Writing good image alt text

Only use an image if it helps people understand the content or service. Before using an image, read Before you use images in web content.

This guidance explains how to write alt text (alternative text) so people can understand images when they cannot see them.

It is for anyone who creates or publishes content on our websites

What alt text is and why it matters

Alt text is a short hidden description used for images that contain information. It helps people who cannot see images understand what the page is saying.

Screen readers read alt text aloud when they reach an image. It may also appear if the image fails to load.

Alt text should not describe every detail. Its purpose is to explain the point of the image clearly and quickly, so people can move on.

Do not add alt text just to meet a requirement. Every image announced by a screen reader demands attention. Unnecessary alt text can slow people down, create noise, and make content harder to use.

The key idea behind alt text

Alt text is about meaning, not appearance.

People who use screen readers want to focus on information they need to understand the page or complete their task.

Only include visual detail when it supports that purpose.

There is rarely a single “correct” way to write alt text. What matters is that it makes sense in context and is quick and easy to understand.

When writing alt text, pause and ask:

Why is this image here on this page?

Your answer is usually what the alt text should communicate.

What good alt text does

Good alt text supports understanding without getting in the way. It helps people understand the content and move through it efficiently.

It should feel quick to hear, easy to follow and useful in context.

Good alt text:

supports how people understand the content
is short and clear
focuses on what matters for that page
avoids guessing gender, emotions, relationships or intent
does not repeat nearby text or captions

How long alt text should be

Alt text is mainly for people using screen readers, so length matters.

In most cases, a single short sentence is enough, usually between 5 and 20 words.

Long descriptions can be hard to follow when read aloud. If more explanation is needed, that detail should be included in the main page content, not hidden in alt text.

Use basic punctuation to support natural pauses when read aloud.

Decorative images

Some images are included only for visual purposes and do not add meaning.

If removing an image would not change what the page is saying, it is decorative. Decorative images should be skipped by screen readers so people can focus on what matters.

For example, consider a page that listed different organisations and their logos. If their names appeared in the main text, their logos would be decorative. Adding alt text would repeat this information without adding value.

On most web pages, skipping decorative images means using empty alt text. On some platforms, such as Microsoft Word, you may need to mark the image as decorative instead.

For guidance on how to do this, read Using decorative images.

One image, different meanings

Alt text should follow the purpose of the page, not describe the image on its own.

Describing everything in an image does not help people understand why it's there.

For example, this image shows a dog standing on grass in a park.

Even though the image contains other visual details, most of them do not matter in every context. The surrounding content decides what information matters.

For example:

On a page about assistance dogs, what matters is the function of the harness.
Alt text: “Assistance dog wearing a working harness.”
On a page about parks, what matters is the setting.
Alt text: “Dog standing on grass in a park.”
On a page about animal intelligence, what matters is the type of animal as a general example.
Alt text: “Dog standing on grass.”
On a page about Labradors, what matters is the breed and colour shown.
Alt text: “Yellow Labrador standing on grass.”

In each case, the image is the same, but the purpose of the page changes what matters. Include detail only when it adds meaning in that specific context.

Colour is included in the last example because it explains the variety shown. Other visual details, like background trees or harness colour, are not included because they do not add meaning.

The same is true for images used on our web pages. For example, on a social care page, what may matter most is the service being delivered in the image, not the setting or clothing.

Using captions with images

A caption is text shown with an image, usually below or above it. It is often announced by screen readers with the image.

Captions can help explain to everyone why an image matters to the content. Alt text explains what matters about the image in this context, using a short, neutral description.

Captions do not replace alt text. Screen readers use captions and alt text for different purposes.

Captions and alt text should support each other, not repeat the same information. Unnecessary repetition can slow screen readers down and make content harder to use.

Alt text should still make sense if there is no caption. It should not rely on the caption to be understood.

For example, consider the image of a dog from the previous section:

Caption: "The same image can mean different things depending on the page it appears on."
Alt text: "Dog standing on grass."

Together, the caption, alt text and the surrounding context on this page explain why the image is there and what it is being used to show.

In this section, this image is marked as decorative because it visually repeats and reinforces what is already explained in the text. It would not add anything if announced by a screen reader.

Read more about images that repeat information in Using informative images in content.

A simple way to decide what to write

Before writing alt text, ask:

Why is this image here on this page?
What would be missing if the image could not be seen?
What is the shortest, most neutral way to explain that meaning?

These questions help you make better choices when deciding what to write.

Some tools may generate alt text automatically, but they cannot judge what the image means for the content. Use these questions to review, edit or remove any suggested alt text.

If you are unsure, keep it simple. A short, neutral description is usually best.

Why alt text does not describe everything

Alt text does not replace sight. It replaces the information the image provides for that page.

People who can see an image naturally ignore most visual detail and focus on what matters first. Alt text needs to do this on purpose.

Trying to describe everything can overwhelm screen reader users and slow them down.

Focusing only on what matters keeps content usable.

Avoid phrases like “image of” or “photo of” unless they add useful information, as screen readers already announce images.

Include the type of image only when it helps people understand or interpret it.

For example:
"CGI illustration of proposed housing development with homes, roads and central green space."

How to check alt text on a live page

Once a page is live, it can be helpful to check how images and alt text work with captions and the wider page context. This is especially useful when reviewing example images or deciding whether an image should be treated as decorative.

You can use browser extensions to help with this.

Image Alt Text Viewer browser extension (Chrome Web Store) shows alt text alongside images so you can check whether it makes sense with the image and surrounding content.

Web Developer browser extension (Chrome Web Store) lets you check the page without images:

Open the extension.
Go to Images.
Select Disable Images.
Refresh the page.

This helps you check whether the page still makes sense without images and whether any important information is missing. When finished, remember to untick Disable Images and refresh the page again.

If the page still works without the image, and the alt text fits the page’s purpose, it is likely doing its job well.