Mastodon post image size
1200 × 675px
Aspect ratio
16:9
Use 1200 × 675 px (16:9) for Mastodon images — it matches the timeline preview, and the focal-point picker controls any crop.
Last verified June 2026
Mastodon is decentralized, which means image rules are really server defaults rather than platform absolutes — but the defaults are consistent enough to design for. A 1200 × 675 px (16:9) export matches the timeline preview shape in the official web app and most clients, so nothing important gets cropped away. Posts carry up to four images, and Mastodon’s underrated weapon is the focal-point picker: you choose the exact spot every crop centers on.
| Variant | Dimensions | Ratio |
|---|---|---|
| Landscape (recommended) | 1200 × 675 pxMatches the default timeline preview — displays without cropping. | 16:9 |
| Square | 1080 × 1080 px | 1:1 |
| Portrait | 1080 × 1350 pxCropped in some clients’ previews — set a focal point. | 4:5 |
| Large photos | up to ~4096 px wideVery large uploads are downscaled server-side; limits vary by instance. | varies |
Different Mastodon clients preview images at different ratios, so the only universal safe zone is the center: keep the subject and any text in the middle 70% of the frame, then set the focal point on it after upload (edit the attachment and drag the marker). That one step makes portrait and oddly shaped images preview correctly in every app that honors focal points — which the official apps and most major clients do.
No network takes image descriptions more seriously than Mastodon. Many users won’t boost an undescribed image, several popular accounts exist purely to remind people, and some servers surface missing alt text in moderation culture. Two or three plain sentences describing what the image shows isn’t just accessibility — on Mastodon it directly determines how far the post travels.
The 16 MB default is exactly that — a default. Some instances cap lower to save storage; a few allow more. If you post for a brand, check your home server’s stated limits once, then export comfortably inside them (a 1200 px-wide JPEG rarely exceeds 500 KB anyway). Federation re-serves your media to other servers, so lean files are good citizenship too.
After attaching an image, open the edit view and drag the focal point onto the subject. It takes three seconds and fixes the most common Mastodon image complaint — faces cropped out of preview thumbnails. Schedulers that publish via the Mastodon API preserve your uploaded framing, so a 16:9 export plus a set focal point covers every client behavior.
1200 × 675 px (16:9) — it matches the default timeline preview, so the full image shows without cropping. For other shapes, set the focal point after upload to control how previews crop.
Default-configured servers accept PNG, JPG, HEIF, WEBP, AVIF, and GIF up to 16 MB per image, four images per post. Limits are server-set in the decentralized model, so your instance may differ slightly.
Servers resize very large uploads to conserve storage and bandwidth across the federated network. Export at sensible web resolution — around 1200–2000 px on the long side — and the server will leave your image essentially untouched.
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