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Alt Text Builder

Answer a few short prompts and get alt text that follows W3C guidance. Check it against each network's alt text limit, catch common mistakes and preview how a screen reader would read it. No AI and no upload: your image never leaves your device.

Start with the thing itself. Skip "image of".

Type it exactly as it appears. Leave empty if there is none.

One short sentence. This is what separates a useful description from a list of objects.

Your alt text

0 characters. You can edit this text directly.

Checks

  • Alt text is empty. If the image carries meaning, describe it; if it is purely decorative, choose that image type.

Length by network

  • X0 / 1,000
    Per the X API media metadata schema.
  • Instagram0 / 1,000
    Image posts only; Reels and Stories do not take alt text through the API.
  • Pinterest0 / 500
    Per the Pinterest API schema.
  • Mastodon0 / 10,000
    Default in the current code; an instance can set a lower limit.
  • Pixelfed0 / 1,000
    Default config value; an instance can change it.
  • LinkedIn0 / 4,086
    Multi-image post API maximum; LinkedIn recommends under 120 characters.
  • Blueskyno documented cap
    The protocol defines alt as a required string with no length cap; the app may apply its own limit.

How a screen reader may read it

Nothing is announced. The image is skipped, as long as the platform lets the alt text stay empty.

An approximation. Each screen reader and app announces images a little differently.

How to use the Alt Text Builder

  1. Choose the image type. Pick photo, screenshot, chart, text graphic or decorative. A decorative image should have no alt text at all.
  2. Answer the prompts. Say what the subject is, what is happening, where it is, any words in the image and why it matters for the post. Leave out what does not apply.
  3. Edit the assembled sentence. The tool joins your answers into plain sentences. Adjust the wording until it reads the way you would describe the image to a friend.
  4. Check length and mistakes. See the count against each network limit, fix anything flagged, then copy the alt text into your scheduler.

Where the guidance comes from

What it checks

The checks look for empty alt text, openers like "image of", "picture of" and "photo of", mostly capital letters, hashtags, emoji, web addresses, file names, very short or very long text, and text that nearly repeats the post caption you paste in. The length panel compares the character count with each network limit. The screen reader preview is a plain text approximation of how an image is announced.

Sources

Writing advice follows the W3C Web Accessibility Initiative Images tutorial at w3.org/WAI/tutorials/images, which covers informative, decorative, text and complex images. Network limits come from the X API media metadata schema, the Instagram Graph API media reference, the Pinterest API description, the Mastodon and Pixelfed source code, the Bluesky app.bsky.embed.images lexicon and the LinkedIn multi-image post API documentation, as checked in October 2026.

Limitations

Screen readers differ in what they announce, so the preview is an approximation. Networks and instances can change limits without notice, and a network app may enforce a shorter limit than its API. A passing check does not guarantee the description is good, so read it aloud once. For charts and diagrams with a lot of data, the W3C suggests a short alt text plus a longer description elsewhere on the page.

Frequently asked questions

How do I write alt text for social media?

Describe what the image shows and why it is in your post, in one or two plain sentences. Start with the subject, not with "image of" or "picture of", because screen readers already announce an image. Include any words that appear in the image. Leave out hashtags and emoji, and do not repeat your caption. This tool walks you through those steps and flags the common slips.

How long can alt text be on each network?

Per official specifications and source code checked in October 2026: X allows 1,000 characters, Instagram 1,000 on image posts, Pinterest 500, Mastodon 10,000 by default (an instance may set less), Pixelfed 1,000 by default, and LinkedIn multi-image posts up to 4,086 with a recommendation to stay under 120. Bluesky defines alt as a required string with no length cap in its protocol. The tool checks your text against each.

How long should alt text actually be?

As short as it can be while still giving the same information the image gives a sighted reader. The W3C does not set a character limit. Most good alt text is a sentence or two. Move long explanations, such as the full data behind a chart, into the post text and keep the alt text to the main point.

What is decorative image guidance?

W3C guidance says an image that adds no information, such as a purely visual flourish or a repeat of nearby text, should have empty alt text so screen readers skip it. Many social networks do not let you mark an image decorative, so on those the best option is a short, honest description or posting without the image. Choose the decorative type here for the guidance.

Does this use AI to describe my image?

No. The tool never sees your image. You answer the prompts, and plain code joins your answers into sentences and runs rule based checks. That means the description comes from a person who knows why the image is in the post, which is what good alt text depends on.

Should I put the text from the image in the alt text?

Yes, if the words matter, such as a quote card, a sign or a chart label. Alt text should give the same information as the image. For a screenshot full of small text, summarise the point and put the long text in the post, because alt text is read aloud in full.