Quick answer: size Mastodon profile pictures at 400×400, the header at 1500×500 (3:1), and post images at 1080×1080 square, 1920×1080 landscape, or 1080×1350 portrait. The one thing no guide can give you is the file cap on your specific server — that is set by the instance admin, not the app.

How Mastodon handles images

Mastodon is open-source software running on thousands of independent servers. The default interface renders avatars at 400×400 and headers at 1500×500, and those numbers are consistent across the network. What changes per server is the upload limit: a stock Mastodon config historically allowed 2 MB for profile images and up to 16 MB for post attachments, while many large instances now accept 8–16 MB for everything. Check your instance's about page before you export.

The avatar is shown as a rounded square in the web UI and as a circle in many third-party apps, so keep the subject centered with padding. The header overlaps the avatar at its lower-left corner, which is the same layout X uses — the left edge is not a safe place for a logo.

The size table

Asset Dimensions Ratio Notes
Profile picture 400×400 1:1 Rounded/circle crop; 2–8 MB by server
Header 1500×500 3:1 Avatar overlaps lower-left; 2–8 MB
Post image (landscape) 1920×1080 16:9 Downscaled to ~1280 px display
Post image (square) 1080×1080 1:1 No crop in the timeline
Post image (portrait) 1080×1350 4:5 Taller images get cropped
Link preview 1200×630 1.91:1 Pulled from Open Graph
Post attachments Up to 4 — 16 MB each on stock servers

Post images and the timeline

Mastodon displays attachments at roughly 1280 px on the long edge, so uploading a 4000 px camera file just wastes bandwidth and gets resized. The timeline crops tall images to a 4:5-ish box with a click-to-expand, which means a 9:16 phone screenshot shows only its center in the feed. If the screenshot carries text, export it at 1080×1350 or accept that followers open it.

One quirk worth knowing: Mastodon preserves image order and lets you add alt text per attachment, which is more than most networks do. The alt text is indexed by the instance's full-text search, so it is both an accessibility feature and a discovery one.

When a post includes a URL, Mastodon fetches the page's Open Graph metadata and builds a preview card at 1200×630, same as Discord and Slack. A missing og:image means a text-only card. The Open Graph image size guide covers the tag, and the aspect ratio explainer explains why 1.91:1 keeps recurring.

Fediverse launches

Developer tools and indie products launch on Mastodon the way they launch on Hacker News — a text post with a screenshot, no marketing gloss. The image set that works is a square or 16:9 product shot plus a proper OG card, and the fediverse audience reads alt text before it reads the pitch. Neatico generates the ratio variants and the OG card together from one product description, which keeps the post from looking like a campaign.

Frequently asked questions

What is the Mastodon avatar size? 400×400 pixels. It renders as a rounded square in the web app and a circle in many mobile clients, so center the subject with 10% padding.

What size is the Mastodon header? 1500×500 at 3:1. The avatar overlaps the lower-left, so keep logos and key text toward the center or right.

Why does my upload fail at 8 MB? The file cap is set per server. Stock Mastodon allowed 2 MB for profile images; check your instance's about page or ask the admin.

Does Mastodon crop post images? The feed crops tall attachments to roughly 4:5 until clicked. Export at 1080×1350 portrait or 1920×1080 landscape; the social media image sizes hub has the full network table.