Alt text is the written description behind an image that assistive technology reads aloud when a person cannot see the picture itself. The single rule that matters above all others: write it to explain the image’s purpose on that specific page, not to please a search algorithm. Get that right and accessibility improves first, with any SEO advantage following as a welcome side effect rather than the goal.
TL;DR: Over 2 million people in the UK live with sight loss, emphasizing the importance of accurate alt text for accessibility and fallback purposes. Decorative images should have empty alt attributes to be skipped by screen readers, while functional and informative images require descriptive, concise alt text. Good alt text should name the subject, explain its role on the page, and avoid unnecessary words or assumptions, typically using five to twenty words. For complex visuals like charts or infographics, include a short alt and provide a detailed explanation in the page content; avoid embedding lengthy descriptions solely within the alt attribute. Consistent decision-making and auditing—deciding if images add meaning, then properly describing or omitting—are key to maintaining accessible, user-friendly websites.
TL;DR:
Screen readers cannot interpret pixels. When they reach an image, they read out whatever sits in the alt attribute, or say nothing at all if that attribute is intentionally left blank. This is why the W3C Web Accessibility Initiative states that every image should carry an alt attribute, even when that attribute is empty for purely decorative pictures. Leaving it out altogether forces a screen reader to guess, often reading the file name instead, which helps nobody.
Over 2 million people in the UK live with sight loss, according to RNIB, a figure that underlines why this small piece of code carries real weight. Alt text also steps in as a fallback whenever an image fails to load, whether that is a slow connection or a broken file path, so the reader still gets the meaning without the visual. Treating alt text as an accessibility requirement first tends to produce better, more usable content for everyone, including readers who never touch a screen reader.
The decision comes down to one question: does the image add information the page would lose without it? If yes, describe that meaning. If the image is purely decorative, such as a background flourish or a repeated icon with no function, use an empty attribute, alt="", rather than skipping it entirely.
alt=""
This structure keeps you consistent across a whole site rather than making a fresh judgement call every time you upload a picture.
Good alt text is short, specific and written for the page it sits on, not recycled from a caption or a file name. Most images need somewhere between five and twenty words, roughly one plain sentence with normal punctuation, since screen readers use full stops and commas to pace their reading naturally.
As the GOV.UK Design System puts it, alt text should explain an image’s meaning in context, with longer explanations moved into the surrounding page text rather than crammed into the attribute itself.
Pro Tip: Read your alt text aloud without seeing the image. If it makes sense on its own, it will probably make sense to a screen reader too.
Some images resist a simple one-line description, and public-sector accessibility guidance offers a consistent approach for each.
Nottinghamshire County Council’s guidance recommends exactly this split for complex visuals: a brief label in the attribute, with the substance carried in a structured table or paragraph nearby.
Seeing the fix next to the fault makes the rules easier to apply than reading them in the abstract.
Each fix follows the same pattern: name the subject, add the missing context, and cut anything the reader does not need.
Adding alt text well takes minutes once the habit is set, and checking it takes even less time.
<img src="chair.jpg" alt="Red leather armchair in a lounge setting">
Automated description tools can produce a useful first draft, but accessibility guidance from government accessibility blogs warns they often miss context or produce literal descriptions that need a human edit before they go live.
We run every image through three steps: decide, write, audit. Decide whether the image adds meaning the page needs or is purely decorative. Write a concise description for the meaningful ones, pointing to a longer explanation in the page text for anything complex, such as a chart. Audit before publishing, checking that decorative images carry an empty attribute and functional images describe their action. It is a small discipline that keeps accessibility consistent across a whole website rather than left to guesswork on each new upload.
— Rob
Most guidance on this topic treats alt text as a search engine optimisation trick with an accessibility footnote attached, when it should run the other way round. Write for the person using a screen reader first, and the search benefit tends to follow because clear, specific descriptions are exactly what a well-structured page needs anyway. The advice to “add keywords naturally” is where most writers go wrong: it produces alt text that reads like it was written for a machine, because it was.
What the research actually supports is far simpler than most style guides suggest. A five-word sentence describing what is visible, written the way you would say it to a friend on the phone, beats a carefully engineered phrase every time. If you take one thing from this guide, prioritise the decision step: is this image decorative or does it carry meaning? Get that single call right and most of the rest follows naturally.
If auditing every image on your site sounds like a slow job to tackle alone, that is exactly the kind of technical clean-up our website development team handles as part of a wider build or refresh.
We fold accessibility checks, including alt text review, into the sites we design and build, so the fix happens once rather than image by image. Get in touch for a free audit and we will show you where your current site stands.
This article draws on the W3C Web Accessibility Initiative, the GOV.UK Design System, RNIB, and Nottinghamshire County Council’s accessible content guidance. Government accessibility blogs and Home Office design guidance informed the sections on automated tools and images of text. Readers wanting a broader compliance view can also see our piece on website accessibility guidelines.
Alt text is a short written description added to an image’s code that screen readers announce aloud and browsers display if the image fails to load. It exists to convey the image’s meaning to someone who cannot see it, per W3C guidance.
In HTML, add it directly inside the image tag as alt="your description". In WordPress, enter it in the Media Library’s alt text field or the Block Editor’s image settings, duplicating the file if you need different wording elsewhere on the site.
alt="your description"
Automated tools can produce a first-draft description, but accessibility guidance cautions that they often miss context or describe images too literally. A human check, focused on what the image means on that specific page, is still necessary before publishing.
A strong example names the subject and adds page-relevant context in one short sentence, such as “Red leather armchair in a lounge setting” rather than “chair” or “image1234.jpg”. For charts or infographics, a brief alt plus a fuller description in the surrounding text works best, as recommended by Nottinghamshire County Council.
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