What happens when you leave it empty
Instagram and Facebook fill the gap with their own guess from the picture, a list of things they recognize, like a person and some food.
That tells someone who can't see it almost nothing about why you posted it.
X and Bluesky make no guess at all.
Without your words, a screen reader just announces an image and moves on.
How to write a good one
Write what someone would need to know if the picture didn't load.
Start with the thing that made you post it, then add the detail that matters.
- "Cake" says almost nothing. "A three-layer lemon cake on a white stand, cut open to show the yellow filling" says what the photo is for.
- Skip "image of" and "photo of", because the screen reader already says it's an image.
- If the picture has words on it, like a price or the steps of a recipe, type those words in. They are often the whole point.
- One or two sentences is usually plenty. WebAIM, who teach web accessibility, say a few words often do the job.
- Hashtags and jokes belong in the caption.
It is the quickest of the small jobs before you press Post, about ten seconds,
and it gives people who use a screen reader the same post everyone else got.
Where the box is, app by app
The box sits a tap or two away from the screen where you write the caption.
App updates move it now and then, so if a step below doesn't match, look for the word ALT or Accessibility.
- Instagram: on the caption screen, open Advanced settings, then Accessibility, then Write alt text. For a post that's already up, tap the three dots, then Edit, then Edit alt text.
- Facebook: on a computer, point at the photo in your new post and press Edit, then Alternative text. For a post that's already up, open the photo and look for Change alt text in its menu.
- LinkedIn: after you add the picture, press the Alt text button on it, before you post.
- X: press +ALT on the picture. The box takes up to 1,000 characters.
- Bluesky: press ALT on the picture. The app takes up to 2,000 characters.
Videos are a different job
Most apps have no alt text box for a video.
There, the same work is done by captions and by a first line that says what the video shows.