Mastodon Image Sizes 2026 — Avatar, Header & Post Dimensions
Mastodon's image limits are set per server (instance), but the defaults used by mastodon.social and most instances are consistent. This guide covers the exact dimensions for your avatar, header, and post images, plus the file-size caps to stay under so your media isn't downscaled.
Mastodon image sizes — quick reference
| Placement | Size | Ratio | Notes |
|---|---|---|---|
| Avatar (profile picture) | 400×400 px | 1:1 | Rounded square |
| Header (banner) | 1500×500 px | 3:1 | Behind profile |
| Post image | Up to 3840×2160 px | Any | Default max 16 MB |
| Feed preview crop | Displayed ~16:9 | 16:9 | Full image on click |
| Link preview image | 1200×630 px | 1.91:1 | From your OG tags |
Per-instance limits (important)
Unlike a single centralized network, Mastodon limits depend on the server you're on. The figures here are the common defaults (mastodon.social and the standard configuration): 16 MB per image and downscaling of anything above roughly 8 megapixels (about 3840×2160). Some instances raise or lower these, so check your server's about page if an upload is rejected.
Avatar and header
The avatar is shown as a rounded square at 400×400 px — upload at least that size, square, with the subject centered. The header sits behind your profile details as a 1500×500 px (3:1) strip, so keep key elements toward the center and avoid putting text where the avatar and name overlap it.
Post images and formats
Mastodon accepts JPEG, PNG, GIF, and (on most modern instances) WebP and HEIC. Images above ~8 MP are downscaled automatically, so resizing to 1920×1080 or 2048-wide before posting keeps you in control of quality. Always add a description (alt text) — the Mastodon community strongly expects it for accessibility.