Writing alt-text

Published on: 04/02/2025 · Last updated on: 04/02/2025

Introduction

alt-text (alternative text) is a text description of an image for people who are unable to see them. This includes photos, diagrams and charts – anytime an image is used. It is essential for people who use screen reader technology so they can access the content. Check out the video below to understand how the alt text is vital for students using assistive technology.

Why do it?

• “A picture is worth a thousand words” This old adage is only true if a user is able to see the image.
• For people with blind or low vision who use screen reader software, an alternative text description is essential so they can access all visual properties and text in the image.
• Alt text is also useful fall back if the image fails to load properly.

How do I add include alt-text?

To add alt-text to any image in Microsoft Word or PowerPoint, right click on the image, then select “Edit Alt-text” from the dropdown menu and enter the description into the box. 

In Moodle, you will be prompted to add alt-text when you add an image. You can edit existing alt-text by selecting an image, clicking the Image button.

How do I write effective alt-text?

Alt-text is a brief description of the content and a few sentences maximum.

1. Context: the description will depend on the context it is used in: is it decorative, an illustrative example or a central learning concept?
2. Concise: be succinct and avoid repeating information presented in the main text.
3. Objective: describe physical appearances and relationships. Don’t interpret or analyse, but allow readers to form their own opinions.
4. Move from generic to specific. Begin with high-level context, then expand to describe detailed information.

A useful evaluation check is to imagine what would happen if all your images were replaced with alt-text: is there enough information to access the meaning?

Here are some useful resources on writing alt-text with examples

• WebAim Guide to Writing Effective Alternative Text
• Accessible Images Home (Penn State) 

When should I set an image as decorative?

Decorative images don’t add any new information to the content. For example, the information provided by the image might already be given using adjacent text, or the image might be included to make the things more visually attractive. If this is the case, it is not useful to add alt-text but this must be marked as decorative so a screen-reader user knows they can safely skip the content.

Decorative Images ((WAI) | W3C) provides guidance and examples on when it’s appropriate to mark something as decorative.

How do I support complex images?

Alt-text is often the simplest and fastest way to provide information to a reader who can’t access it visually. However, some complex images (e.g. diagrams and charts) can’t be easily be described in a few short sentences for alt-text. It is still essential to provide alt-text for images, but some additional considerations may need to be put in place to support the accessibility.

The recommended approach to alt-text is outlined Describing complex images resource:

1. Can teaching points be described in a few sentences?​ If so, just use normal alt-text.
2. Is the image just a graphical summary of a nearby text explanation?​ If yes, use the alt-text to signpost the text description (e.g. see body text for description).
3. Is the information in the image available in other resources – for example ebooks, audio files, websites?​ If yes, use the alt text and a caption to signpost to an alternative format (alt text = “see caption for link to alternative format”)
4. Do AI tools (e.g. Microsoft CoPilot) create a good description of the main teaching points?​ If yes, edit for accuracy, and then upload this separate online description to Moodle and reference to it with alt-text and caption.
5. Use the suggested Alt Text formula outlined in Suggested workflows (Describing complex images).

Also see our guidance on How to create accessible figures and diagrams.

What is the difference between a caption and alt-text?
A caption appears next to an image and supplements an image for all users. It could contain a title, overview, context, copyright/source information or extra information about the image.
Alt-text replaces the visual aspects of an image through a text description for users who can’t see the image. A caption can be useful for readers, but can never replace the need for alt-text.

How can I spot these issues?

• Use the Microsoft Accessibility Checker (see later Checking your content section) to find images with missing alt-text within you files (Word, PowerPoint and Excel).
• Use Accessibility+ Toolkit to find and fix issues with missing alt-text on Moodle.

Additional support

• Improve image accessibility in PowerPoint (including for diagrams made of groups of shapes)
• Accessible Figures Guidance – University of Bath Teaching and Learning Hub.
• The DIAGRAMcenter.org have a series of examples sorted by figure type: for example bar graphs, line graphs,  relational diagrams with multiple start points/paths, etc.