How does a Fitbit know how many lengths you've swam?

[Cocolapew]

I'm baffled.

In my experience it doesn't! I've found it wildly inaccurate. But I think it's trying to do it from when you change directions, which it can tell from your arm movements.

In the settings you can set pool length, so it works it out from that once it picks up that you're swimming.

Although I don't know how accurate it is because it thinks I'm swimming when I'm washing up or washing my hair!

It tries to work it out by assessing when your arms stop moving as you push off at the end of each length. I find it fairly accurate (I do sets so I know the distance I'm swimming anyway) but imagine that if you don't have much of a glide then it's not as accurate.

If it's the same as my garmin, it seems to work out when you stop briefly and start again.
It gets confused if you change strokes mid-length or pause to cough up pool water (I'm not a great swimmer...)

Thanks all. I sometimes swim on my back and not use my arms does that mean its not counting it? I have visions of myself flapping my left arm about like Nemo.

My Garmin counts lengths by recognising the turns. It's really inaccurate. When I did 5k for swimathon and a had an actual human standing at the end of my lane tallying my lengths it was over 1000m different.

I find my garmin is pretty accurate, i did 5km and it was only a length out (I paused it several times along the way to keep a check on it). However it's not good if you do drills, use a float or anything other than "standard" strokes so I assume it works on arm movements.

Mine is a bit rubbish, it thinks I'm riding a bike when I'm pushing a pram. Did a very intense caroline girvin kettlebell workout yesterday which it registered as a 30 min walk. It does know when I'm swimming but thinks I do about 9 lengths instead of 40! I only wear it for the step count really.

Mine is rubbish as well, but it's better than me trying to count myself and losing count.

Also it often thinks I am on the elliptical when I am actually going for a walk.

My fitbit versa didn't survive swimming. I had to request a replacement from fitbit as it kept glitching after being wet although I always followed the instructions.

My Apple Watch seems to count my lengths very accurately, including if I’ve turned half way through a length to avoid a slower swimmer in the lane.

Conversely, if I take the dc swimming I spend most of my time pretty stationery watching them jump and splash and yet after an hour in the pool with them somehow my watch still thinks I’ve done about 30 lengths!

So to conclude, I’m not really sure what it’s doing

I have a misfit swim and it's pretty accurate. You tell it how long a length is and where on your body you are wearing it. It can't recognise yoga though.