Alt text is one of those things almost everyone has heard of, but very few people use properly. It often gets treated as a checkbox task or ignored completely, even though it plays a direct role in accessibility, search visibility, and how content is understood by machines.
In simple terms, alt text helps search engines and assistive technologies understand what an image represents. When done well, it improves user experience, supports accessibility standards, and strengthens on-page SEO. When done poorly, it does the opposite.
This guide breaks down what alt text actually is, how it works, why it matters for SEO and accessibility, and how to write alt text that is genuinely useful.
What Is Alt Text?
Alt text, short for alternative text, is a written description added to an image in HTML using the alt attribute.
Its primary purpose is to describe the content and function of an image when the image itself cannot be seen. This includes situations where images fail to load, as well as cases where users rely on screen readers.
A basic example looks like this:
The text inside the alt attribute is what screen readers read aloud and what search engines use to understand the image.
What Does Alt Text Do in Practice?
Alt text serves three main functions:
- It provides context for users who cannot see images
- It helps search engines interpret visual content
- It acts as a fallback when images fail to load
Each of these functions is important on its own. Together, they make alt text a core part of accessible, search-friendly content.
Why Alt Text Matters for Accessibility
Accessibility is not optional. For many users, it is the only way they can consume content.
People using screen readers rely on alt text to understand images. Without it, the screen reader may announce the image file name, a URL, or nothing at all. That results in missing information and a broken experience.
How Screen Readers Use Alt Text
Screen readers read alt text aloud when they encounter an image. If the alt text is missing, the user loses context. If the alt text is vague or incorrect, the information becomes misleading.
For example:
- Bad alt text:
alt="image" - Bad alt text:
alt="photo" - Good alt text:
alt="Person typing on a laptop at a desk with a notebook and coffee"
The second example provides actual meaning and context. This is what accessibility guidelines aim for.
Accessibility Standards and Alt Text
The Web Content Accessibility Guidelines (WCAG) explicitly require text alternatives for non-text content.
According to the WCAG 2.1 guidelines from the World Wide Web Consortium (W3C), non-text content must have a text alternative that serves the equivalent purpose.
This means alt text is not a nice-to-have. It is a requirement for compliance.
Why Alt Text Matters for SEO
Search engines cannot see images the way humans do. They rely on surrounding text, file names, structured data, and alt text to understand what an image represents.
Alt text helps search engines:
- Understand image context
- Associate images with page topics
- Rank images in image search
- Improve overall page relevance
Alt Text and Google Search
Google has publicly stated that alt text helps them understand images and how they relate to content.
In Google’s image SEO documentation, they recommend using descriptive alt text to improve accessibility and image search performance.
Alt text does not magically boost rankings on its own. However, it contributes to better content understanding, which supports SEO as a whole.
Alt Text vs Image File Names
Alt text and file names are related, but they are not the same thing.
An image file name like IMG_3829.jpg provides no context. A file name like red-running-shoes.jpg helps a little. Alt text adds a clearer, human-readable explanation.
Best practice is to optimize both:
- Use descriptive file names
- Use meaningful alt text that explains the image
They work together, not in isolation.
What Makes Good Alt Text?
Good alt text is descriptive, specific, and written for humans first.
Key Characteristics of Good Alt Text
- Describes what is actually in the image
- Matches the context of the surrounding content
- Is concise but meaningful
- Avoids unnecessary filler words
- Does not stuff keywords
For example:
- Weak alt text:
alt="SEO image" - Strong alt text:
alt="Example of well-written alt text explaining an image for accessibility"
The second version actually explains something.
How Long Should Alt Text Be?
There is no strict character limit, but most screen readers handle alt text best when it is under 125 characters.
That does not mean you should aim for a number. It means you should aim for clarity.
If the image is complex, a short alt text plus additional surrounding explanation may be better than forcing everything into one line.
Alt Text Best Practices for SEO
Alt text should support SEO without turning into keyword stuffing.
Do This
- Use keywords naturally where relevant
- Describe the image honestly
- Keep it aligned with page intent
- Use different alt text for different images
Avoid This
- Repeating the same alt text across images
- Stuffing keywords unnaturally
- Writing alt text that does not match the image
- Using phrases like “image of” or “picture of”
Search engines already know it is an image.
Examples of Good and Bad Alt Text
Example 1: Blog Image
Image: Screenshot of a website analytics dashboard
- Bad alt text:
alt="analytics" - Better alt text:
alt="Website analytics dashboard showing traffic and engagement metrics"
Example 2: Decorative Image
If an image is purely decorative and adds no informational value, the correct approach is to use empty alt text:
<imgsrc="divider.png"alt="">
This tells screen readers to skip the image entirely.
Alt Text for Different Image Types
Product Images
Alt text should describe the product clearly.
Example:
alt="Black leather backpack with front zipper pocket"
Infographics
Alt text should summarize the purpose, not every data point.
Example:
alt="Infographic showing steps for image optimization on websites"
Buttons and Icons
Alt text should describe the function, not the appearance.
Example:
alt="Download PDF"
Common Alt Text Mistakes
These mistakes show up on even well-designed websites.
- Leaving alt text empty on important images
- Copying captions directly into alt text
- Writing overly long descriptions
- Using keywords instead of descriptions
- Forgetting alt text on icons and buttons
Each of these reduces usability and clarity.
Alt Text and Image SEO Beyond Google
Alt text also plays a role in:
- Bing image search
- Voice assistants
- AI-powered search summaries
- Assistive browsing tools
As search evolves toward more multimodal and AI-driven systems, clear image descriptions become even more important.
Should You Use an Alt Text Generator?
Alt text generators can help speed up workflows, especially for large websites or image-heavy content.
However, they should assist, not replace, human review.
A good alt text generator should:
- Create descriptive, readable text
- Avoid keyword stuffing
- Allow manual editing
Generated alt text should always be reviewed to ensure it matches context and intent.
How to Audit Alt Text on Your Website
A basic alt text audit can reveal major issues quickly.
Steps:
- Crawl your site using an SEO tool
- Identify images with missing alt text
- Review duplicated alt attributes
- Check decorative images
- Update key pages first
This alone can improve both accessibility and content quality.