Mastodon Image Sizes & Dimensions (2026 Guide)

Mastodon profile picture size (400x400px)

Mastodon stores every avatar at exactly 400x400 pixels. Upload something larger and the server crops it to a square and scales it down; upload something smaller and it gets stretched. Starting from a 400x400 export is the only way to control what the crop keeps.

• Non-square images are cropped, not letterboxed. A 1600x900 upload loses the left and right thirds. Square your image before uploading.
• The mask changes per client. Mastodon’s default web interface renders avatars as rounded squares; iOS/Android apps and many third-party clients render them as circles. Keep faces and logos inside a centered circle so both look right.
• It displays small. In a timeline the avatar is only a few dozen pixels wide. Fine detail and small text disappear — use a bold, high-contrast subject.
• Animated GIF avatars become static. Mastodon generates a still PNG version, so don’t rely on motion for the first frame to read well.

Mastodon header image size (1500x500px)

The Mastodon header — the banner across the top of your profile — is 1500x500 pixels, a 3:1 aspect ratio. Like the avatar, it is resized server-side on upload, so exporting at the exact dimensions avoids a second round of compression.

• Your avatar overlaps the bottom-left. Keep text and logos in the center or right two-thirds of the frame so nothing is hidden behind the profile picture.
• The visible strip shrinks on mobile. Narrow screens crop the header horizontally rather than shrinking it, so anchor important content near the middle.
• 3:1 matches Twitter/X. One 1500x500 export covers both platforms — see Twitter/X image sizes for the rest of that set.
• Headers are optional. Servers show a flat color block if you skip it, which is better than a stretched low-resolution image.

Mastodon post image size and aspect ratios

Mastodon does not force a single aspect ratio on post images. It keeps the ratio you upload and only intervenes when an image exceeds 8,294,400 pixels in total area — the equivalent of 3840x2160 — at which point it is scaled down proportionally. In practice any sensible export sails under that cap.

• 16:9 (1280x720) is the safest default. It fills the timeline width without dominating the column.
• 1:1 (1200x1200) works well for graphics, quote cards, and screenshots.
• 4:5 (1200x1500) claims more vertical space, the same trick that works on Threads and Bluesky.
• Up to 4 images per post. Multiple images render in a grid, so give them a shared aspect ratio or the grid looks ragged.
• Very tall images are cropped in the preview. The timeline thumbnail is regenerated at roughly 230,400 pixels of area, and readers tap through to see the full image.
• Set a focal point. Mastodon lets you click the spot in an image that must survive cropping. Use it for anything where the subject is off-center.

Alt text (Mastodon calls it a description) accepts up to 10,000 characters, and the culture on the fediverse strongly expects it. Write it — it is the single most-noticed difference between a Mastodon post and a cross-post from elsewhere.

Mastodon image file size limits

These are stock Mastodon’s defaults. Because every instance is independently operated, an admin can set them lower to save storage — or higher, if they are feeling generous. Check the about page of your own server if an upload is rejected.

• Animated GIFs become silent MP4 videos. Mastodon transcodes them on upload, so a 15MB GIF may still work — but an actual video file is a better source.
• EXIF metadata is stripped. Location and camera data are removed on upload, though orientation is respected. Don’t count on copyright fields surviving.
• Federation copies your file. Every server that sees your post caches the image, which is why admins care about upload size. Being economical is good fediverse manners.