The Complete Guide to Alt Text for Images (2026)

Alt text (alternative text) is a written description of an image embedded in HTML using the alt attribute, consumed by screen readers and search engines to understand visual content. The WCAG 2.2 AA standard requires all non-decorative images to have a text alternative that serves the equivalent purpose — not just a literal description of pixels, but a functional equivalent of what the image conveys in context. A 2024 WebAIM million-page analysis found that 22.1% of all home-page images were missing alt text, making it the second most common accessibility failure on the web.

Why Alt Text Matters: Accessibility + SEO

Alt text serves two distinct audiences: people using assistive technology and search engine crawlers. For the 2.2 billion people worldwide with visual impairments — including 253 million with moderate to severe vision loss according to the WHO — alt text is the only way to understand images when using screen readers like JAWS, NVDA, or VoiceOver.

For SEO, Google has confirmed that alt text is used as a ranking signal in image search and helps the algorithm understand page content. Pages with properly implemented alt text tend to perform better in both web and image search results, though Google's John Mueller has stated it's a "very lightweight" ranking factor compared to content quality and backlinks.

The accessibility requirement is not optional. The Americans with Disabilities Act (ADA) and Section 508 of the Rehabilitation Act have both been used in lawsuits against organizations whose websites lacked proper alt text. In 2023, over 4,600 digital accessibility lawsuits were filed in U.S. federal courts according to accessibility.com tracking data.

How to Write Alt Text for Different Image Types

Informative images that convey concepts or information need alt text that describes the key takeaway, not every visual detail. For a photo of a crowded subway, "Commuters packed into a Tokyo subway car during rush hour" is better than "Many people standing in a train."

Functional images used as links or buttons must describe the destination or action, not the image itself. A magnifying glass icon linking to search should have alt="Search" not alt="Magnifying glass."

Complex images like charts and infographics require structured descriptions that convey the data or relationships. A bar chart showing quarterly revenue growth should communicate the trend and key data points, not just "A bar chart with four bars."

Decorative images that add no informational value should use alt="" (empty alt) so screen readers skip them entirely. A purely decorative background pattern or stock photo that doesn't contribute meaning is a prime candidate for this treatment.

Alt Text Length: Best Practices

The widely cited 125-character guideline originates from early screen reader behavior — JAWS and NVDA would truncate alt text at around 125 characters in certain modes. Modern screen readers can handle longer descriptions, but shorter is generally better. Aim for 5-15 words depending on image complexity.

For images that genuinely require longer descriptions — such as infographics, charts, or complex diagrams — use the longdesc pattern or provide the detailed description in adjacent body text with a concise alt attribute that references it. A 2023 Nielsen Norman Group study found that users of assistive technology prefer concise alt text (under 20 words) for most images, with longer descriptions reserved for content where visual relationships are essential to understanding.

WCAG Requirements for Alt Text

WCAG 2.2 Success Criterion 1.1.1 (Non-text Content, Level A) requires all non-text content presented to the user to have a text alternative that serves the equivalent purpose. This covers images, charts, graphs, icons, and CAPTCHAs.

Key requirements:

• All informative images must have appropriate alt text
• Decorative images must use alt="" or aria-hidden="true"
• Image buttons must describe the action (e.g., "Submit search")
• CAPTCHAs must provide alternatives for multiple sensory modalities
• The alt attribute must be present on every <img> element — missing alt is a WCAG failure

The WebAIM Million 2024 report showed that pages with correctly implemented alt text had an average of 50.6 accessibility errors per page compared to 73.1 for pages with missing alt text, demonstrating that proper alt text implementation correlates with better overall accessibility practices.

Common Alt Text Mistakes

Keyword stuffing remains the most visible error in alt text writing — phrases like "blue running shoes for men buy now 2026 best price" hurt both accessibility and SEO. Screen reader users hear every irrelevant keyword, and Google's algorithm penalizes manipulative alt text practices.

The "image of" or "picture of" prefix is redundant because screen readers announce "graphic" before reading the alt attribute. Writing alt="Photo of a sunset over the ocean" wastes characters — alt="Sunset over the ocean" is cleaner.

Blank alt text on informative images is a critical WCAG failure. If the image conveys information, it needs a text alternative. If it's decorative, use alt="" deliberately rather than omitting the attribute entirely.

Using file names as alt text — alt="IMG_8472.JPG" or alt="blue-sneakers-final-v3.jpg" — provides zero value to either accessibility or SEO and is one of the most common automated generation errors found in the WebAIM Million data.

Alt Text vs Captions vs Descriptions

These three elements serve different purposes and should not be conflated:

• Alt text is machine-readable metadata for screen readers and search engines. It is not displayed visually on the page unless the image fails to load.
• Captions are visible text displayed alongside an image, typically providing context, attribution, or commentary. Captions are read by everyone, including sighted users.
• Image descriptions are longer, detailed written explanations placed in the surrounding content, often used for complex images where alt text alone would be insufficient.

Best practice is to use all three where appropriate, with each serving its distinct role. A caption can provide context while the alt text delivers the functional equivalent, and neither should simply repeat the other.

FAQ

What is the ideal length for alt text?

The widely used guideline is 125 characters or fewer, as early screen reader versions like JAWS 8 and older NVDA releases truncated at this threshold. Modern screen readers handle longer text, but brevity remains best — most images need only 5-15 words. Reserve longer descriptions for complex data visualizations.

Does alt text help with SEO rankings?

Yes, but indirectly. Google uses alt text to understand image content for image search rankings, and properly described images can appear in Google Image Search results. Google's John Mueller has described alt text as a "very lightweight" ranking factor — useful but not a primary ranking driver compared to content quality and relevance.

Should I use "image of" or "picture of" in alt text?

No. Screen readers already announce "graphic" or "image" before reading the alt attribute. Starting alt text with "image of" or "photo of" is redundant and wastes the limited characters that assistive technology users have to process.

What is WCAG 1.1.1?

Success Criterion 1.1.1 (Non-text Content, Level A) requires all non-text content presented to the user to have a text alternative that serves the equivalent purpose. This is the foundational accessibility requirement for images and applies to all web content under WCAG 2.0, 2.1, and 2.2.

Do decorative images need alt text?

Yes, but they need empty alt text (alt=""). This tells screen readers to skip the image entirely. Omitting the alt attribute entirely is not the same as empty alt — some screen readers will read the file name if alt is missing, creating a poor experience.

Can alt text be the same as the caption?

No. Alt text and captions serve different purposes. Alt text is invisible metadata for assistive technology and search engines. Captions are visible text for all users. They should complement each other, not duplicate — if the caption already describes the image fully, the alt text can use a concise summary that references the caption context.

How often should alt text be updated?

Whenever the image changes. Product photos that get replaced, infographics that are updated with new data, and screenshots that show different versions of software all need their alt text reviewed. A 2023 accessibility audit of 500 e-commerce sites found that 34% had alt text describing products that were no longer displayed, creating confusion for screen reader users.