Alt Text Overview
What Is Alt Text?
Alt text (alternative text) is a short written description of an image that is read by screen readers to visually impaired users. Alt text also appears when an image fails to load, which provides additional information to all users.
Canvas, Word, PowerPoint, etc., all have fields similar to the one below where you can enter alt text for an image, or you can mark the image as decorative.
Alt Text Basic Principles
- Keep alt text under 120 characters when possible.
- Describe the meaning of the image in its context, not just what it looks like.
- Do not include phrases like "a picture of." Screen readers already announce that it's an image.
For support in writing alt text, try tools like the AI Image Accessibility Generator to help draft descriptions that you can review and edit for accuracy.
Which Images Need Alt Text?
You should add alt text to all meaningful images in Canvas, Microsoft Word, PowerPoint, PDFs, etc. Use the following as a guide:
Informative Images
If an image conveys information, enter concise alt text that explains the image’s purpose and context, not just what it looks like.
Alt Text example (short summary in the Alt Text field): Illustration of water evaporating from a surface into a cloud, with raindrops falling back down, representing the processes of evaporation and precipitation.
Complex Images
If a chart or diagram is too complex to describe in alt text, provide a short summary or link to a more detailed explanation near the image.
There are three different approaches you can take to document complex images such as charts, graphs, equations, etc.
Alt Text example (short summary in the Alt Text field): Diagram of Earth's water cycle. See below for full description.
Long Description example (separate doc with link adjacent to image): Long Description Alt Text Example
Alt Text (short summary in the Alt Text field): Diagram of the Earth's water cycle showing pools and fluxes. A full description is provided in the text below.
Long Description example (in surrounding text after the image): Description of the Water Cycle Diagram: This detailed illustration from the U.S. Geological Survey depicts the Earth's water cycle, showing how water moves through various natural systems and human uses. The diagram identifies pools (places where water is stored) such as oceans, lakes, rivers, wetlands, snowpack, glaciers, soil moisture, and groundwater. It also illustrates fluxes—the ways water moves between these pools—including precipitation, evaporation, runoff, snowmelt, groundwater recharge and discharge, and evapotranspiration.
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- Human uses of water are shown as well, including:
- Agricultural water use (e.g., irrigation)
- Municipal water use (e.g., homes and cities)
- Industrial water use (e.g., factories)
- Domestic water use (e.g., household consumption)
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The diagram uses arrows to show the direction of movement—such as how precipitation over land becomes runoff, which may enter rivers, lakes, or groundwater, eventually flowing back to the ocean. It also shows atmospheric transport of moisture and exchange between ocean layers (mixed zone and deep water zone).
This technique is only suitable if the appropriate information can be conveyed succinctly.
Alt Text (short summary in the Alt Text field): Water cycle diagram showing water movement through oceans, land, atmosphere, and human uses like farming and cities.
Which Images DON'T Need Alt Text?
Decorative Images (images that are purely decorative and don’t convey meaning) do not require alt text. Instead, you must mark the image as Decorative to tell screen readers to skip that image.
For example, a page on the water cycle might contain a picture of water simply for visual interest. Since this image doesn't convey additional information to the reader, it is considered decorative.
