Why do my images need alt text?
Because without it, some people get nothing at all — and everything else that reads your page gets nothing either.
Because a screen reader has nothing else to work with. Without alt text it announces "image" or reads the filename aloud, so the information the image carried is simply lost. It also matters when images fail to load, on slow connections, and to every machine that reads your page.
The people it affects
Around two million people in the UK live with sight loss, and roughly forty million people worldwide are blind. Many use screen readers, which convert a page to speech or braille. An image is opaque to that process unless you describe it.
It is not only screen reader users. Alt text is shown when an image fails to load, used by people who disable images to save data, and read out by voice assistants.
What good alt text does
Describe the information and function, not the appearance.
| Context | What to write |
|---|---|
| A chart | The finding, not the format: "Revenue rose from £1.2m to £2.8m across 2024" |
| A product photo | What a buyer needs: colour, material, size, notable features |
| A logo that links home | The destination: "Example Ltd — home" |
| A button rendered as an image | The action: "Search" |
| A decorative flourish | alt="" — deliberately empty |
| An image whose caption already describes it | alt="", to avoid hearing it twice |
The empty alt is not the same as no alt
This distinction does real work. alt="" tells assistive technology "this carries no information, skip it" — which is exactly right for decoration. Omitting the attribute entirely leaves the technology guessing, and it commonly falls back to announcing the filename.
Hearing "I M G underscore four four seven one dot j p g" is worse than hearing nothing.
Writing it well
- Skip "image of" and "photo of" — the screen reader already says it is an image.
- Keep it under about 125 characters. Longer descriptions belong in the page text.
- Do not repeat the adjacent caption.
- Do not stuff keywords. It degrades the experience for the people who depend on it and is a recognised spam signal.
- For complex images — a detailed diagram, a data table rendered as a picture — put the full description in the page, where everyone benefits, and keep the alt short.
The secondary benefits
Image search relies on alt text to understand what a picture shows. Answer engines and link preview generators use it to understand what your page contains. These are real, and they are secondary — the reason to do it is that without it, some of your visitors get nothing.
What our audit reports about this
Every item below is measured directly, not inferred. Run it against your own site and the result names the exact rule or header responsible.
- Every image with its alt text, and a count of those with none.
- Alt text that is a filename, or that starts with redundant phrases.
- Whether decorative images correctly use an empty alt.
- Whether images declare dimensions, which affects layout stability.
For agents and scripts, the same measurement is at
/api/v1/a11y?url=yoursite.com —
see the API documentation.
Related questions
What about purely decorative images?
Use alt="". That is the correct answer, not an omission — it tells assistive technology to skip the image rather than guess.
Do background images need alt text?
CSS background images cannot have alt text and are invisible to assistive technology, which is correct for decoration. If an image carries information it belongs in an <img>, not a stylesheet.
Is alt text a ranking factor?
It matters for image search directly and contributes marginally to page understanding. The accessibility case is far stronger and should be the reason you do it.
Read next
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ReadWhat is a skip link and do I need one?
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ReadHow can I improve my SEO?
The work that actually moves rankings, ordered by effect per hour spent — and the popular tasks that do almost nothing.
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