Alt Text Best Practices: The Complete Guide to Better SEO, Accessibility & Image Search

Alt Text, Alt Tags and Alt Descriptions: What Is the Difference?

These terms are often used interchangeably, but they do not always mean exactly the same thing.

<img
  src="mobile-tire-technician-edmonton.jpg"
  alt="Technician installing a winter tire on a customer's vehicle in Edmonton."
  width="1200"
  height="800"
>

The code does not need to describe every pixel. It needs to replace the information the image contributes in that specific context. Harvard’s accessibility guidance emphasizes that the same image may require different alt text on different pages because context changes which details matter.

Alt Text Is Still One of the Web’s Most Common Accessibility Gaps

The 2026 WebAIM Million study automatically evaluated the home pages of the top one million websites. Automated testing cannot measure every accessibility issue, but it provides a useful large-scale view of detectable failures.

WebAIM concluded that more than one in four images on popular home pages had missing, questionable or repetitive alternatives. The issue is therefore not limited to small or outdated websites.

How Does Alt Text Support SEO?

Google Search Central says that Google uses alt text together with computer-vision algorithms and surrounding page content to understand image subject matter. It also describes alt text as the most important image attribute for providing additional metadata.

Should keywords appear in alt text?

A relevant search term can appear naturally when it accurately describes the image. Google explicitly advises writing useful, information-rich alt text with keywords used appropriately and in context. It also warns against filling alt attributes with repeated keywords because that harms users and may be treated as spam.

Does Every Image Need Written Alt Text?

Every HTML <img> should have an alt attribute, but not every image should have a spoken description. The correct value depends on the image’s purpose.

How to Write Alt Text That Works for People and Search Engines

There is no single perfect character count for every image. Harvard recommends keeping alt text brief, usually one or two sentences. The Government of Canada advises concise text and recommends a long description when a complex image cannot be represented adequately in a short alternative. British Columbia’s guidance suggests trying to remain under about 15 words for ordinary images.

• Describe the image’s purpose in the current page, not every visible detail.
• Write for a person first. Use search terms only when they are accurate and natural.
• Be specific enough to distinguish the image from other images on the same page.
• Do not begin with “image of” or “photo of” unless identifying the medium is important.
• Do identify a logo, screenshot, diagram, illustration, painting or chart when the medium matters.
• Do not repeat a nearby caption, heading or paragraph word for word.
• For image links and buttons, describe the action or destination.
• Use punctuation so synthesized speech pauses naturally.
• Use empty alt for decorative images rather than inventing unnecessary descriptions.
• Review AI-generated descriptions before publishing them, especially for technical, medical or sensitive images.

Before-and-after examples

When Is a Long Description Needed?

Complex images contain more information than can reasonably fit into a brief alt attribute. W3C identifies charts, graphs, maps, flowcharts and information-heavy diagrams as common examples. In these cases, use a two-part approach: a short alternative that identifies the image and a detailed text equivalent that communicates the essential data.

<figure>
  <img
    src="organic-traffic-growth-chart.png"
    alt="Organic sessions increased steadily from January to June. Detailed monthly data follows."
  >

  <figcaption>
    Organic sessions by month after the technical SEO launch.
  </figcaption>
</figure>

<table>
  <caption>Organic sessions from January to June</caption>
  ...
</table>

A visible data table is often the strongest solution because it can support screen-reader users, people with cognitive disabilities, users who prefer exact values and search systems that can process structured text.

Alt Text Audit Checklist for Business Websites

• Crawl the website to identify images with missing alt attributes.
• Separate decorative images from informative and functional images.
• Review linked images first because missing alt can create unnamed links.
• Replace filenames and generic descriptions such as “image,” “graphic” or “photo.”
• Check product galleries for duplicated alt text across different views.
• Review icons used as buttons or links and provide accessible action names.
• Confirm that charts, maps and infographics have equivalent long descriptions or data tables.
• Keep important visible text as real HTML whenever possible instead of embedding it in an image.
• Use descriptive filenames and place images beside relevant page content.
• Check both desktop and mobile templates for equivalent image metadata.
• Test representative pages with VoiceOver, NVDA or another screen reader.
• Validate changes with automated tools, but do not rely on automation alone.
• Review image impressions and clicks in Google Search Console after implementation.
• Add alt-text quality checks to the publishing workflow so the problem does not return.