Design4 min read
Alt Text Character Limits on Every Network
Instagram stores 100 characters, LinkedIn 120, X 1,000, Bluesky 2,000. A description written for one and pasted into another is cut mid-sentence, and nothing tells you.
The alt text character limit is different on every network and the spread is twenty-fold: Instagram stores 100 characters, LinkedIn 120, Pinterest 500, X 1,000, Mastodon 1,500 and Bluesky 2,000. Facebook and Threads publish no figure at all.
Every alt text character limit, in one table
| Network | Limit | What happens past it |
|---|---|---|
| 100 | The field stops accepting input; pasted text is cut | |
| 120 | Cut, often mid-word | |
| 500 | Cut | |
| X | 1,000 | Cut |
| Mastodon | 1,500 | Cut. Other instances may be configured lower |
| Bluesky | 2,000 | Cut |
| Not published | Your text replaces Facebook's auto-generated description | |
| Threads | Not published | — |
| A web page | None | Nothing. HTML sets no limit |
Instagram's own accessibility help documents the 100-character field (Instagram Help), and it is the tightest of the group by a wide margin.
Why the numbers are so far apart
They are answering different questions.
Instagram's 100 characters is a caption-field decision: alt text there is one line in a composer designed for phones, and the product treats it as a label. Bluesky's 2,000 is a protocol decision — alt text is a first-class field in the record, the app shows an ALT badge on any image that has one, and the design assumes people will write properly.
Neither is more correct. What matters is that a description written to fit Bluesky is over Instagram's limit by a factor of twenty, and the cut is silent.
The cut is silent, which is the actual problem
Alt text is the only field on a social post that nobody proofreads, because nobody can see it.
The caption is right there in front of you. The alt text sits behind a menu, is entered once, and is never rendered on screen again — so a description truncated at 100 characters looks exactly like one that fit. The only people who find out are the people using a screen reader, and they will not tell you.
FreeMentioned in this article
Alt Text CheckerReads your image description the way a screen reader will.Paste the description, pick the network, and it tells you whether it fits and shows you exactly where it would be cut. It also checks the nine rules that go wrong more often than length does.
Writing to the tightest limit
If the same image is going to more than one network, write to 100 characters and stop. It is the only length that survives everywhere, and the constraint is more useful than it sounds.
A hundred characters is roughly fifteen words. That is enough for one clear sentence about what is in the frame, which is what alt text is for. Descriptions get worse, not better, past about twenty words — a listener following a single unbroken sentence with no way to skim it loses the thread, and the detail you are trying to add belongs in the caption where everyone can read it.
Two exceptions, and they are the cases where you should use the longer field:
- The image carries text. Every word in a quote card, a screenshot or an infographic has
to be in the alt text, because for a listener that text is the image. On Instagram this is frequently impossible in 100 characters, which is a real argument for not putting essential words in an image at all.
- The image is a chart. Give the trend and the numbers that matter. That does not fit in a
hundred characters either.
What the limit does not solve
Fitting is necessary and nowhere near sufficient. The faults that actually make alt text useless have nothing to do with length: starting with "image of" when the screen reader has already said it, pasting the caption in again, or filling the field with hashtags.
Those are covered with the fixes in social media alt text, and the Instagram-specific version — where the field is and what to do about the automatic description — is in instagram alt text.
How long can alt text be on Instagram?
100 characters. It is the tightest limit of any major network, which is why a description written for X almost never fits.
What is the alt text limit on X?
1,000 characters - the most generous of the group, and ten times Instagram's.
Is there a character limit for alt text in HTML?
No. HTML sets none. The convention of staying under about 125 characters comes from older screen readers and is a readability guideline rather than a limit.
What happens if alt text is too long?
It is cut, silently. Because the field is invisible after you save it, nothing on the post indicates that the description ends mid-sentence.
Paste a description into the alt text checker and pick the tightest network it is going to.
Put it to work
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