What Is Alt Text and How Do You Write It for SEO?

Alt text — short for alternative text — is the written description you add to an image on a web page. It serves two primary purposes: providing a text alternative for users who cannot see the image (because they use a screen reader, or because the image failed to load), and giving search engines the context they need to understand what the image depicts.

Despite being one of the simplest optimisations available to any website owner, alt text is consistently neglected. Many sites either omit it entirely, fill it with generic file names, or stuff it with keywords in a way that helps no one. This guide covers how to get it right.

Why Alt Text Matters

Search engines cannot see images the way humans do. They rely on the textual information around an image — the alt attribute, the file name, the surrounding copy, and the page context — to infer what it shows. Without alt text, an image is essentially invisible to Google.

This matters for two SEO reasons. First, images with relevant alt text can rank in Google Image Search, which can drive meaningful traffic for product images, infographics, diagrams, and other visual content. Second, alt text contributes to the overall relevance signals on a page — helping Google understand what the page is about.

Accessibility is equally important. Screen readers used by visually impaired users read alt text aloud. Poor or missing alt text makes your site less usable for a significant portion of your potential audience. In the UK, the Equality Act places obligations on service providers to make reasonable adjustments for accessibility — and alt text is a minimal, cost-free adjustment.

How to Write Good Alt Text

Good alt text describes the image accurately and concisely. Aim for one to two sentences at most. Describe what is actually in the image — the subject, the setting, any relevant action — rather than writing a general description of the page topic.

Include relevant keywords naturally where they fit the description. If you run a Norfolk-based plumbing business and the image shows a plumber fixing a boiler, "plumber repairing a combi boiler in a Norwich kitchen" is accurate and contains relevant terms. Don’t force keywords in — "boiler repair Norwich plumber Norwich emergency plumber" is keyword stuffing, not alt text.

Don’t start alt text with "Image of..." or "Photo of..." — screen readers and Google already know it’s an image. Go straight to the description. Also avoid writing the same alt text for every image on a page — each image should have a unique description that reflects what it actually shows.

Special Cases and Common Mistakes

Decorative images — background patterns, dividers, purely aesthetic visual elements — should have an empty alt attribute: alt="". This tells screen readers to skip the image rather than reading out a file name or description that adds no value. Never leave the alt attribute absent entirely; that is not the same as an empty alt and will cause screen readers to read the file name.

For complex images like charts, graphs, or infographics, alt text alone is rarely sufficient. Provide a more complete description in the surrounding body text, a caption, or a linked data table. Alt text is a summary, not a full transcription.

Avoid using images of text for important content. Search engines cannot read text embedded in images as reliably as HTML text, and screen readers cannot read it at all. If you must use an image of text, ensure the alt text reproduces the text content exactly.

Regularly audit your site for missing or poor-quality alt text. A crawl with Screaming Frog will surface all images with missing alt attributes, empty alt attributes, or file-name alt text that crept in through bulk uploads. Fixing these is a quick win with direct SEO and accessibility benefits.