Convert FIT (device activity file) to CSV (spreadsheet)

Spreadsheets are still the fastest way to actually dig into a workout's numbers — sort by power, chart heart rate against elevation, filter out a stretch of bad GPS — and FIT's binary structure makes that impossible without converting first. This tool reads every record message a FIT file contains and writes it out as one row per point, with a header naming exactly the columns your file has data for: timestamp, elapsed seconds, latitude, longitude, elevation in metres, distance, speed, and then whichever of heart rate, cadence, power, and temperature your device actually recorded. Nothing is invented to fill a gap — a channel your watch didn't log simply isn't a column in the output, rather than showing up as a field full of blanks. Because CSV is FitFileHub's own schema rather than an external standard, every channel FIT supports maps onto it fully, including power and temperature, which get dropped or degraded converting into some of the other formats on this site. Lap boundaries survive too, though not as a true lap structure — CSV represents them as a per-point lap-index column, so you can still group or filter by lap in a spreadsheet even though there's no nested lap object the way TCX has. Timestamps keep FIT's original millisecond precision rather than being rounded to the second. The whole conversion — reading the binary FIT stream and generating the CSV text — happens locally in your browser using a background worker, so a long ride with hundreds of thousands of trackpoints doesn't lock up the tab, and your file is never uploaded anywhere. Free for files up to 25 MB, three at a time; supporting the project removes those limits.