What is Alt Text? SEO Guide for Beginners

Learn what alt text is, why it matters for SEO and accessibility, and how to write effective image alt attributes.

Alt text (alternative text) is the value of the alt attribute on an HTML image element that provides a text replacement for what an image shows. It is read aloud by assistive technologies used by people who cannot see images, and it displays on the page when the image fails to load because of network errors, content blocking, or link rot, per MDN. For SEO, Google uses alt text along with its computer vision algorithms and the surrounding page content to understand the subject matter of an image, which influences whether and where that image appears in Google Images.

Why Alt Text Matters for SEO

Google Image Search drives a significant amount of traffic. According to Sparktoro, Google Images accounts for over 20% of all web searches. Without alt text, your images are invisible to this entire channel. Properly optimized alt text gives your images a chance to rank and pull in visitors who would never find you through standard web search.

Alt text also provides context to search engines about your page content. If you have a blog post about "best standing desks" and your images have alt text like "ergonomic standing desk with monitor arm in home office," Google gets a stronger signal that your page is genuinely about standing desks. This reinforces your on-page relevance.

From an accessibility standpoint, alt text is not optional. Screen readers depend on it to describe images to users who cannot see them. Sites that meet web accessibility standards (WCAG) perform better in search because Google favors user-friendly experiences. In some jurisdictions, accessibility is also a legal requirement.

When images fail to load due to slow connections or broken URLs, alt text serves as a fallback that keeps your page informative. Users still understand what was supposed to be there instead of staring at a broken image icon.

How Alt Text Works

You add alt text using the alt attribute inside an <img> tag: <img src="desk.jpg" alt="Wooden standing desk with dual monitors">. Most CMS platforms provide an alt text field when you upload or insert an image.

Google crawls your alt text along with the surrounding content, the image file name, the page title, and any captions or nearby text. All of these signals together help Google understand and categorize your image.

Alt text should describe the image accurately and concisely. MDN states that an alt value "should provide a clear and concise text replacement for the image's content" and "should not describe the presence of the image itself or the file name of the image." There is no hard character cap in the HTML spec or in Google's guidance. The widely repeated "125 character" figure is a screen reader usability convention, not a real limit. It traces back to older JAWS versions that read alt text in 125 character chunks, and accessibility experts have explicitly debunked the idea that browsers or screen readers truncate at that count. Keeping informative alt text in roughly that range is a reasonable readability habit because a screen reader user cannot pause or rewind mid description, but a longer alternative is read in full when the image genuinely needs it.

Decorative images that serve no informational purpose, like background patterns or dividers, should have an empty alt attribute (alt=""). The W3C WAI alt decision tree directs you to use empty alt when an image is purely decorative or redundant to nearby text, which tells assistive technology to skip it and avoids audible clutter. MDN reinforces that alt="" explicitly marks an image as not a key part of the content. Note the difference between alt="" and omitting the attribute entirely: the attribute should always be present, either with a real description or intentionally empty, never left off.

How to Improve Alt Text on Your Site

Be specific and descriptive - Instead of "dog," write "golden retriever puppy playing fetch in a park." The more context you provide, the more queries your image can match. Describe what is actually happening in the image, not just the subject.
Include relevant keywords naturally - If the image supports your target keyword, work it into the alt text. For a page about "home office setup," an alt like "minimalist home office setup with standing desk and plant" adds keyword relevance without feeling forced.

Avoid starting with "image of" or "picture of" - Screen readers already announce that they are describing an image. Starting with "image of" is redundant. Jump straight into the description. Instead of "Image of a laptop on a desk," write "Laptop open on a wooden desk with coffee cup."

Audit your existing images with Screaming Frog or Ahrefs - Run a crawl of your site and filter for images with missing or empty alt attributes. Prioritize fixing alt text on pages that already get organic traffic, since those improvements can unlock additional image search visits.

Use unique alt text for every image - Just like title tags and meta descriptions, duplicate alt text wastes an opportunity. If you have five product images, each one should describe the specific angle, color, or feature shown in that particular image.

Common Mistakes to Avoid

Leaving alt text blank on important images: Every informational image needs alt text. Running a site crawl typically reveals dozens or hundreds of images with missing alt attributes. Fixing these is one of the easiest SEO wins available.
Keyword stuffing the alt attribute: Writing "standing desk best standing desk buy standing desk cheap standing desk" as alt text is spam. Google can penalize pages with over-optimized image attributes. One natural mention of your keyword is enough.
Using the file name as a substitute: File names like "IMG_3847.jpg" or "screenshot-2026-02-14.png" provide zero context. Rename your image files to something descriptive before uploading, and still write proper alt text separately.

Key Takeaways

Alt text helps Google understand image content and is a key signal for appearing in Google Images
Keep descriptions specific and concise with natural keyword inclusion; there is no hard character limit, but roughly 125 characters is a readable target for informative images
Every informational image on your site needs unique, descriptive alt text, while decorative images use an empty alt=""
Alt text serves double duty by improving both SEO performance and web accessibility (WCAG) compliance

In Practice

Say you publish a buying guide and embed a product photo. Compare a weak implementation against a corrected one.

Before, the image is informative but the markup gives Google and screen readers nothing to work with:

<img src="/uploads/IMG_3847.jpg" alt="image">

After, the file is renamed to something descriptive and the alt value is a clear, concise replacement that names the subject and key details without keyword stuffing:

<img
  src="/images/oak-standing-desk-dual-monitor.jpg"
  alt="Oak standing desk raised to standing height with dual monitors and a desk lamp"
  loading="lazy">

For a purely decorative divider in the same article, the correct choice is an empty alt so assistive technology skips it rather than announcing noise:

<img src="/images/section-divider.svg" alt="">

The descriptive version follows Google's guidance to use information-rich content in context of the page, the renamed file follows Google's "my-new-black-kitten.jpg is better than IMG00023.JPG" filename advice, and the empty alt="" on the divider matches the W3C WAI decision tree path for decorative images.

What is Image SEO?, the broader practice of optimizing images for search, of which alt text is one part
What is Accessibility?, why alt text matters for screen reader users and WCAG conformance
What is a Title Tag?, another descriptive on-page element Google reads to understand content
What is a Meta Description?, like alt text, a per-page text field that should be unique and not stuffed
What is Core Web Vitals?, image handling also affects loading performance metrics that influence ranking