What Is a THM File? File Format Explained

If you've stumbled across a .thm file on your computer, camera, or media device and aren't sure what to do with it, you're not alone. THM files are one of those quiet background formats that show up without much explanation. Here's what they actually are, where they come from, and what affects how useful — or useless — they might be for you.

The Basic Answer: THM Files Are Thumbnail Images

A .thm file is a thumbnail image file. The name comes from "thumbnail," and that's exactly what it contains: a small, compressed preview image associated with a larger media file — typically a video.

These files use the .thm extension but are almost always JPEG images underneath. If you rename a .thm file to .jpg, most image viewers will open it without any trouble, because the format itself is standard JPEG — only the extension is different.

Where Do THM Files Come From?

THM files are generated automatically by certain devices and software. You didn't create them manually, and you usually won't need to. The most common sources include:

• Canon digital cameras and camcorders — Canon devices generate .thm files alongside video files (typically .avi or .mov) as a visual reference for the clip
• Sony cameras and camcorders — Similar behavior, pairing thumbnails with video recordings
• Garmin GPS devices — Garmin uses .thm files as thumbnail images associated with map themes or custom vehicle icons
• Other digital cameras and media recorders — Various manufacturers adopted .thm as a convention for pairing preview images with video content

The logic is practical: when you're browsing footage on a small camera screen or in media management software, loading full video files for every clip is slow. The .thm file gives the software something quick to display instead.

How THM Files Are Paired With Other Files 🎥

THM files almost never exist on their own. They're companion files — always paired with another file that shares the same base name. For example:

The pairing is by matching filename. If you move or rename one file without the other, software that relies on the thumbnail association may stop displaying the preview correctly — though the actual video file remains completely intact.

Can You Open a THM File?

Yes, and it's straightforward. Since THM files are standard JPEG data, your options include:

• Rename the extension from .thm to .jpg — most image viewers, including Windows Photos, macOS Preview, and any browser, will open it
• Use an image editor like GIMP, Photoshop, or Affinity Photo, many of which will open .thm files directly
• Use a dedicated media manager — software like Adobe Bridge, digiKam, or the camera manufacturer's own tools often reads .thm files natively without any conversion

The image quality inside a THM file is generally low resolution — think 160×120 pixels or similar — because the only purpose is a quick visual reference, not a usable photograph.

Should You Keep or Delete THM Files?

This is where your own situation starts to matter. The answer depends on a few variables:

Disk space: THM files are tiny — typically a few kilobytes each. Even thousands of them won't meaningfully impact storage. Deleting them purely for space reasons rarely makes a practical difference.

Workflow and software: If you use camera manufacturer software, video editing tools, or media asset managers that rely on .thm thumbnails for navigation, deleting them may break the preview display in those applications. The underlying video is unaffected, but browsing becomes slower or less visual.

Archiving practices: If you're archiving raw camera footage long-term, keeping .thm files preserves the original folder structure exactly as the device created it. Some archivists prefer this; others strip companion files for cleaner organization.

Transfer and backup: When copying camera footage to a hard drive or cloud storage, .thm files copy along with everything else automatically. No special handling is needed.

THM Files and Garmin Devices 🗺️

It's worth separating the Garmin use case because it's different from camera thumbnails. On Garmin GPS devices, .thm files serve as theme preview images — small visuals that represent how a map skin or custom vehicle icon will look. These files sit inside the device's theme folders and are referenced by the device's UI.

In this context, deleting or modifying a .thm file can affect how themes display on the device. The variables here include which Garmin device model you have, which firmware version it's running, and whether you're installing custom map themes from third-party sources.

Factors That Shape How THM Files Affect Your Workflow

Not every user encounters .thm files the same way. The experience varies based on:

• Device type: Camera users see them routinely; Garmin users less so; most desktop-only users may never encounter them
• Operating system and file view settings: Windows shows .thm files in folders by default; some configurations hide them if file extensions are hidden
• Media management software: Some applications handle .thm natively; others ignore them; a few flag them as unknown
• Transfer method: Dragging folders manually vs. using import software affects whether .thm files end up in your destination folder at all
• Storage organization style: Flat folder structures vs. date-sorted or project-sorted archives change whether the pairing stays intact

Someone editing wedding videos from a Canon camera in Premiere Pro has a very different relationship with .thm files than someone who occasionally pulls clips from an old camcorder onto a Mac.

The format itself is simple and stable — but what it means for your specific files, devices, and tools depends entirely on your setup. 📁