Which format is best for uploading to Strava?

FIT. Strava ingests FIT natively and it is the only one of the three that reliably carries heart rate, power, cadence, laps and session summaries in a single file. GPX uploads to Strava lose most of that, and TCX is the legacy fallback.

Can a GPX file store heart rate or power?

Not in any way most software honors. GPX 1.1 has a TrackPoint extension mechanism that can technically carry heart rate, but support across platforms is inconsistent at best. If you care about workout metrics, keep the FIT original.

How much smaller is FIT than GPX?

Usually 5 to 10 times smaller for the same recording. FIT is a compact binary format; GPX is verbose XML text. A three-hour ride might be 25 MB as GPX and 3 MB as FIT.

Will Garmin Connect accept a GPX upload?

It accepts the file, but it treats GPX as a route or course, not as a recorded activity. If you want it to appear in your activities feed with distance and time, upload a FIT or TCX instead, or convert first.

Is TCX still worth using?

Rarely. It dates from the Garmin Training Center era and newer formats have superseded it. It remains useful for a handful of older devices and legacy coaching software, so we keep a converter around for exactly those cases.

FIT vs GPX vs TCX: Which GPS File Format Should You Use?

If you have ever exported a ride or a run from a GPS watch, you have met at least one of these three formats. They all describe the same basic thing — where you went, when, and (sometimes) how hard you worked — but they are not interchangeable, and picking the wrong one silently drops data.

Here is the short version: FIT is the archive format, GPX is the exchange format, TCX is the legacy format. The rest of this guide explains what that means in practice.

What each format actually contains

FIT — the full recording

FIT (Flexible and Interoperable Data Transfer) is a compact binary format that Garmin designed for devices. A FIT activity file is not a flat list of points. It carries a file header, a session message summarizing the whole activity, lap messages describing structure, and a dense stream of record messages — every GPS fix, heart rate sample, power reading and cadence value the device logged.

Because it comes straight off the device, FIT is the most complete record you will ever have of a workout. That is why every piece of advice about data archival starts with keep your original FIT files.

The trade-off: being binary, FIT is not human-readable, and software support outside the sports world is thin. GIS tools, mapping sites and most route planners simply do not read it.

GPX — the universal exchange format

GPX (GPS Exchange Format) is XML, standardized long before FIT, and supported by essentially everything that touches a map: OpenStreetMap editors, route planners, Google Earth, hiking apps, cycling sites. A GPX file holds waypoints, routes and tracks — point sequences with timestamps and elevation.

What GPX does not hold, in any widely honored way, is workout data. Heart rate and power can technically ride along in extension elements, but support is patchy. When you convert FIT to GPX, you keep the track and lose the training metrics. That is a property of the format, not a flaw in the converter.

TCX — the legacy bridge

TCX (Training Center XML) is Garmin's older XML format from the Training Center era. It can represent activities and courses, and unlike GPX it does carry heart rate and power data. It was the upload format of choice for training platforms in the late 2000s.

Today almost everything that reads TCX also reads FIT, so TCX survives mainly on older devices and in legacy coaching workflows. It has no meaningful capability that FIT lacks.

A decision table for real situations

Situation	Use	Why
Archiving your original recordings	FIT	Complete data, compact, device-native
Sharing a route with a friend	GPX	Every mapping tool reads it
Uploading to Strava or Garmin Connect	FIT	Native ingest, metrics survive
Moving a track into a GIS tool	GPX	Universal support
Feeding an older device or coaching app	TCX	Compatibility with legacy software
Building a course to navigate	FIT (or GPX)	Devices accept both; FIT carries turn data

What conversion actually loses

Every conversion between these formats is lossy in one direction or another:

FIT → GPX: track, timestamps and elevation survive; heart rate, power, cadence, laps and session summaries do not. (Convert FIT to GPX)
GPX → FIT: you get a valid FIT file containing exactly what the GPX had — a track. There are no heart rate samples to restore, because they were never in the source. (Convert GPX to FIT)
TCX ↔ GPX: workout metrics ride along in TCX but vanish in GPX, same as above. (TCX to GPX, GPX to TCX)
FIT → TCX: most training data survives, but TCX cannot represent everything modern FIT files carry, notably developer-defined fields. (Convert FIT to TCX)

One more subtlety: timestamps. FIT stores UTC plus a timezone-offset message; GPX timestamps are UTC with no offset. If a converted file shows your evening ride at the wrong time of day, timezone handling — not data loss — is usually the reason.

The practical rules

Never delete your original FIT files. Storage is cheap; a re-encoded GPX is not a substitute for the source.
Convert on demand, not ahead of time. Keep originals, and produce a GPX or TCX when a specific tool needs one — our FIT to GPX converter and its siblings run in your browser precisely so this is a two-minute task, not a commitment.
Match the format to the destination, not the other way around. Uploading to a training platform? FIT. Handing a track to a mapping tool? GPX. Running a ten-year-old head unit? TCX, and only then.

Get those three habits right and the format question stops being something you think about — it becomes something you look up once, here, and move on.