Alt text, captions and image descriptions are often treated as the same thing, and on some platforms the names are even used interchangeably. They are not the same. Each one reaches different people and does a different job, and an image often benefits from more than one. This post sorts out the terms, then shows how they fit together on a website and in a social post.
The short version
- Alt text replaces the image for people who cannot see it. It lives in a hidden field on the image, mainly screen reader users get it, and a sentence or two is usually enough.
- A caption adds context to the image for everyone. It sits next to the image, or it is the post text, and it is usually a sentence or two.
- An image description describes the image in more detail, in visible text in the page, the post or a reply, and can be as long as it needs to be.
The rest of this post explains each one and where the lines blur.
Alt text
Alt text is a text alternative attached to the image itself. On a website it is the alt attribute of the image. On social media it is the field you fill in while posting, which our guide to adding alt text shows platform by platform.
Its job is to stand in for the image. A screen reader reads it out when it reaches the image, and a browser shows it when the image fails to load. WebAIM's guide to alternative text lists these uses, and the W3C's images tutorial gives the most useful test for what to write: imagine reading the page aloud to someone over the phone.
Who can see alt text varies by platform:
- On X, an ALT badge on the image shows the description to anyone who selects it. Bluesky and Mastodon show an ALT badge on described images too, and Bluesky has a setting to make the badges larger.
- On Instagram and Facebook, the help pages say the description will only be read if someone is using a screen reader.
- On LinkedIn, alt text is not visible to members who do not use a screen reader.
So write alt text for someone who cannot see the image at all, but do not assume nobody else will read it.
Captions
A caption is visible text shown with the image, read by everyone. It adds what the image cannot show by itself: who took it, where, why it matters, what to notice.
On a website, a caption is usually a figcaption inside a figure. MDN's figcaption reference describes it as a caption or legend for the figure's contents. On social media, the caption is the post text that goes with the image.
A caption does not replace alt text. WebAIM is direct about this in the figure and figcaption section of its guide: the image still needs alt text, and the two should not repeat each other. The caption gives context, the alt text gives what is in the image.
Here is how that works on a news photo:
- Caption: "The new footbridge opened to the public on Saturday morning."
- Alt text: "A narrow steel footbridge over a river, crowded with people walking across. Several are stopping to take photos of the water."
Read together, nothing is repeated. A screen reader user gets the caption from the page and the scene from the alt text, the same as a sighted reader.
A common mistake is the reverse: alt text that copies the caption word for word. A screen reader user then hears the same sentence twice and still does not know what the photo shows.
Image descriptions
"Image description" has two meanings, and it helps to know which one someone means.
The platform name for alt text
Some platforms simply call alt text an image description. X's help says that image descriptions, also called alternative text, make images accessible. Mastodon's documentation calls it a media description. In these cases, image description and alt text are the same field.
A longer, visible description
The second meaning is a description written in the visible text, for everyone. It is used when alt text is not enough or not reachable:
- Complex images on websites. The W3C's tutorial on complex images recommends a short alt text plus a long description for charts, diagrams and maps, ideally shown on the page so everyone can use it. Our post on alt text for charts and graphs covers this.
- Social posts. Some people add "Image description:" followed by a description in the post itself or in the first reply. This reaches people who do not use a screen reader but still benefit from a description, such as people with low vision or those who find images hard to interpret, and people on apps that do not show alt text.
A visible description is not a reason to leave the alt text field empty. The alt text is what a screen reader announces with the image. If you write a full description in the post, the alt text can be short and point to it: "Comic strip in four panels. Full description in the post."
What about video captions?
On video, "captions" means something else again. The W3C describes captions as a text version of the speech and other important sounds, synchronised with the video, for people who are Deaf or hard of hearing. They are not about images at all. The closest video equivalent of alt text is audio description, a narration of the important visual information.
How to use them together
For most images, the combination is simple:
- Always write alt text for an image that carries meaning. Describe what the image shows, and keep it to what someone who cannot see it needs. The guide to alt text length has examples.
- Add a caption or post text for context: who, where, when, why. Do not repeat it in the alt text.
- Add a visible description when the image is complex, such as a chart, a diagram or a dense infographic, or when you want everyone to have the detail.
And for the one case where you need none of them: a purely decorative image that adds nothing to the content, such as a divider or a background pattern. The W3C's decorative images guidance says to give it an empty alt="" on a website so screen readers skip it. On social media, almost every image you post is there for a reason, so it needs alt text.
Where AltKit fits
AltKit writes the alt text part: a description of what the image shows, written to stand in for it. In the web creator, signed-in users can add context, such as the caption or where the image will appear, so the alt text adds to the caption instead of repeating it. The caption and any longer description are still yours to write.