Search for "alt text character limit" and you will find tables that disagree with each other, often because they were copied from an older table. Limits change: Bluesky and Mastodon have both raised theirs. This list gives the limits as of October 2026 and links each one to the platform's own documentation or source code, so you can see where the number comes from. Where a platform does not publish a limit, it says so instead of guessing.
The limits at a glance
| Platform | Alt text limit (October 2026) | Source |
|---|---|---|
| Bluesky | 2,000 characters | Bluesky app source |
| X | 1,000 characters | X Help Center |
| Mastodon | 10,000 characters from version 4.6; 1,500 on older servers | Mastodon source |
| Not published for the app; 4,086 through the API | LinkedIn Image API | |
| Not published for the app; 1,000 through the API | Instagram Platform docs | |
| Not published | Facebook Help Center |
A limit is a ceiling, not a target. Most images need far less than any of these, and the guide to alt text length explains why. The details below matter mainly for screenshots, charts and other images with a lot of text.
Bluesky: 2,000 characters
Bluesky's alt text dialog allows 2,000 characters. The number is set as MAX_ALT_TEXT in the constants file of the Bluesky app. Older guides list 1,000, which was the limit when users asked for more room in 2024.
Posts themselves are much shorter: the same file sets them at 300 characters. That is why the full text of a screenshot usually goes in the alt text rather than the post.
X: 1,000 characters
X's help article on making images accessible gives the limit as 1,000 characters on iOS, Android and the web, and the step-by-step article shows the count in the corner of the description box. The limit applies to each image, so a post with four images can carry four descriptions of up to 1,000 characters each.
Mastodon: 10,000 or 1,500, depending on the server
Mastodon is the one platform where the answer depends on where your account is. Each server runs its own copy of Mastodon, and the limit changed with the software:
- Mastodon 4.6, released in June 2026, sets the limit to 10,000 characters (
MAX_DESCRIPTION_LENGTHin the 4.6.0 source). - Mastodon 4.5 and earlier use 1,500 characters (4.5 source).
Large servers such as mastodon.social already run a version with the higher limit, but smaller servers update on their own schedule. To check yours, open https://your.server/api/v2/instance in a browser and look for description_limit under media_attachments. The field is documented in the Mastodon instance API.
If you write long descriptions, keep in mind that your post is also read on other servers. Some older versions rejected posts from elsewhere that carried long media descriptions, which the Mastodon changelog lists as fixed. Staying within 1,500 characters is the safe choice when a description has to reach everyone.
LinkedIn: no published limit in the app
LinkedIn's help article on adding alternative text explains where the field is but does not give a limit. The only official number is in LinkedIn's developer documentation: the Image API accepts alt text up to 4,086 characters and recommends keeping it under 120.
That recommendation is worth reading as advice about readability rather than a rule. Tables that list 300 or 1,000 for LinkedIn do not cite a LinkedIn source, and we could not find one.
Instagram: no published limit in the app
Instagram's help article on editing alt text does not state a limit for the app. Meta's developer documentation for publishing through the Instagram API accepts alt text "up to 1000 character" for image posts, and notes that Reels and stories are not supported.
You will often see 100 characters quoted for Instagram. We found no official source for it, so treat it with caution. If the app stops you, it will show you where.
Facebook: no published limit
Facebook's help article on editing alternative text explains how to replace the automatic description but gives no limit. Numbers quoted elsewhere are not backed by a Facebook source.
What happens when you go over
Platforms handle a description that is too long in different ways, and not all of them stop you:
- Bluesky lets you keep typing, then shows "Alt text will be truncated. Limit: 2,000 characters." When you save, the alt text dialog cuts the text at the limit and adds an ellipsis, so the end of your description is lost.
- X shows a running count in the corner of the description box, as its help article illustrates with "178 characters of the 1,000 characters available".
Either way, the end of a long description is the part at risk. That is one more reason to put the most important information first.
Why these numbers drift
Three reasons the tables you find online disagree:
- Limits change. Bluesky and Mastodon have both raised theirs since the first round of guides was written.
- The app and the API differ. LinkedIn and Instagram document an API limit but not one for the field in the app. Some tables report one as if it were the other.
- Some numbers have no source. A limit that appears without one has often been copied from another table that did not cite one either.
When a number matters, test it: paste a long description into the field and see what the platform does with it. The platform's own counter or warning is the final word.
What fits on every platform
If you post the same image to several platforms, write for the strictest field you use. A description of two or three sentences fits every limit above with room to spare. For a text-heavy image, staying under 1,000 characters keeps it within X, Instagram's API limit, and Mastodon on any version.
AltKit uses that ceiling: the alt text it writes is never longer than 1,000 characters, so a description from the web creator or the browser extension stays within every published limit above.