravel racers — riders who are used to laying down longer, very evenly paced efforts.

All this information aside, if you’re already used to one particular testing format, stick with it! It’s important to stay loyal to one testing format so you can compare your previous testing results to your most recent.

Once you’ve decided on the testing format that’s right for you and scouted out your stretch of flat road, here’s how to execute the day of the test:

  • Warm up for about 20 minutes. Don’t soft pedal the whole time, mix in some bursts of intensity.
  • Do your 8- or 20-minute effort. If you’re doing the 8-minute effort, take 10 minutes to recover in between your two efforts. You’ll want to take your best average power of the two efforts to calculate your FTP.
  • Cool down for 10 minutes.
  • Calculate the average power of your test effort(s) and reduce it by 5 or 10% depending the test you did. Reduce your best average power by 10 percent if you did an 8-minute effort and by 5 percent if you did a 20-minute effort.

What other variables should you consider when testing outdoors? Although it’s more difficult to reduce variables when performing your FTP test outdoors compared to indoors, there are still factors you should keep in mind and control as much as possible. Here are just a few:

  • Testing location. This almost goes without saying, but each time you perform your FTP test it should be done on the same stretch of flat road, and whenever possible, under similar weather conditions.
  • Nutrition. This is a big one. Leading up to an FTP test, make sure you’ve fueled and hydrated in a similar manner to your previous tests. And if you’ve made a mistake previously, don’t make that mistake today of all days.
  • Build up. Playing off of the above bullet, try to keep your lead up to each FTP test as consistent as possible. For example, if you typically test best after two days of rest beforehand, avoid doing a really hard workout the day before your next test. It’s best to keep the week leading up to an FTP test fairly the same. You don’t have to do the same workouts, but it should be typical of your regular routine. If you’re on one of our structured training plans, you don’t have to worry about this as your FTP tests are scheduled for you at the most appropriate times.
  • Power source. If you change your power source, you will need to reassess your FTP. Whether you’re testing outdoors with a power meter or indoors with VirtualPower, you can’t compare your FTP results from two different power sources. If you look at power meters alone, every option on the market is slightly different and will give varying power readings. So, stick with one option and if you change and/or upgrade, reevaluate your FTP with your new power source.

How to Calculate Your FTP Indoors

When performing your 2×8 or 1×20 FTP test indoors, you have the advantage of limiting the number of variables like traffic, wind, road quality and variations in pitch you would otherwise experience outside. For a scientific test like the FTP test, this is key.

If you do not use TrainerRoad, you can apply the same testing structure we recommended for outdoors (see above) on the trainer: 20 minutes of warming up, followed by your testing efforts, concluding with a 10-minute cool down. After your test, calculate the average power of your testing interval(s) and reduce the number by 5 or 10 percent. Again, reduce by 10 percent if you did an 8-minute effort and by 5 percent if you did a 20-minute effort.

If you use TrainerRoad, you can expect a more strategically structured warm up when you perform one of our FTP tests. Each FTP test includes a warm up that consists of a few big efforts — these peaks of intensity are specifically designed to prepare you for the testing intervals ahead.

The warm up for the 8-minute test includes intervals that are higher, shorter and more intense compared to the 20-minute test. In either case, the intense warm-up efforts we prescribe are driven by one goal: to thoroughly prepare your body and mind for the upcoming intensity. After you’ve completed your FTP test on TrainerRoad, we will present you with your new FTP. Once you accept it, all your workouts will be automatically customized to your new FTP.

Now that you know how to perform an FTP test indoors, adding to the list above, here are a few more variables to consider that pertain specifically to testing indoors:

  • Temperature. I’m talking about the ambient temperature in the room you’re testing in. You do not want to be testing in a super humid room and then in a super dry room. Try to keep the climate controlled and the same each time you test. Also, get yourself a good fan. When riding indoors, it’s essential. Place it at the base of your bike and aim it at your body, or center it in front of you on its high setting.
  • Tire and trainer pressure. Especially if you’re using VirtualPower, tire and trainer pressure (how tight the tire is against the trainer) is a pretty big deal because it will directly affect your power readings. That said, whatever power source you’re using, it’s important to have those two variables nailed down so that they’re the same each time you test.
  • What you wear. We mentioned this in our winter training guide, but what you should wear when training indoors really comes back to cooling. You should be wearing as little as possible, this—especially applies to testing. Of the items you do wear, they should be your best, most comfortable pieces. Many cyclists leave their old cycling kit (worn-out jerseys and bibs) for the trainer — this couldn’t be further from the best practice. When you’re outdoors, mostly because you’re in and out of the saddle and moving more, it’s a little more forgiving if you have an old chamois. But indoors, because you’re sitting in the same position for your entire workout, it’s best to wear the good stuff. You don’t want to set yourself up to get a saddlesore, rawness or something worse during your test.

The Differences Between Testing Outdoors V. Indoors

A lot of athletes will report they have a lower FTP when they test indoors than when they test outdoors, and sometimes vice versa. There could be a number of reasons for this, but a good one to look to first is thermoregulation. About 75% of your body’s energy consumption when cycling goes to cooling, or maintaining your body’s temperature. When you’re outdoors this is better achieved thanks to wind and sometimes temperature. But indoors you’re reliant on a fan or another cooling mechanism to keep your body from having to work so hard to stay cool.

We could go on and on in this section and try to dissect every little variable that could lead to a difference in test results, but ultimately I think it’s better to end this post simply: Because there are less variables to consider, assessing your FTP inside will provide more sound data, i.e. accurate assessment results, than what you’ll get outside. What’s more, because there are so many variables and differences to consider between the two approaches, you simply cannot compare your test data from an outdoor test to your test data from an indoor test, and vice versa.

If you have any questions about FTP testing, feel free to leave them for me below. Also, if you’re interested in more info on FTP testing, be sure to check out the resources listed below.

Other Helpful Resources on FTP Testing

We’ve written about FTP testing many times on our training blog. Here are some of our most popular posts on the topic:

Listen to Certified Cycling Coaches Discuss FTP Testing

FTP testing is just one topic we covered in episode 30 of the Ask a Cycling Coach podcast. Listen to the episode’s full recording below to hear this and other questions from cyclists get answered by our certified cycling coaches.

Additional Notes

TrainerRoad’s Ask a Cycling Coach podcast is dedicated to making you a faster cyclist. It gives you the chance to get answers to your cycling and triathlon training questions from USAC certified coaches Chad Timmerman, Jonathan Lee and special guests. Learn more about other topics we covered in the latest episode with our resources below:

  • How to use a smart trainer with TrainerRoad
  • How cadence affects electronic trainers
  • How to lose weight while retaining muscle
  • How to count calories for cyclists
  • How to know if you are sugar adapted
  • How to reduce reliance on sugar
  • How to use TrainerRoad with a coach
  • How to make your own custom workouts
  • How to use VirtualPower
  • Why does indoor FTP differ from outdoor FTP?
  • How to set up your trainer
  • How to adjust the intensity of a workout in TrainerRoad
  • What cadence should you ride at?
  • Is low cadence better than high cadence?
  • What gear should you use on an electronic trainer?
  • How to use your power meter with TrainerRoad
  • How to use PowerMatch

If you have a question that you’d like to ask Coach Chad, submit your question here. We’ll do our best to answer them on the next episode of the Ask a Cycling Coach podcast.

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Chad Timmerman

Chad Timmerman is the Head Coach and Co-Founder of TrainerRoad — cycling’s most effective training system. He has nearly 10 years of coaching experience as a Level I USA certified Cycling and Triathlon coach. When he’s not developing structured training plans for TrainerRoad, you can catch him sharing his coaching advice on the Ask a Cycling Coach podcast. To get Chad’s best cycling knowledge delivered to your inbox, sign up for his free 6-part email course Train Smart, Get Fast.