How Does Garmin Run Indoor App Compute Distance

A common question I receive from many of my athletes regards treadmill running. This is a question that I've had myself and you have possibly had as well. Most runners use a Garmin or other GPS watch to track their runs – including for indoor running. The Run Indoor setting on many GPS watches allows you to track and record your treadmill run, just as you would an outdoor run. However, several of the runners I coach ask me about the accuracy of the Garmin Run Indoor mode. More often than not, the Garmin treadmill sensor records a different distance – and therefore a different average pace – than the treadmill.

So which is more accurate – the Garmin or the treadmill? Why is there a discrepancy in the distance between the two? Why is the discrepancy even more prominent on interval workouts than easy runs? (Do note this post refers to a motorized treadmill – non-motorized treadmills may render different metrics).

How Accurate is Garmin Indoor Run Mode on the Treadmill?

Is Your Watch or Treadmill More Accurate?

Unless you are using an old treadmill, the treadmill will be calibrated for the appropriate distance. This is especially true of newer treadmills, especially home models. The advanced technology on these machines yields accurate data.

If the treadmill feels harder or easier than outdoors, that does not mean the treadmill is slower or faster. A 2019 meta-analysis published in Sports Medicine concluded that while VO2max and heart rate tend to be similar on the treadmill compared to outdoor running, endurance is poorer on the treadmill. This could be due to the lack of air circulation, monotony of the treadmill, or other factors. Even if eight miles "feels" easier on the treadmill, you still ran eight miles if your treadmill says so.

How Does Garmin Indoor Mode Work?

Unless you utilize a foot pod, a GPS watch tracks distance indoors using an accelerometer. Accelerometers calculate the distance covered based upon your stride length, stride frequency, and arm swing. The accelerometer calibrates based on your form and cadence on outdoor runs. It then counts your steps on a treadmill run and estimates your distance in the recorded time frame.

Running on the treadmill is relatively the same as outdoors in terms of heart rate and energy output. However, many runners do slightly alter their stride on the treadmill. As a 2017 study published in the International Journal of Exercise Science notes, stride length and stride frequency tend to differ when running on the treadmill than outdoors.

The difference in gait on the treadmill versus outdoors makes sense. Even if your overall form is similar (as this study inThe Journal of Sport Rehabilitation determined), many runners slightly alter their form based on the different environmental cues. The confined space of the treadmill can also cause many runners to change their form. You may do this if you are worried about flying off the back of the treadmill and subconsciously alter your stride. If you are used to running on rolling hills and set the treadmill to 0% incline the whole way, your form will be different than outdoors.

If the Garmin treadmill mode is calculating the distance run based on your stride and arm swing, and you change your stride and arm swing on the treadmill, it therefore makes sense that the distance calculated is different than the treadmill reads. The treadmill calculates distance based on the number of revolutions of the belt – unless the treadmill is very old or miscalibrated, it should be relatively accurate.

You will know this is happening to you if your cadence tends to be lower on treadmill runs compared to outdoor runs. If your cadence is relatively the same yet numbers still differ, know that it is likely the shortcoming of the technology. Your cadence may only vary 15 steps per minute between easy and hard paces; that is certainly not precise enough to accurately capture the difference between a 7:45/mile and 8:15/mile, for example!

Many newer models of Garmin, Coros, and other GPS watches allow you to calibrate the distance after you complete the run. Some (such as Coros ) even use an algorithm to learn from each of your indoor runs to provide a more accurate read in the future.

But most of us runners are analytical numbers geeks – we want accurate data and we want to be able to easily track our workouts. These few tips will help you minimize the discrepancy between your watch and the treadmill – and help you get the most out of your indoor run.

How to Optimize Garmin Indoor Mode Accuracy

Focus on Your Form

If the indoor mode of your GPS device and your treadmill read statistically significant differences (say, more than 15% of the distance of the run), your form may drastically different on the treadmill than outdoors.

For example, most runners will have a similar cadence on the treadmill than outdoors. However, there are outliers in both directions. If you notice that your Garmin treadmill mode reads slower than outdoors, you may have a lower cadence on the treadmill than indoors. If that is the case, be mindful to maintain quick turnover on the treadmill.

Whether or not you care about the GPS data, pay appropriate attention to your form on the treadmill. Avoid holding onto the front bars or running at the very front of the belt. Running at the front of the belt can shorten your stride and inhibit your arm swing. You want to run in the middle of the belt, giving yourself room for your stride and arm swing to open up more naturally. Run tall, avoiding hunched shoulders. Cue yourself to "run light," which encourages shorter ground contact time and a quicker cadence. Don't obsessively check your watch, as bringing your wrist forward and craning your head down throws off your form.

Run For Time Instead of Distance

If even a minor distance discrepancy is going to mess with your mind, change your metric and run by time instead of distance. After all, the duration of the run is ultimately what your body responds to. For example, if your normal easy run pace is approximately a 9-minute mile and you want to run 5 miles, run for 45 minutes on the treadmill. If your watch reads short or long, it really doesn't matter – you will have achieved your prescribed workout.

For speedwork, use time-based interval workouts such as fartleks rather than distance-based intervals. Faster paces may exaggerate cadence and form differences. In this case, your watch will likely be more inaccurate during interval runs. If you are using the interval function on your watch, time-based intervals will align with the time readouts on the treadmill. Try a fartlek workout or one of these treadmill workouts for runners.

Remember the Purpose of Your Workout

In the age of GPS watches and social training logs, we can become slightly obsessive over the preciseness of data and numbers. Ultimately, what matters is the workout itself: how much time you spend running at a particular intensity. Distance and pace are one method of measuring volume and intensity, but so are duration and perceived effort.

For example, 5 mile run and 5.3 mile run at an easy effort render the same workout. Choose which way you want to measure that run on the treadmill and then disregard any minor discrepancy. Don't obsess too much if your watch and treadmill are slightly different on distance, especially if it's an easy run. The discrepancy does not negate the purpose of your workout.

Treadmill Inclines Will Affect Accuracy

When you run uphill or downhill, your gait naturally changes. However, your watch cannot differentiate flat, uphill, or downhill gradient on a treadmill. Rather, it simply notices the changes in your stride. If you change the incline often on the treadmill, this can affect the Garmin indoor mode accuracy. However, this is no reason to avoid inclines on the treadmill! Simply use auto lap (see below) and calibrate your watch after the run. (For more on the benefits of treadmill incline workouts, read here!)

Utilize the AutoLap

Want accurate splits for your Strava or Final Surge data? Utilize the auto lap function on your watch. You can hit auto lap each mile based on the treadmill reading. You will still want to calibrate the distance at the end, but then at least the splits are accurate.

A Final Note on Indoor Mode

In addition to the treadmill, Run Indoor mode functions well for the other type of indoor running, the indoor track. Run Indoor mode will be more accurate on the indoor track than the treadmill. This is because indoor track running is overground running, just like running outside. You can also use the precisely measured distance of an indoor track (a majority are 200ms per loop) to confirm the distance and pace of your run. (If you are training on the indoor track, try one of these indoor track workouts!)

How Accurate is the Indoor Run Mode on Garmin?

For more on treadmill running, check out these blog posts:
How to Survive – and Enjoy – Treadmill Long Runs in Winter
Runners Share their Best Treadmill Tips
Ask A Running Coach: Common Treadmill Questions

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Do you use your Garmin on treadmill runs? Have you noticed issues with Garmin indoor mode accuracy?
Do you run by time or distance on the treadmill?

27 Responses

My Garmin always measures a little extra distance on the treadmill. I try to use it so that I can keep track of my mileage on Garmin connect, but I tend to go by the treadmill distance and pace. I also tend to forget to stop my Garmin when I run on the treadmill, so that messes up my data too!
I don't use my Garmin on the treadmill because the treadmill display tells me everything I might want to know. The track at my gym, though, is 12 laps = 1 mile (about 135 m/lap I think). I would have to use my Garmin if I wanted to run any significant distance there, because I'd quickly lose track of the number of laps!
My treadmill and my garmin are both way off. I sometimes put an old bike wheel on a trainer with a speed sensor on the hub and push it against the back of treadmill belt. It is probably the most accurate way to get true speed. It shows how far off the garmin and treadmill are.
1. Thanks.this was a great solution for me . Tested it right way !
I've been using it a lot if you go fast say 15 kmph the garmin is slower if you go dead slow the garmin is faster There is a distance where it is accurate for me about 13.8 kmph but if you swing your arms more you can speed it up more it's still a good idea but if your treadmill out then who knows lol
Great article. I will note that Garmin has an option on treadmill runs after you complete to 'save and calibrate'. This option allows you to enter the treadmill difference before you save to get an accurate pace and distance.
This is suppose to allow Garmin to adjust future treadmill runs to be closer to what the machine actually says.
1. Thank you! I haven't seen that option and will need to look more into it!
2. Well, yes. But I keep calibrating it every time because of the different ways of running (speed, fartlek, long easy runs, etc) so it seems it gets confused and keeps measuring wrongly because of the different references it is using to calibrate. So I try to run outside as much as possible and not to take in count the data for indoor runs. Also, when you use "save&calibrate", it does not change your lap's info… which is confusing. Also, it does not change your records or personal bests, which is annoying… 🙂 Anyway, good luck and wish you a lot of miles for 2020!!!!
  1. Do you think the incline on the "dreadmill" has anything to do with it? I usually keep it at 1.5 because I feel like that's more like "outside." Any run on any treadmill results in less distance on my Forerunner 35. This morning I wanted to run 6 miles, but at that point on the treadmill my watch only said 5.46. Often the farther I go the larger the discrepancy – like the first mile might be only .04 short, but after the second I'm now .1 short and by the third maybe it's then .17 short (though it doesn't necessarily always increase the per-mile discrepancy each mile). I am a Galloway runner, so I usually walk one minute at 3.8 mph then three minutes at 6.2 mph. Sometimes I increase to 6.3 mph on mile 2, 6.4 mph on mile 3 etc. Anyway, every 4th minute is the 3.8 mph walk. When I awkwardly try to sneak glances at my watch on the walk it says something like 15:45 minute pace, which is about right, but my run pace is usually something over 10 minutes, maybe even as much as 10:40, when at 6.3 mph it should be closer to 9:52 minutes. Is this because of the incline, the intervals, the fact that my arm stops moving when I have to blow my nose (which happens frequently on indoor runs)? All of the above? I always feel like I have to run extra so my watch can catch up.
    1. At least in my observations of both my own Garmin and the data from my athletes, intervals absolutely affect the accuracy of indoor mode. I just always trust the treadmill – most are well-calibrated machines.
    2. Thank you for sharing! I'm definitely like you.. On my long runs, I run for 2 miles at 6 mph on the mill, then walk 1 minute at 4 mph, then repeat this run/walk method until I'm finished. I have used the Garmin Forerunner 230 for 4 years to track my indoor/outdoor mileage. I love to use my Garmin to keep track of my trainings, but I always end up going off what the treadmill says in regards to distance. Ultimately, I feel like it's more accurate than the watch.
3. Hi how do you calibrate if you have already done your first run and did not calibrate correctly? Does not give me the option now? Thank you!
  1. At the end of the run, scroll down past "Save" to "Calibrate and save". This updates the total distance for the current run, but all other distances (laps, etc.) is using the previous calibration.
I find that if I calibrate at a specific pace, my next run at that pace will be spot on (compared to the treadmill that was also used for the calibration), but if I run faster the distance is short and when running slower, the distance is long. It seems the calibration only use step length and do not take account of the fact that my step length tends to increase with speed.
This is a very interesting article, Laura!
I don't use a treadmill and I have never tested Garmin's indoor run mode against the treadmill's data. But it makes sense that it differs since a runner's form is slightly different on a treadmill compared to overground running.
I wonder if the very latest Garmin models are better at calibrating treadmill runs?
For most of my runs my Garmin measures slightly longer- sometimes I calibrate it and other times I just don't worry about it. There was one random time that I ran 10 miles and both my watch and treadmill had the same distance! I have no idea how that happened!
I have never worn my Garmin while on the treadmill because I figured the treadmill was accurate enough (at least regarding distance). I have always, and I am not exaggerating, felt that my treadmill runs were more difficult than any outdoor runs. My gait/form always feels compromised…glad to see it wasn't all in my head LOL
I am an outdoor runner only but I also notice some weird Garmin accuracy. If I am running with a friend and we are both using a Garmin and run the same distance, we should have the exact same distance recorded. 99% of the time my friend's distance is about a 1/4 of a mile longer than mine!
Good question.

But I don't have a treadmill and when I ran on one in a gym, I didn't wear my Garmin.

I do often use my Apple Watch for mileage and I know that it is not as accurate as a Garmin.

I always say that I ran 10ish miles and I'm ok with that. lol
This is a great post (as always) Laura. I do use the indoor mode on my Garmin — my treadmill is also VERY old (about 20 years). I know for sure I'm not running as fast as my Garmin says I am.

I like the idea of using time. I have been doing that sort of anyway recently — on the treadmill, not so much outside.

I'm not training for anything, though, and don't anticipate any real training, so I'm not so hung up on the numbers.
Interesting. The few times I run on the treadmill, I don't think my data is TOO far off, but I don't do it often enough to say for sure.

I know I DO run at the front of the belt. I have a long legs and I am terrified of sliding off the back. Whoops.
I rarely run on the treadmill and if I do I just let the treadmill tell me how far/fast I've run. I'm not sure I understand why one would need the Garmin data. For tracking, you can add it in yourself. Maybe it's because I'm older and I just don't worry about my pace anymore. 🙂
This is so interesting. My Garmin, similar to what Al said above, has an option that allows me to save and calibrate my run with the mill. Typically my Garmin and treadmill aren't too far off from each other.
Great article. The tip about cadence is very helpful, I'll be watching my form next time I run on the treadmill ( when the gyms finally open again safely)!
While a Garmin isn't exactly necessary for a solid workout, the way that it tracks information and saves data makes it great for those that are more dedicated to their regimens. Good information to have here!
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