What Is Alt Text? Writing Image Descriptions That Work

Aug 10, 2026 · SEO Glossary

Close your eyes and have someone read a web page to you. That's the everyday reality for millions of people using screen readers — and every time the reading voice hits an image, it says whatever the developer put in one small HTML attribute. If that attribute is empty or lazy, the listener hears "image" or a filename like "IMG_4032.jpg" and learns nothing.

That attribute is alt text. So what is alt text, formally? It's the alt attribute on an image tag — <img src="photo.jpg" alt="your description here"> — a written substitute that conveys an image's content and purpose when the image itself can't be seen. Screen readers speak it, browsers display it when an image fails to load, and search engines read it to understand what a picture shows.

What is alt text for, and who actually uses it?

Three audiences, in order of importance:

1. People using assistive technology. Screen reader users navigate your page by ear. The WebAIM Million study — an annual accessibility survey of the top million homepages — consistently finds missing alt text among the most common failures on the web, appearing on over half of the homepages tested. Writing it isn't a nice-to-have; in many jurisdictions it's a legal accessibility requirement under standards like WCAG.
2. Anyone whose images don't load. Slow hotel Wi-Fi, a broken image path, an email client blocking images by default — the alt text shows in the empty frame and the content still makes sense.
3. Search engines. Google's image understanding has improved enormously, but alt text remains a direct, controllable signal about what an image depicts. It's a meaningful factor in Google Images rankings and adds context to the surrounding page.

Notice the order. Write for the human listener first, and the SEO benefit follows naturally — the reverse approach produces keyword-stuffed junk that serves nobody.

How to write alt text that actually works

The test I teach: if you read your alt text aloud to someone who can't see the image, would they know what matters about it? Some before-and-afters from real audits:

• Bad: alt="dog" → Good: alt="Golden retriever puppy asleep in a food bowl". Specific beats generic.
• Bad: alt="chart" → Good: alt="Line chart showing organic traffic rising from 2,000 to 9,500 monthly visits between January and December". Data images need their takeaway spelled out.
• Bad: alt="best plumber sydney emergency plumber cheap plumber sydney" → Good: alt="Plumber replacing a corroded pipe under a kitchen sink". The first one is keyword stuffing — screen readers actually speak that gibberish aloud, and Google treats it as spam.
• Bad: alt="Photo of a picture of an image of our team" → Good: alt="The five-person Acme support team in the Manchester office". Never start with "image of" or "photo of" — the screen reader already announces it's an image.

Length guidance: most good alt text lands between 5 and 15 words. Screen readers historically truncated around 125 characters, and while modern ones handle more, brevity respects the listener's time. A complex diagram that needs 100 words of explanation should get that explanation in the page text or a caption, with alt text that summarizes.

When to leave alt text empty (yes, really)

Here's what people get wrong most often: not every image deserves a description. A purely decorative image — a divider flourish, an abstract background blob, a stock photo that adds mood but no information — should have an empty alt attribute: alt="". That tells screen readers to skip it silently.

The distinction matters. alt="" means "decorative, skip me." A missing alt attribute means the screen reader falls back to reading the filename, which is how listeners end up hearing "hero dash banner dash final dash v2 dot png." Empty and absent are not the same thing.

Two edge cases worth knowing. Images inside links take their alt text as the link's accessible name — so a logo linking home should read alt="Acme Widgets homepage", describing the destination, not the artwork. And images containing text (badges, screenshots of quotes) need that text reproduced in the alt, because it's invisible to everyone who can't see the image.

Alt text and image SEO

Google Images drives real traffic for the right niches — recipes, products, tutorials, interiors, charts. Alt text is one input among several that decide whether your images surface there:

• Alt text describes the image content directly.
• Filenames add a weak signal — walnut-dining-table.jpg beats DSC01822.jpg.
• Surrounding text and captions give context; Google reads the paragraph around an image.
• Page relevance matters most of all — an image ranks for "walnut dining table" far more easily on a page about dining tables.

Include a keyword in alt text only when it's an accurate description of the image. On a product page for a walnut dining table, alt="Six-seat walnut dining table with tapered legs" is both honest description and useful SEO. That's the whole trick — there is no tension between accessibility and optimization when you describe things truthfully. Pair well-described images with a solid title tag and page copy, and the image results take care of themselves.

Finding the gaps at scale

Nobody audits alt text by right-clicking every image. Chrome DevTools helps for a single page (inspect an image and check its attributes), and the Lighthouse accessibility audit flags missing alts page by page. For a whole site, audit every page in one pass — the crawl lists every image with missing or empty alt attributes across all your pages, so you can separate the decorative ones (fine) from the informative ones that need writing.

Then work through them in priority order: product images and images in money pages first, blog illustrations second, decorative flourishes marked empty and forgotten. A 200-page site usually takes an afternoon. Your screen reader users, your image traffic, and the next person who browses your site on train Wi-Fi all collect the benefit.