Incorrect OpenWater Swimming total distance and speed


I did a comparison of the Fenix 7 with my TomTom Adventurer during open water swimming. It appeared that the distance logged by the Fenix (on the watch as well on Garmin Connect) was 2231 m whereas the distance logged by the TomTom was 3.16 km. I know from experience that the TomTom is right, hence the Fenix logged distance is to far to short (29% to short!!).

The speed recorded by the Fenix appears 2.37 min/100m which is 2.3 km/h which is also to low since I swim about 3.2 km/h. Makes sense, since the speed is derived from the distance.

Note that I attached both watches to my swimming buoy (20 cm distance between them) for better satellite signal reception. They were above water all the time with a clear 180/360 degree sky view.

Software version: 8.18


Exporting both files to GPX (using own software tools) and showing the activities in Oziexporer reveals  some remarkable issues

In red the TomTom, in blue the Fenix.


  • Some parts of the track do not contain trackpoints, revealing as straight lines. Especially in the northern part, which is the first part of swim and in the south-west corner.
  • Other parts of the track show trackpoints each second, whereas the Setting: Data Recording -> Frequency is set to 'Smart'. Usually it would skip trackpoints
  • Based on this, Oziexplorer reports a distance of 3.05 km for the Fenix, which confirms the 2231 m of the Fenix/Garmin Connect is far to low.

I enclose the original FIT file of the activity:

On close analysis the FIT file contains 1053 erronous [lat, lon] coordinates [180, 180]. This cannot be due to poor satellite reception since the watch was on top of the buoy giving it a 180 degrees sky view 360 degrees around (the best you can get). 

Note that in the conversion to GPX format, I filtered the [180, 180] coordinates out.

The GPX contains 2448 correct coordinates. Assuming each coordinate represents the same distance and a wrong coordinate is not counted in the total distance, this would mean the total distance calculated is 2448/(2448+1053) * 3.16 km =  2209 m, which is pretty close to the reported 2231 m by the Fenix and Garmin Connect.

I checked another Open water Swimming activity in which I compared the TomTom (on buoy) and Fenix 7 (on wrist).

TomTom (blue): 2.17 km

Fenix (red): 2.06 km, 17 malformed [180, 180] lat/lon coordinates

I checked a trail run: 1 malformed [180, 180] coordinate

I checked a generic cycling event: 0 malformed coordinates

I checked a short hike: 1 malformed coordinate


  • The Fenix logs wrong coordinates [180, 180] in all activities. 
    In Open Water Swimming they appear to be numerous. 
  • Calculation suggests that this results in wrong distance calculations on the Fenix, which renders the watch not very usable for the serious sporter!!
    Other posts on this forum also report wrong total distance for other activities.


  • Solve the bugs so coordinates are correctly logged
  • Make sure that if occasionally a wrong coordinate occurs, it is omitted for distance calculation. In other words calculate the distance only by calculating the distance between correct coordinates. This gives better results than current situation.
  • You’re not the first to report erroneous distances with Garmin watches in the open water and you will not be the last. However, others, and that includes me, find Garmin watches in the open water, including the Epix/F7 to be accurate. For your distance to be so far off, and the number of errors you are reporting, I’d first be looking to the settings. For OWS you should be on 1-sec recording and All Systems (+ Multiband if you have it). If you still have problems contact Garmin support to be added to their case for this.

    There will be someone posting here shortly lambasting my reply. However, if I swim a marked and measured 2k course and always get within 150m depending on tide and my tracking then I am obviously quite happy with the results. Some people simply cannot get a good result for some reason. Garmin have a case open to try and find out why. 

  • Thank you for the response. Few remarks:

    1. Using GPS on the wrist for swimming is quite challenging, since GPS signal does not penetrate water. My TomTom performs badly showing a track jumping in all directions. So reporting wrong distances is to be expected. The Fenix 7 performs very well. The track is pretty smoothed though, which I would expect (smoothing is the way to get rid of deviations).
      However, that is not an issue here since I took the Gamin out on my buoy. Better circumstances are not possible: no trees, no body that shields the GPS.

    2. I have 25+ years experience in software development, so I have a pretty good 'feeling' on what takes place inside the device software (as I showed above I hope). I therefore believe Garmin can get rid of the wrong calculation very easily (my second advise), even if occasionally the GPS hickups. I proved it by showing the good coordinates in Oziexplorer: the distance based on good coordinates is 3.05 km which is pretty close to the tomtom value of 3.16 km.

    I hope Garmin reads this forum... ;-)

    You write: 

    'if I swim a marked and measured 2k course and always get within 150m '

    I guess it is always short, right?

    Do you take your Garmin connected phone with you?

    What device do you use?

    Could you provide me with a FIT file of such a swim that is off by 100-150m (to verify whether the missing coordinates is an issue of my device)? I can send you my e-mail via a PM I guess.

    150m on 2 km is 7-8% When I compare GPSes they are usually within 1% of the distance. Can be due to the swimming inaccuracy mentioned above or due to the missing coordinate issue...

  • guess it is always short, right?

    No. While the course might be accurately measured at a point in time, I don’t swim in a perfectly straight line and the tide differences will always yield a distance that is 2000m or thereabouts. Garmin suggest around 10% error margin for open water swimming should be acceptable given the difficulties of getting the signal as you point out. That is why Garmin have developed algorithms over time using a combination of GPS signal and accelerometry etc to calculate swimming distance in open water That said, there are some who have never been able to get anywhere near accurate distance with any Garmin watch while some of have. Since your distance is so far out the best thing to do is to contact Garmin and get added to the case. 

    Do you take your Garmin connected phone with you?

    Swimming? In the water? No

  • I noticed same as you, an I could tell a difference only by my felling. Before tactix 7 I have 6X and measured swim distance is not the same between these two. Great work!

  • The case described above appears to be the worst of the 35 activities I logged so far. Maybe the GPS had a bad hair day or something...

    The table below shows the good and wrong coordinates and the percentage wrong coordinates in the FIT file.

    Usually when there is 1 or 2 coordinates wrong, these are the first coordinates logged.

    Swimming appears to be on top of the list followed by hiking

    Activity type Subtype Good Wrong Percentage wrong SW
    Swimming Open_Water 2448 1053 30% 8.18
    Hiking Generic 66 6 8%
    Swimming Open_Water 875 38 4%
    Hiking Generic 1608 36 2%
    Hiking Generic 934 17 2%
    Swimming Open_Water 2490 39 2%
    Cycling Mountain 1858 19 1%
    Swimming Open_Water 2685 17 1%
    Swimming Open_Water 2363 14 1%
    Hiking Generic 1167 6 1%
    Hiking Generic 248 1 0%
    Hiking Generic 377 1 0%
    Running Trail 916 1 0%
    Hiking Generic 1845 2 0%
    Hiking Generic 956 1 0% 8.21
    Hiking Generic 1154 1 0%
    Hiking Generic 2493 2 0%
    Cycling Generic 2418 1 0%
    Hiking Generic 2762 1 0%
    Swimming Open_Water 4263 0 0% 8.21
    Cycling Generic 1865 0 0% ?
    Running Generic 575 0 0%
    Hiking Generic 1825 0 0%
    Cycling Mountain 8271 0 0%
    Cycling Mountain 9050 0 0%
    Cycling Mountain 3435 0 0%
    Cycling Mountain 5929 0 0%
    Hiking Generic 4 0 0%
    Generic Navigate 12 0 0%
    Cycling Mountain 6148 0 0%
    Cycling Gravel_Cycling 3005 0 0%
    Cycling Generic 2804 0 0%
    Cycling Gravel_Cycling 2811 0 0%
    Cycling Generic 1615 0 0%
    Cycling Generic 1372 0 0%
  • There is an open ticket on beta forum , add your vote here if you are enrolled in beta program. 

    However there are many thread open regarding this issue inside fr945 , fenix 6 and fenix 7 forums.

    If you want to do a quick test, just use open water swim activity on a known path and than try to use walk or run activity on the same path. Or simply walk for 100m using walk activity and than come back with open water activity,  you will see a very big discrepancy. I made also a video with a delta solar (fenix 7 acts the same) :

  • If you want to do a quick test

    Try actually swimming in the water. That’s the only way to provide useful information to Garmin so they can work out why OWS works for some but not for others. People who are not swimming in the open water do not have a pony in the show. 

    While most of the people I regularly swim with are not seeing major errors in the open water, I am aware of at least one other who has reported similarly large errors to Garmin along with the activity files. 

    This morning’s swim - (Epix) and (F7) demonstrate once again that the inaccuracy that some experience is not universal, as is often the case. The best way to get a resolution is for Garmin to have as many examples as possible of good and bad swims. Walking on land simply does not represent swimming in the water. If you are swimming in the open water (not swimming on the land) and have issues with accuracy contact Garmin Support, provide them with recorded data from open water swims and help resolve the problem. 

  • Thanx! I've left a vote and a remark pointing to the analysis above.

  • Great summary - thanks

    As a triathlete, I moved over from Polar almost 15 (?) years ago, and am, like the readers of this forum, a raving fan of Garmin.  My only gripe….it is shite for open water swimming.  My Apple Watch is close to perfect in the water, which tells me somebody somewhere can get the software/hardware/GPS issue right.

    Last week I went for a 1500m open water swim with new Fenix 7X Sapphire Solar, it measured 682m for the 25mins…wasn’t going to post that on Strava ;)