Mastering Alt Text for SEO: Improve Your Accessibility and Search Visibility

Alt text is a key WCAG requirement, but many developers and website owners don’t realize that it can have a positive impact on SEO too.

To help you make the most of your alt text efforts, we’ve unpacked the effects it can have from an SEO perspective and the steps you can take to get the best of both worlds – web accessibility compliance and greater search visibility.

What Is Alt Text and Why Does It Matter for SEO?

Alt text, short for “alternative text,” is a brief description that’s added to the HTML of an image on your site. Its primary function is to describe the content and function of an image for people who can’t see it, whether due to a visual impairment or because the image fails to load.

From an SEO perspective, alt text plays a leading role in how search engines understand and index the visual content on your site. Unlike humans, search engines can’t “see” images, so they rely on alt text to interpret what an image represents. 

When written well, alt text helps clarify the context of a page, improve its relevance for certain search terms, and support the overall structure of your content, making it a vital component of an on-page SEO strategy.

Does Alt Text Help SEO?

Optimizing your image alt text isn’t just about accessibility, it’s also a powerful tool for improving your site’s search engine performance. Here’s how strategic alt text can enhance your SEO efforts:

Alt Text Helps Search Engines Understand Images

Search engines rely on textual cues to understand what an image depicts. Alt tags offer this context, ensuring search engine crawlers can accurately index your visual content. This can strengthen the relevance of any given page for specific keywords, especially when the images support the surrounding written content.

Alt Text Is a Ranking Factor

While alt text alone won’t skyrocket a page to the top of search results, it is a known ranking factor, particularly for image search. Google uses alt text as one of the signals to determine what images to show in response to queries. Well-optimized alt text also indirectly supports broader web search rankings by reinforcing the topical relevance of a page.

Alt Text Drives Additional Traffic Through Google Images & Visual Search

When your images are properly labeled with descriptive alt text, they stand a better chance of appearing in Google Images and visual search tools like Google Lens. This opens up another avenue for attracting visitors, especially for content that’s visually driven, such as e-commerce products, recipes, infographics, or how-to guides. 

How to Write Alt Text for SEO: Essential Best Practices

Crafting effective alt tags for SEO isn’t complicated, but there are a few key rules to follow if you want to get it right for both SEO and accessibility.

Rule #1: Be Descriptive and Specific

Describe what’s actually in the image using clear, specific language. Think about what someone would need to know if they couldn’t see the image. Instead of saying “dog,” say “golden retriever puppy playing with a tennis ball in the grass.” The more accurate the description, the more useful it is for both users and search engines.

Rule #2: Keep it Concise

Alt text should be brief but informative. Aim for around 5–15 words or under 125 characters. This keeps the description short enough for screen readers, while still providing enough detail to be meaningful. Avoid cramming in too much information or turning alt text into a full sentence unless it adds clarity.

Rule #3: Use Keywords Naturally

Incorporate relevant keywords when they make sense, but don’t force them in. Keyword stuffing, like repeating the same phrase unnaturally, can hurt the user experience and you may be penalized by search engines. Focus on writing for humans first, search engines second.

Rule #4: Provide Context

Think about why the image is on the page and how it connects to the surrounding text. Your alt tags should reflect the image’s purpose in context. For example, if the image supports a blog about DIY gardening, describe what the image shows in a way that supports that narrative—e.g., “woman planting herbs in a raised garden bed.”

The Synergy Between Alt Text and Web Accessibility

Alt text doesn’t just support SEO, it also helps create a more inclusive web. When written with accessibility in mind, alt text enhances the online experience for users with visual impairments while also improving your site’s overall usability and structure.

Helping Screen Readers Do a Better Job

Screen readers rely on alt text to communicate the content of images to users who are blind or have low vision.

When an image is encountered, the screen reader reads the alt attribute aloud, allowing the user to understand the image’s purpose or content. Well-written alt text ensures that visual elements don’t create gaps in comprehension for those navigating a site without vision.

Creating a Human-First Experience

Great alt text prioritizes clarity, context, and user experience, all of which align with what search engines reward.

Writing image alt tags for people, not algorithms, leads to more natural phrasing, stronger relevance, and better comprehension. When you write with accessibility in mind, you’re inherently improving your SEO by making your content easier to understand and navigate.

Complying with WCAG Guidelines

The Web Content Accessibility Guidelines (WCAG), a global standard for web accessibility, emphasize the importance of alternative text.

Under WCAG 2.2, providing alt text for non-text content is a Level A requirement, which is the minimum level of accessibility compliance. Sites aiming for AA or AAA compliance must ensure all meaningful images are described and decorative ones are properly marked with null alt text.

Common Alt Text Mistakes to Avoid

Even with the best intentions, it’s easy to make mistakes when writing alt text, especially if you’re trying to optimize for both accessibility and SEO. Here are some of the most common pitfalls to watch out for:

• Leaving Alt Text Blank. Unless the image is decorative and intentionally marked with alt=””, skipping alt text can make your site less accessible and leave gaps in content understanding for screen reader users.
• Keyword Stuffing. Cramming multiple keywords into the alt attribute makes the text unreadable and you may be penalized by search engines. Keep it natural and relevant.
• Using “Image of…” or “Picture of…”. Screen readers already announce that it’s an image, so phrases like “image of a flower” are redundant. Instead, simply write “red tulip blooming in spring garden.”
• Being Too Vague or Too Long. Alt text like “photo” or “nice view” provides no real value. On the flip side, overly wordy descriptions can overwhelm screen reader users. Aim for a specific, concise summary that fits within 125 characters.

How to Audit Alt Text On Your Site

Regularly auditing your site’s alt text is essential for maintaining both accessibility and SEO performance. To make things easier, here are some tools and techniques to help you evaluate and improve your alt text usage:

• AccessibilityChecker.org. Our automated tool can scan your web pages for accessibility issues, including missing or poorly structured alt text. It provides a clear breakdown of what’s wrong and where, making it easier to prioritize fixes and improve compliance with WCAG standards.
• WAVE. Offered by WebAIM, the WAVE tool highlights images with missing, empty, or suspicious alt attributes directly on your web page so you can spot gaps instantly.
• Lighthouse in Chrome DevTools. Google’s built-in auditing tool (found under the “Lighthouse” tab in DevTools) provides an accessibility score for your page and flags images without alt text.
• Manual Review with Screen Reader Tools. For a human-centered perspective, try navigating your site using a screen reader like NVDA (Windows) or VoiceOver (Mac). This can reveal how useful or frustrating your alt text is in practice.

Advanced Alt Text Considerations

Once you’ve mastered the basics of writing effective alt text, there are more nuanced situations to consider.

Alt Text for Complex Images

For images that convey dense or layered information, like charts, graphs, or infographics, standard alt text alone may not be enough. These visuals often require:

• Concise alt text that identifies the type of graphic and its purpose (e.g., “Bar chart showing 2024 monthly sales by region”).
• A longer description in the surrounding text, a linked page, or using ARIA longdesc attributes to explain the full data set or insights.
• Data tables as alternatives, when appropriate, to ensure the same information is accessible in a screen-readable format.

The goal is to ensure users who can’t see the image can still fully understand the information it presents.

Alt Text vs. Image Titles vs. Captions

Alt text often gets confused with image titles and captions, but each serves a different role:

• Alt Text: Invisible to sighted users, read by screen readers and used by search engines.
• Title Attribute: Optional metadata that appears as a tooltip when a user hovers over an image. Not consistently supported by all browsers or screen readers.
• Captions: Visible text near the image that describes or contextualizes it for all users. Captions are great for storytelling or additional context but don’t replace alt text.

Best practice: Always use alt text for accessibility and SEO, and only use titles and captions when they serve a clear purpose.

The Role of AI in Generating Alt Text

AI-based tools can automatically generate alt text based on image recognition, which is helpful for large websites or content libraries. However, there are key trade-offs:

Pros:

• Speeds up the process of labeling large volumes of images.
• Useful for content creators with limited accessibility knowledge.
• Can catch missing alt text across a site.

Cons:

• Descriptions can be vague, generic, or inaccurate (e.g., “a person” instead of “a nurse assisting a patient”).
• AI often lacks context, especially for branded content or emotional nuance.
• Still requires a human review to meet accessibility and SEO standards.

AI can be a helpful assistant, but it shouldn’t replace thoughtful, human-written alt text, especially for meaningful or complex images.

In Closing

Adding alt text to your images is just one part of providing a more user-friendly experience on your site or app and complying with web accessibility standards, but it has great importance. And, in the bigger scheme of things, it can also help drive more traffic to your site, if implemented correctly, that is.