How to Write Alt Text for the Image and Its Purpose

When you add an image to a web page, its alt text is the written alternative stored in the image’s HTML attribute. Screen readers use it to describe relevant images to people who cannot see them, and search engines use it as one clue about what an image shows.

For search engine optimization (SEO), write alt text to explain what the image contributes in its specific context. Describe its meaningful content, label its action if it functions as a link or button, and leave it empty when the image is purely decorative or repeats nearby information. Relevant alt text helps Google understand images, but it does not guarantee higher rankings or more traffic.

How alt text contributes to image SEO

Google says it uses alt text, computer vision (technology that analyzes image content), and the surrounding page text to understand an image. It also recommends placing images near relevant text on pages that match their subject. Together, these details help Google interpret the image’s topic and context. The documented mechanism supports writing accurate descriptions; it does not establish a ranking boost from changing alt text alone. See Google’s Image SEO best practices for its guidance.

Alt text has a second SEO function when an image is the only content in a link. Google uses that text as anchor text, or the link label that describes where the link goes. For example, if a product image links to the product page, the alternative should help explain the link’s destination. Google describes this use in its link best practices.

These uses do not make alt text a place to repeat target keywords. Google warns that stuffing it with keywords harms the experience and may cause a site to be seen as spam. Describe the image’s relevant meaning or function instead.

Choose the wording by the image’s purpose

Start by asking what a reader would lose if the image disappeared from the page. The answer determines whether the image needs a description, an empty value, or text about an action. The W3C image guidance organizes this decision around an image’s purpose and context.

For an informative image, describe the detail that matters to the page. A photo in an article about puppy training might use alt="Dalmatian puppy catching a red ball on grass" if that activity supports the surrounding point. If the article discusses the puppy’s coat pattern, describe that relevant feature instead. A description should match the image and the reason it appears there, rather than inventory every visible detail.

For a decorative image, use an empty value: alt="". This tells assistive technology to skip the image. If you omit the alt attribute, a screen reader may announce the image’s filename or give no useful description. W3C explains when and how to use empty alt text for decorative images.

For an image that acts as a link or button, describe what the control does or where it leads. An image-only search button needs a label such as “Search,” rather than “magnifying glass.” For a linked product image, describe where the link leads. W3C’s guidance on functional images covers these cases.

For a chart, map, or other complex image, keep the alt text brief and make the important information available elsewhere on the page. A chart’s alt text can identify its subject, while nearby text or a table presents its values and relationships. A single short description cannot reliably replace detailed data; W3C explains approaches for complex images.

Check the alt text on the published page

Read the alt text with the nearby heading, paragraph, and caption. Ask whether it gives a reader who cannot see the image the information or function they need, whether it matches the actual image, and whether it repeats something already stated in the text. The HTML Standard also recommends avoiding alt text that merely repeats a caption or nearby text. Its image guidance treats alt text as a substitute for the information the image adds to the page.

Then inspect the published page, not just the content-management field. Confirm that the intended alt value appears in the rendered HTML and that the page includes the relevant image. Google’s image guidance recommends standard HTML image elements for discoverability and notes that it does not index CSS images in the same way.

To assess search performance, use the Search Console Performance report and filter for image search. The report shows how often pages appeared in results (impressions) and how many clicks they received. It also reports click-through rate, the share of impressions that led to clicks. The report includes average position too. Its Pages view shows page URLs, so it does not identify which individual image file appeared in results. Compare relevant pages and time periods, but do not credit an alt-text edit alone for a change in those metrics. Search Console describes the report’s image-search performance data.

What alt text cannot guarantee

Google’s documentation explains how alt text contributes to image understanding, but it does not quantify the ranking or traffic effect of changing alt text alone. It also gives no universal ideal character or word count. Write enough to explain the image’s purpose. If the image contains more information than a short alternative can convey, explain the rest in the page text.

Descriptive alt text also does not guarantee that Google will index or show an image. Search visibility depends on more than the alt attribute, including the image and its page context. Likewise, Google’s guidance for generative AI features recommends established SEO practices, including its existing image recommendations, but it does not provide an alt-text formula that guarantees inclusion in an AI-generated result. Google’s AI search guidance says eligibility does not guarantee crawling, indexing, or serving.

Key Takeaways

• Write for purpose and context: Describe the image detail that supports the page, or describe the action or destination when the image is a control.
• Use alt="" for decoration: An intentional empty value lets assistive technology skip an image that adds no information.
• Keep keywords natural: Google uses alt text with visual and page context, and warns that keyword stuffing harms the user experience.
• Check the rendered page: Confirm the published HTML contains the right alternative and that the surrounding text does not repeat it unnecessarily.
• Measure without overclaiming: Search Console reports image-search performance for pages, but its metrics do not prove that an alt-text change caused a result.

What I'm building

Delegate tasks. Get software.

Give Vroni a GitHub issue, bug report, spec, or rough idea. It reads the repo, plans the change, writes code, runs checks, and works toward a review-ready pull request.

Take a look at vroni.com