I tried it on 100m, 400m and 1km swims. In each case it miscounted my turns and distance which I kept in my head to verify its accuracy. In one 100m it missed two turns claiming I did 50m and not 100m. That's patently wrong since I know the pool I go to is 25m and I can count to 4. All my turns were flip turns under water with relatively good technique (think advanced amateur who almost made it to swim team in college)