Mastodon isn’t one platform in the way Instagram or X are, it’s a network of independently run servers that all speak the same protocol. The dimensions below are the ones the standard Mastodon software uses, but individual servers set their own file size caps on top of that, which is why an image that uploads fine on one instance can be rejected on another.
Here are the dimensions Mastodon’s default software uses.
| Type | Size | Aspect ratio |
|---|---|---|
| Profile Photo | 400 x 400 | 1:1 |
| Header | 1500 x 500 | 3:1 |
| Post Image | 1920 x 1080 | 16:9 |
| Post Image (Square) | 1080 x 1080 | 1:1 |
| Post Image (Portrait) | 1080 x 1350 | 4:5 |
Which size to actually use
1920 x 1080 is the default worth reaching for on a standard post image. It displays cleanly in the timeline without being cropped by most Mastodon clients, and it matches the wide shape that link previews already use across the network.
Square at 1080 x 1080 and portrait at 1080 x 1350 both work, and portrait takes up more vertical space in a timeline the same way it does on Instagram. Because different apps render Mastodon differently (the official web client, Ivory, Ice Cubes and others), a standard ratio behaves more predictably than an extreme one, an ultra-tall image can display inconsistently across clients in a way it wouldn’t on a single-app platform.
Format and common mistakes
Export as JPEG for photos and PNG when you need transparency or sharp text. The most common mistake on Mastodon specifically is not realizing your home server has its own upload size cap, some instances allow much larger files than others, so an image that works fine on one server gets rejected on a stricter one. A social media image resizer gets your source image to a sensible size before you find out the hard way.
Run an image through a Mastodon image size checker if you want to confirm it before posting, and use the social media image size guide if you’re keeping several platforms’ specs straight at once.
An image compressor is worth using here more than on most platforms, since a stricter server-side file size cap is more likely to reject an oversized upload outright rather than just recompress it for you.