Why won’t my .fit file open

Start by finding out which problem you have. Drop the file below — if any of it is readable, you will see what is there, and if none of it is, you will see why.

How often files are actually broken

We parse a corpus of 68,068 real .fit files — 602 MB from 46 manufacturers, covering 241 versions of the format. Of those, 4 are unreadable. That is 0.006%.

So the prior is strongly against your file being broken. Far more often, the file is fine and the software is refusing a legal feature of the format. Four causes, in the order they actually occur.

1. The file is truncated

The commonest by a wide margin. A FIT header declares how many bytes of data follow; if the watch lost power, filled its storage, or was unplugged mid-transfer, the file is shorter than the header promises. Strict readers treat the mismatch as corruption and refuse the whole thing.

They should not. The records before the cut are complete and readable. This page recovers them — see truncated .fit files for what that looks like and how much you typically keep.

2. The checksum does not match

FIT files carry a CRC. It fails on 3 of our 68,068 files — and all of them parse completely anyway. A checksum protects against corruption in transit; a failed one is a reason to be careful, not evidence that the data is gone. What a CRC mismatch actually means.

3. The reader caches a definition that the file replaced

This is the one that produces the strangest symptoms — a file that opens, shows the first few minutes correctly, and then produces nonsense or stops.

FIT lets a file reuse one of its 16 message slots for a different message type partway through. It is not an edge case: 35,971 of 68,068 files (52.85%) do it, 566,605 times between them. A reader that caches the first definition for a slot reads every later record with the wrong field layout.

If your file opens in one tool and not another, or opens and shows implausible values after a certain point, this is the likeliest cause and there is nothing wrong with your file.

4. It is not a .fit file

Some devices and services hand you a .zip containing the FIT file, or a .tcx/.gpx with a misleading name. A real FIT file has the ASCII bytes .FIT at offset 8 — the tool above checks that rather than trusting the extension, and will tell you if what you have is something else.

What we cannot help with

If the file is zero bytes, or the device wrote nothing before failing, there is no data to recover and no tool can invent it. The page will say the file parsed and contains nothing, which is a different and more useful answer than "corrupt".