Why a pool activity can have the wrong number of lengths
A compatible Garmin watch estimates a pool workout from motion and the pool size you enter. In a simple session, distance is the number of completed lengths multiplied by the pool size. The watch uses changes in arm movement, stroke mechanics, turns, and the push-off from the wall to decide when one length ends and the next begins. That works well when a swimmer turns consistently, but a missed push-off, an early pause, a drill, or a brief interruption can make one real length look like two fragments or make two real lengths look like one long interval.
The file does not contain a hidden video of the swimmer. It contains recorded messages: lengths, laps, timestamps, stroke counts, and summaries. A repair can therefore compare patterns in those messages, but it cannot prove what the watch saw at the wall. That distinction is important. SwimLapFix is designed to make a cautious proposal from measurable timing, not to invent certainty where the export has no sensor ground truth.
Step 1: build a swimmer-specific baseline
The analyzer starts with the active lengths in the uploaded activity. It derives a typical duration for that swimmer and that session rather than applying a universal pace threshold. Obvious outliers are excluded while the baseline is formed, because the very errors being investigated should not define what “normal” means. Rest intervals and incomplete records are treated differently from active lengths when the FIT messages make that distinction available.
This baseline is a measuring stick, not a target. A swimmer can change pace during a workout, and a set can intentionally contain different intervals. The detector therefore looks for a close match to a small number of ordinary lengths and requires more than a length merely being fast or slow. A long length must look like the time of two to four typical lengths, or adjacent short fragments must add up to a plausible normal length. If the evidence is not strong enough, the safer result is to leave the activity alone.
Step 2: apply only bounded corrections
A missed turn can look unusually long
When one recorded length is much longer than the session baseline and its duration closely matches two, three, or four ordinary lengths, SwimLapFix can propose splitting it into that many lengths. The proposed pieces retain the total duration and distribute the existing stroke information conservatively. The comparison view identifies the original long bar and the new lengths so you can decide whether the pattern fits your workout.
A false turn can create short fragments
When adjacent recorded lengths are unusually short and their combined duration resembles one normal length, the tool can propose merging them. This is especially useful when a small arm movement or hesitation at the wall caused the watch to close a length too early. The merge is bounded to neighboring fragments and does not try to rearrange unrelated portions of the session.
These operations are intentionally narrow. The implementation does not ask a language model to interpret your swim and does not rewrite every message in the file. It carries the chosen length and lap structure forward, rebuilds relevant counts and distances, writes a new FIT header and checksum, and decodes the output again to validate that the file is structurally readable.
Step 3: compare before downloading
The result page shows the recorded distance, proposed distance, active-length counts, detected patterns, and a duration chart. Read those details against your memory of the set. A plausible total is not enough if the timing or lap pairing looks wrong. Keep the original Garmin ZIP or FIT file in a safe place; the repaired file is a new copy and should never be your only record.
For a proposed repair, the downloaded file is intended to be imported as a separate activity after you decide how to handle the original. Garmin Connect may identify it as a duplicate. Retaining the original lets you undo a choice, compare metadata, or provide a useful report if the correction does not describe the swim. If the analysis says no high-confidence correction was found, it is deliberately not forcing a change.
Limits worth knowing
- There is no sensor ground truth in an exported FIT file, so the detector cannot guarantee that a proposed split or merge is what happened.
- Ambiguous swims, drills, long stops, mixed sets, and activities with too little consistent data may be left unchanged.
- Pool size, stroke mechanics, push-off quality, and pauses affect the watch’s original length detection; a repair cannot correct a wrong pool-size setting by itself.
- Re-encoding through the public FIT profile can omit unknown Garmin-private fields. Standard timing, length, lap, session, and record data are validated, but the output should still be compared with the original.
Have the context the file cannot provide?
If a correction is wrong, use the report form in the repair flow and include the expected distance or active-length count. A report is optional and requires explicit consent because it retains the original activity for investigation.
Sources and related reading
Garmin’s Using the Pool Swim Activity on My Garmin Watch page explains pool size, accelerometer-based turn mechanics, accuracy, rests, and Drill Log behavior. Its Tips For Using Pool Swim Workouts on a Garmin Watch page adds workout context, while Garmin’s export and manual-upload pages describe the file workflow. SwimLapFix paraphrases those official references and uses the activity export as the source for its own comparison.