Mastodon image sizes
| Use | Size (px) | Ratio | Notes |
|---|---|---|---|
| Post image | Not fixed | Any | PNG, JPG, HEIF, WEBP or AVIF up to 16 MB, downscaled to about 8.3 megapixels (3840x2160). Up to 4 images per post. Animated GIFs: up to 16 MB, under 1 megapixel, one per post. |
| Avatar | 400 × 400 | 1:1 | Cropped to 400x400. Up to 8 MB per the Mastodon source code. |
| Header | 1500 × 500 | 3:1 | 1500x500 header dimensions. Up to 8 MB per the Mastodon source code. |
Which size to use
Mastodon crops avatars to 400 × 400 and uses 1500 × 500 (3:1) for headers. Both can be up to 8 MB, per Mastodon’s source code. Sizing them yourself means you choose the crop instead of the server.
Post images can be any ratio. Mastodon accepts PNG, JPG, HEIF, WEBP or AVIF up to 16 MB and downscales anything above about 8.3 megapixels (3840 × 2160). A post can have up to 4 images, or one animated GIF under 1 megapixel.
The general shapes in the size menu are starting points for posts, not Mastodon rules. Admins can change their server’s limits, so if yours isn’t mastodon.social, check its settings.
Mastodon video limits
The resizer handles still images. For video, these are the limits to plan around.
| Use | Max length | Max file size | Ratio | Notes |
|---|---|---|---|---|
| Video attachment | Not stated | 99 MB | Any | MP4, M4V, MOV or WebM up to 99 MB, transcoded to H.264. One video per post. mastodon.social limits frames to 8,294,400 pixels (3840x2160) and 120 fps. No duration limit is documented. |
Tips for Mastodon images
- Add a media description (alt text) to every image.
- Use the general square for single photos and the landscape shape for screenshots.
- Write multi-word hashtags in CamelCase so screen readers read them correctly.
Schedule Mastodon images with Postocial
Postocial publishes to Mastodon on any instance. Attach your images, add descriptions, set a custom thumbnail for videos and schedule a single post or a whole thread.
See how Postocial works with Mastodon.
Sources
The Mastodon sizes on this page come from these pages. Where a network publishes no size of its own, we say whose recommendation it is. Researched on . Networks change their limits without notice, so check the official pages before a big launch.