If your Apple Watch stops adding distance during a swim, first check whether the workout is paused or the timer is still running. A paused workout and missed pool lengths are different problems. Identify which happened before changing settings or resetting the watch.

Did the timer stop, or only the distance?

  • The timer stopped: look for a paused workout and resume it before continuing. See how to pause and resume a swim.
  • The timer runs but distance stays unchanged: finish at the wall, then compare the saved length count with your own count.
  • The workout ended or the watch restarted: note the battery level, watchOS version, and the time it happened. This is a different failure from one missed turn.
  • Only heart rate is missing: follow the swimming heart rate checks; a gap in that graph does not establish that distance recording stopped.

How can you check whether distance recording stopped?

Record a separate Pool Swim with the correct pool length and units. Swim a small number of complete lengths using one stroke, count them yourself, and rest at the wall. Compare the saved result with that count before trying another change.

For example, eight lengths in a 25-meter pool total 200 meters. If the workout shows seven lengths and 175 meters, investigate length detection. If it shows eight lengths and 400 meters, check the pool-length setting. Keep the test simple enough to distinguish those errors.

Does Reset Fitness Calibration Data fix swimming?

Apple documents a calibration reset, but its calibration procedure concerns walking and running stride measurements. It does not establish that changing your swimming technique creates an outdated stroke profile, or that resetting calibration reliably fixes missing swim lengths.

The setting is in the iPhone Watch app under My Watch → Privacy → Reset Fitness Calibration Data. Treat it as a broader troubleshooting step if Apple Support recommends it, rather than a guaranteed swimming fix. See Apple’s calibration documentation.

What should you save if it happens again?

Save the test result and note the watch model, software version, workout app, pool length, and whether the timer kept running. Restart normally after saving the workout and check for software updates before repeating the same test. Give Apple Support both the expected and recorded distance if the failure persists.

For errors that occur throughout a session, use the complete missing-laps diagnostic guide.