Accuracy Of My Garmin Running Watch

If you are a runner like me, you know that Garmin  watches are probably the best running watches out there. It took me a few years of running before I bought my first professional running watch.

As almost everyone I started with Endomondo about 8 years ago. A year go I bought  myself my first running watch, TomTom Runner. A few months ago I finally got myself Garmin Forerunner 235 without which I couldn’t image running right now. 

Even though the watch is all great and almost perfect, it is still only a measuring tool which is prone to error and inaccuracy.

In today’s post, I will show you the two situations in which my Garmin Forerunner turned out to “lie” a bit about the distance (and pace) and how I lost the chance to beat my PB.

How GPS Running Watches Work

GPS (Global Positioning System) is a technology that really has revolutionized the way we move around Earth and measure our movement. It’s now applied in a variety of devices like cars, phones, watches and what not.

It is simply a network of 24 satellites going around Earth in precisely tracked orbits.  A GPS device simply receives signals from these satellites (at least three) and calculates its position (and other parameters) relative to them. This is exactly what your GPS watch does and presents these data to you.

Accuracy of Garmin running watches

If you want to learn a bit more about how GPS technology and GPS watches work, I suggest you read this article about GPS watches.

Two Situations Where My Garmin Forerunner 235 Lied A Bit

Of course, I knew and I know that as running watches rely on GPS signal they may sometimes be inaccurate. There are situations where four satellites are not available and the watch has to “approximate” the data it has collected. This is exactly what happened in these two situations. Let’s take a deeper look at them.

Running At The Stadium

Running at the stadium is probably the most confusing type of running for your watches. Fortunately, we know the length of the stadium (do we?) we run at, so all we really need is a stopwatch. Anyway, I wanted to measure time, pace and distance with my Garmin and it turned out the watch lied a little bit.

Actual distance (4 km)

The actual distance I ran during this workout was 4 kilometers. My running coach told me to run 1 kilometer four times (with 5-minute breaks in between). It was on a specially designated one-kilometer section of the treadmill.

Actual average pace (4’31”)

It took me exactly 18 minutes and 2 seconds to run these 4 kilometers, which means the pace of 4’31” / km. I am supposed to ran during this type of a workout at a pace from 4’00” to 4’30” so it’s obvious that I ran too slowly and this workout shouldn’t count.

You may think that a difference of 1 second is not much but actually it may make a whole of a difference on longer distances.

How Garmin Measured This Workout (4.05 km at a pace of 4’27”)

According to my running watch I ran not 4 km  but 4.05 km. Of course, this is not a big difference, but in my case this small error changes a lot. Take a look at the summary (I imported this workout to Endomondo).

Accuracy of Garmin running watches

As you can see Garmin says I was running at a pace of 4’27”, which makes a difference of 4 seconds (in my favor) per kilometer. Of course, I didn’t take any shortcuts through the middle of the treadmill (this is probably the approximation my Garmin did). 😀

When I first saw the summary of the workout on the screen of my watch I was very happy because  I thought I managed to run in the pace zones my couch told me to run in. After I took a look at the distance, I realized that my watch was too optimistic about me.

This situation is not a big of a problem because I was running on a treadmill and I could compare the actual distance with what my watch was showing. Unfortunately, I couldn’t do the same on a race I ran a few days ago.

Trying To Beat My PB In A Half-Marathon In Warsaw

A few days ago I took part in the biggest running race in Poland, the Warsaw Half-Marathon (with more than 14 thousand contestants).   What I did in Warsaw last year was unthinkable for me. I improved my PB by more than 18 minutes. This time I only wanted 2-3 minutes of improvement (it’s much harder to improve once you start running at a pace below 5’00”). My plan was to beat my PB which on this distance (21.097 km) is 1:41:00 and finish under 1 hour and 40 minutes.

Accuracy of Garmin Forerunner

This is a selfie I took a few seconds before the start. I was still full of hope. 😀

1 Hour and 39 Minutes at 4’42”

As my Garmin has a lot of interesting in-built features (you can read more about them in my review of Garmin Forerunner 235) I thought I would set it to watch my running pace and alert me when I run too slow or too quick.

I calculated that to finish in 1:39:00 I have to run at an average pace of 4’42”.  I knew that there was some elevation on the race route so I set my watch to alert me when I ran slower than 4’40”. This way I would have some extra time in case I felt worse or faint (there was a huge 400-meter ascent on the 20th kilometer).


I set data screens to show average pace and distance.

I set my watch to show me the distance and average pace fields all the time, so that I can control if I still run within 4’40” zone.

How I Let My Garmin Trick Me

I started the run and for the first 10 kilometers (when I was still fully aware of what was happening) I was happy that I was doing so great. The average my watch showed was 4’35”-4’36”.  It sometimes  alerted me that I was running too slow (indicating 4’42” or even 4’55” sometimes) but I just thought it went crazy with measuring pace at the moment, so I kept watching the average pace.

When at 15th kilometer fatigue started to appear (and pain in my hip and spine), I slowed down a bit, but still watched my average pace not to be above 4’42”.

Unfortunately, after 16th kilometer the pacemaker for 1:40:00 caught up with me. At that moment I realized that Garmin was lying. To finish in 1 hour and 40 minutes, you have to run at an average pace of 4’44”, which was more than what my watch was indicating at the moment.

The pain and fatigue started to become unbearable and I gave up on beating my PB. I just wanted to finish the race in a decent time.  I finally made it in 1:42:33, which wasn’t  the worst result after all. This is the summary of my run (imported to Endomondo).


My Garmin Forerunner tricked me a bit because this time it also added some extra distance to the real distance and, therefore, the average pace (calculated on this false distance) was too quick. If I had run at an average pace of 4’49”, I would have finished in 1 hour 41 minutes and 30 seconds.

Accuracy of Garmin Forerunner 235

I finished in 1 hour 42 minutes and 33 seconds, so I ran at an average pace of 4’52”. This a difference of one minute and one second which in my case was crucial (I wanted to improve my PB by one minute and 1 second).

Important Ramifications for All the Runners Using GPS Watches

It’s obvious that if I were in a better shape, I wouldn’t have had any problems with beating my PB even without any running watch. However, as I wanted to beat my PB by just one minute and one second (and 1 hour and 40 minutes is the so-called “wall” for me now), the difference and error measurement in my Garmin Forerunner turned out to be fatal for me.

Nevertheless, this is an important lesson to learn both for  you and me and here are the most important conclusions to draw.

  • If you want to rely on your running watch, remember that the average pace it shows is probably too optimistic and you have to decrease it by a few seconds at least.
  • It seems that the alerts about the current pace work fine (at least are more accurate than the average pace), so If you want to rely on your watch, rely on its alerts to slow down or speed up, not the average pace of the whole distance it shows.
  • Remember that this difference in pace is the result of an error in measurement of the distance (according to my watch the distance is always a bit longer than in reality).
  • Train hard and you will always beat your PB and your watch! 🙂

I hope these two life scenarios will help you avoid making the mistake I made and you will be able to beat your PB with the help of your Garmin watch.  If you have any questions or comments, feel free to use the comment box below or contact me directly.


  1. Jacek 11 April 2016
    • Olga 13 April 2016

Add Comment