What is alt text and how to use it for better SEO

Alt text is one of the smallest things you can add to a web page and one of the most consistently overlooked. It sits invisibly in your code, most visitors will never know it is there, and yet it quietly does two important jobs at once: it makes your site usable for people who cannot see your images, and it helps search engines understand what those images are.

That combination is exactly why it matters. Few SEO improvements deliver as much value for as little effort, and in 2026, with AI search drawing on image data more than ever, getting it right is genuinely worth the few extra seconds per image. Here is what alt text is, why it counts, and how to write it well.

What is alt text?

Alt text, short for alternative text, is a written description of an image that lives in the image’s HTML. It is the alt attribute inside the <img> tag, and it exists to describe an image to anyone, or anything, that cannot see the image itself.

It serves two audiences. The first is people using screen readers, who rely on alt text being read aloud to understand what an image shows. The second is search engines, which cannot truly “see” an image and instead read its alt text to work out what it depicts and how it relates to the surrounding content. To a search engine, an image with no alt text is close to an unreadable black box.

Why alt text matters for SEO

There are a few distinct reasons alt text earns its place in your on-page SEO.

The first is relevance. By describing an image accurately, you are explicitly telling Google what that image is about, which reinforces the topic of the page it sits on and strengthens your overall topical authority on that subject. The second is image search. Google Images is a real and often underused source of traffic, and alt text is one of the primary signals that determines whether your images surface there. The third is accessibility, which Google actively rewards. A site that follows accessibility best practice signals quality, and alt text is fundamental to that.

There is also a newer reason. AI search tools, including AI Overviews and assistants like ChatGPT, Perplexity and others, draw on alt text to understand and reference images. As more discovery happens through AI-driven surfaces, the description you attach to an image increasingly influences whether that image, and the page around it, gets pulled into an answer.

It is worth being clear that Google’s own image recognition has become very capable, and it can interpret a lot from an image without help. But it supplements alt text rather than replacing it. Writing good alt text remains one of the highest-return on-page tasks available to most site owners.

How to write good alt text

The single most useful question to ask yourself is this: what information would someone miss if they could not see this image? Answer that clearly and concisely, and you have your alt text. Everything else is refinement.

Be specific and descriptive

Describe what the image actually shows, and where relevant, the function it serves on the page. “Black leather backpack on a wooden table against a green background” is far more useful than “bag.” Specificity helps both the screen reader user and the search engine.

Keep it concise

The widely accepted convention is to keep alt text to roughly 125 characters, with many practitioners aiming for somewhere in the 80 to 140 character range. This is not a hard technical limit but an accessibility one, since screen readers read the whole thing aloud and overly long descriptions become a burden rather than a help. Convey the meaning, then stop.

Include keywords naturally, never forcefully

This is where alt text most often goes wrong. It is fine, and even helpful, to include a relevant keyword where it fits naturally into an accurate description. What you must not do is stuff in keywords. Something like “best running shoes cheap trainers buy shoes online sports footwear” offers no accessibility value whatsoever and is treated by Google as a spam signal. Accuracy always comes before keyword density. A good rule of thumb is one relevant keyword, woven in only if the description still reads naturally.

Match the language of the page

If you run a multilingual site, your alt text should be written in the language of the page it appears on. The Spanish version of a page needs Spanish alt text, both so it is accessible to native speakers and so search engines index the image correctly for that locale.

Know when to leave it empty

Not every image needs alt text. Purely decorative images that add nothing to the meaning of the page, such as background flourishes or dividers, should have an empty alt attribute (alt=””). This tells screen readers to skip them rather than announcing something meaningless. Adding descriptions to decorative images just adds noise.

Common mistakes to avoid

A few patterns crop up again and again. Leaving alt text empty across an entire image library is the most common, and it is usually the result of images being uploaded in a hurry over a long period. Industry accessibility reports consistently find that more than one in four images on popular websites has alt text that is either missing or inadequate, which means fixing yours is a genuine competitive opportunity.

The other frequent issues are duplicating the same generic description across many different images, keyword stuffing, and describing the image so literally that you miss why it is actually on the page. Context matters as much as content. An image of a graph is not just “a line graph,” it is “a line graph showing a 40 percent rise in organic traffic over six months,” because that is the point it is making.

A small task with a real payoff

Alt text rarely feels urgent, which is exactly why it gets neglected. But it is one of the few areas where doing the right thing for accessibility and doing the right thing for SEO are the same task. Describe your images honestly and clearly, keep it concise, resist the urge to stuff keywords, and you will improve your image search visibility, your accessibility compliance and your standing with the AI tools that increasingly shape how people find content, all in one go.

If your site has built up a backlog of missing or poor alt text, it is well worth auditing as part of a wider technical SEO review, and getting it right alongside your broader SEO content work. If you would like a hand assessing where your images are letting you down, or you want to build image optimisation into a fuller SEO services strategy, get in touch with the team at TAL and we will help you put a plan in place.

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