Mastodon Media Upload Fails With File Too Large: Fix

When you try to attach an image or video to a Mastodon post and see the error “File too large,” your upload is being rejected by the server. Mastodon instances enforce a maximum file size limit to control storage costs and bandwidth usage. This limit is set by your instance administrator and cannot be changed from your account settings. This article explains why the limit exists, how to check your instance’s specific limit, and the exact steps to resize or compress your media so the upload succeeds.

Why Mastodon Rejects Large Media Files

Mastodon instances run on servers with finite storage and bandwidth. Each uploaded file consumes disk space and bandwidth when other users view it in their timelines. To keep the instance running smoothly, administrators set a maximum upload file size. This limit typically ranges from 4 MB to 16 MB for images and 40 MB to 100 MB for videos, depending on the server’s resources and policies.

When you upload a file that exceeds this limit, the Mastodon server returns the “File too large” error before the file is stored. The error appears as a red banner at the top of the compose box. No part of the file is saved, so you must resize or compress the file locally before trying again.

The limit applies to the total file size, not the image dimensions. A 20 MB photo with 6000×4000 pixels will fail the same as a 20 MB video. You can reduce the file size by lowering resolution, reducing quality, or changing the file format.

Steps to Fix the File Too Large Error on Mastodon

1. Check your instance’s upload limit
   Open Mastodon in your browser. Go to Preferences > Appearance > Instance details. Scroll to the Server section and look for the line that says “Maximum file size for uploads.” The value is shown in megabytes. If you cannot find it, contact your instance admin or check the server’s about page at yourinstance.social/about/more. This number is your target for the next steps.
2. Resize an image using Windows Photos app
   Right-click the image file and select Open with > Photos. Click the ellipsis menu (three dots) in the top-right corner and choose Resize. Select a predefined size like 1920 x 1080 or choose Custom to set a max width of 1200 pixels. Check the box “Keep aspect ratio” to avoid distortion. Click Save a resized copy. The new file should be smaller. Check its size by right-clicking the file and selecting Properties. Repeat if it is still above the limit.
3. Compress an image using an online tool
   If resizing is not enough, use a free web-based compressor like TinyPNG or CompressJPEG. Drag your image onto the website. The tool reduces file size by removing metadata and lowering quality. Download the compressed file. Verify the size is under your instance’s limit. Do not upload sensitive images to online tools if privacy is a concern.
4. Reduce video file size with HandBrake
   Download and install HandBrake (free, open source). Open HandBrake and drag your video file into the Source Selection window. Under Presets, choose Fast 1080p30. This preset reduces resolution to 1920×1080 and uses efficient encoding. In the Summary tab, adjust the Quality slider to a value between 20 and 25 for a good balance of size and quality. Click Start Encode. The output file will be significantly smaller than the original. Check its size in File Explorer before uploading to Mastodon.
5. Upload the compressed file to Mastodon
   Return to the Mastodon compose box. Click the attachment icon (paperclip). Select your resized or compressed file. The upload should proceed without the “File too large” error. If it still fails, repeat the compression steps with a lower quality setting or smaller dimensions.

Common Pitfalls When Reducing File Size for Mastodon

Image dimensions are reduced but file size stays the same

Reducing pixel dimensions does not always lower file size if the image contains complex details or high color depth. A 1000×1000 pixel photo of a dense forest can still be 8 MB. In this case, also lower the JPEG quality to 80% or use a compression tool to strip metadata like GPS coordinates and camera settings.

Video compression removes audio or reduces quality too much

Some compression presets drop the audio track or use very low bitrates. In HandBrake, check the Audio tab to ensure the track is included. For the Quality slider, a value below 20 produces very large files. A value above 30 may look blocky. Stick to 22-25 for most content.

Uploading via mobile app shows a different error

The Mastodon mobile app may display “Upload failed” instead of “File too large.” The cause is the same. Use a photo editor app like Snapseed or a video compressor app like Video Compress to shrink the file before uploading from your phone.

Mastodon Instance Upload Limits: Common Ranges

Item	Small Instance	Large Instance
Image max size	4 MB	16 MB
Video max size	40 MB	100 MB
Typical resolution limit	1280×720	1920×1080
Admin configurable	Yes	Yes

If your instance’s limit is very restrictive (4 MB for images), consider using a different instance with higher limits or hosting your own instance where you control the limit. Many Mastodon instances publish their limits on their About page.

You can now resize and compress any media file to fit within your Mastodon instance’s upload limit. Start by checking your server’s maximum file size in Preferences > Instance details. Then use Windows Photos, TinyPNG, or HandBrake to reduce the file. If you frequently upload large videos, ask your instance admin to increase the limit or switch to an instance with a higher cap. A useful tip: set your camera or phone to record video at 1080p instead of 4K to avoid compression work later.