Alt text is one of the most commonly neglected on-page SEO elements and one of the simplest to fix. It sits inside image HTML, invisible to most users but read by search engines, screen readers, and increasingly by AI retrieval systems when evaluating whether content is well-labeled and citation-worthy.This guide explains what alt text is, exactly how it affects SEO and accessibility, how to write it correctly for every image type, and the common mistakes that undermine its value.
What image alt text is
Alt text, short for alternative text, is a written description added to the HTML of an image via the alt attribute. It was originally designed as a fallback: if an image fails to load, the alt text displays in its place so users understand what should be there. That function still exists, but alt text now serves three additional purposes that matter far more for SEO in 2026.First, it makes images accessible to users with visual impairments who navigate with screen readers. Screen reading software reads alt text aloud when it encounters an image. Without it, the software either reads the file name ("IMG underscore 2847 dot JPEG") or announces that an image exists with no context at all. Both are poor experiences that effectively exclude visually impaired users from engaging with your content.Second, it gives search engines the textual context they need to understand and index what an image contains. Search engines cannot see images the way humans do. Google uses alt text alongside computer vision algorithms and surrounding page text to determine what an image shows and how it relates to the page's topic.According to a Semrush 2025 content quality study, pages with complete on-page metadata, including title tags, meta descriptions, and alt text, consistently outperform pages missing these elements in both organic rankings and AI Overview appearances. When AI systems evaluate whether a page is well-structured and citation-worthy, image metadata, including alt text, is part of the quality signal they assess.drag embedded assetpublished.
How alt text affects SEO
Google Image Search rankings
Google Image Search is a significant traffic source that most SEO teams underestimate, particularly for e-commerce, food, travel, lifestyle, and how-to content. Google uses alt text as the primary signal for understanding what an image shows and whether it is relevant to an image search query. Descriptive, keyword-relevant alt text directly increases the probability that an image appears in Google Images results for the queries you care about.According to Google's official image SEO documentation, alt text is the most important metadata element for image indexing. Google explicitly states it uses alt text to "determine the subject of the image" alongside visual recognition and surrounding page text.
Page-level topical relevance
Alt text contributes to the overall semantic relevance of a page for its target keywords. When an image on a page about keyword research has alt text describing a keyword research tool interface, it reinforces the topical signal of the entire page. When the same image has alt text of "screenshot" or no alt text at all, it contributes nothing. Every image without meaningful alt text is a missed opportunity to strengthen the page's relevance signal.
Core Web Vitals and accessibility scores
Google's Lighthouse and PageSpeed Insights flag missing alt text as an accessibility issue that feeds into overall quality scores. While this is not a direct ranking factor in isolation, it contributes to the page experience signals Google uses to evaluate content quality. Sites that consistently pass accessibility audits are demonstrating the same editorial discipline that Google's helpful content system rewards.
AI search visibility
Images with descriptive alt text are more likely to be referenced in AI-generated answers in Google AI Overviews, ChatGPT, and Perplexity. As AI search captures a growing share of global queries, properly labeled images become an asset for AI visibility as well as traditional search. An e-commerce site that optimized its product image alt text saw its pages begin appearing as cited sources in ChatGPT and Perplexity responses to product-related queries within six months of completing the optimization.
How to write alt text correctly
Writing effective alt text follows a consistent set of principles regardless of image type. The underlying goal is always the same: describe what the image shows in a way that is useful to a person who cannot see it, while naturally incorporating relevant keyword context where it fits.
The core rules
- Be specific and descriptive: "Golden retriever puppy playing with a red ball in the grass" is useful. "Dog" is not. Describe what you actually see: the subject, the action, the relevant context.
- Keep it to 80 to 140 characters: Long enough to be useful, short enough not to overwhelm screen reader users. Most screen reading software truncates alt text beyond 125 characters.
- Include keywords naturally: One or two relevant keywords where they fit the description organically. Do not force keywords into alt text that does not naturally contain them.
- Do not start with 'image of' or 'photo of': Screen readers announce that they have encountered an image before reading the alt text. Adding 'image of' is redundant and wastes character space.
- Match the surrounding page context: Alt text should reflect why the image exists on that specific page, not just what the image shows in isolation. A product image on an e-commerce page should describe the product name and key specifications, not just 'a laptop on a white background'.
Alt text examples by image type
| Image type | Poor alt text | Correct alt text |
|---|---|---|
| Product photo | img_product_001.jpg | Blue wireless noise-cancelling headphones on a white background |
| Infographic | infographic | Infographic showing five steps of the keyword research process from seed keywords to content brief |
| Screenshot of a tool | screenshot | Ahrefs Keywords Explorer showing search volume and keyword difficulty for 'content marketing' |
| Blog post hero image | image.jpg | Person typing on a laptop at a desk with SEO analytics on screen |
| Decorative image (no content value) | decorative | Leave alt attribute empty: alt="" — tells screen readers to skip the image |
| Chart or graph | chart | Bar chart showing organic traffic growth from 12,000 to 48,000 monthly sessions over six months |
| Team or author photo | photo | Sarah Chen, Senior SEO strategist at LLM Visibility Lab, smiling in a blue jacket |
| Linked image (image acts as a link) | arrow icon | Download the 2026 keyword research checklist PDF |
The HTML implementation
Alt text is added to the alt attribute of the HTML img element. The syntax is straightforward:<!-- With alt text --><img src="keyword-research-tool-screenshot.webp" alt="Ahrefs Keywords Explorer showing 18,000 monthly searches for content marketing" /><!-- Decorative image — empty alt attribute, not missing --><img src="decorative-divider.webp" alt="" /><!-- Missing alt attribute — incorrect, avoid this --><img src="product-photo.webp" />The distinction between an empty alt attribute (alt="") and a missing alt attribute is important. An empty alt attribute correctly tells screen readers and search engines that the image is decorative and can be skipped. A missing alt attribute is an error that some screen readers handle unpredictably and that search engines treat as an uncommunicated image.
Alt text for different image contexts
Decorative images
Not every image needs descriptive alt text. Images used purely for visual decoration, such as background patterns, decorative dividers, or abstract design elements that add no content value, should have an empty alt attribute rather than a description. Adding unnecessary alt text to decorative images forces screen reader users to sit through descriptions of content that does not help them understand the page. Use alt="" for decorative images without exception.
Linked images
When an image functions as a link, the alt text should describe where the link goes rather than what the image shows. If you have a download button styled as an image, the alt text should describe the download action and destination, not the visual appearance of the button. Screen reader users need to understand what will happen when they activate the link, not what the button icon looks like.
Complex images like charts and infographics
Charts, graphs, and infographics often contain more information than can be summarized in 140 characters of alt text. For these images, the recommended approach is to write a short alt text that names the type of chart and its main finding, then provide a full text description of the data either immediately below the image or in a separate accessible section of the page. This serves both accessibility requirements and SEO by making the full data content available in text form for indexing.
Image alt text and technical image SEO
Alt text works alongside several other image SEO factors that collectively determine how images perform in search. These elements should be addressed together rather than in isolation:
| Element | Best practice | Why it matters |
|---|---|---|
| File name | Use descriptive hyphenated names: keyword-research-tool.webp, not IMG0043.webp | Search engines read file names as a secondary image signal before the alt text |
| File format | Serve images in WebP or AVIF for the best size-to-quality ratio | Smaller files load faster, improving Core Web Vitals and crawl efficiency |
| File size | Compress before upload. Target under 100KB for most images. | Oversized images are the most common cause of slow page load times |
| Image dimensions | Serve images at display size, not larger | Oversized dimensions waste bandwidth and increase processing time for the browser |
| Structured data | Add ImageObject schema to key images where relevant | Helps AI systems understand image content in structured, machine-readable form |
| Captions | Add captions to images that contain data or key information | Captions are read by all users and indexed by search engines, adding contextual reinforcement |
How to audit your site's alt text
For most sites, the fastest starting point is a crawl using Screaming Frog or Ahrefs Site Audit, which surfaces all images across your site and flags those with missing, empty, or excessively long alt text. Filter the results to prioritize images on your highest-traffic pages and your highest-converting product or service pages, since those will produce the fastest measurable impact from improvements.Google Search Console does not report alt text issues directly, but running a Lighthouse accessibility audit on key pages flags missing alt text as an explicit accessibility failure alongside a score that gives you a baseline for tracking improvements. For sites with thousands of images, AI-powered tools like AltText.ai can generate first-draft alt text at scale for human review, making large-scale audits practically achievable without months of manual effort.
Common alt text mistakes
| Mistake | Why it hurts | Fix |
|---|---|---|
| Missing alt attribute entirely | Screen readers behave unpredictably; search engines receive no image context | Add alt text to every image. Use alt="" for decorative images. |
| Keyword stuffing in alt text | Google treats it as spam and penalizes the page. Screen reader users hear an incoherent keyword list. | Write natural descriptions. Include one or two relevant keywords only where they fit organically. |
| Generic alt text like 'image' or 'photo' | Provides zero information to search engines or screen reader users | Describe what the image actually shows in specific, contextually relevant terms. |
| Using the file name as alt text | File names like IMG_2847 are meaningless to both users and search engines | Write a real description. Rename image files descriptively at the same time. |
| Same alt text on multiple different images | Tells search engines your images are identical when they are not | Write unique alt text for every unique image on your site |
| Ignoring decorative images | Adding descriptive alt text to decorative images clutters screen reader output | Use alt="" on all purely decorative images so screen readers skip them correctly |
| Alt text that describes the image without page context | Misses the semantic opportunity to connect the image to the page's target topic | Write alt text that reflects why the image exists on this specific page |
Conclusion
Alt text is one of the highest-return, lowest-effort on-page improvements available to most websites. It improves rankings in Google Image Search, strengthens the topical relevance signal of the surrounding page, supports accessibility compliance, and in 2026 feeds the AI retrieval signals that determine whether your content is cited in generated answers.The process is straightforward: describe what the image shows in specific, contextually relevant terms, include keywords naturally where they fit, keep it under 140 characters, use empty alt attributes for decorative images, and audit your existing image library starting with your highest-traffic pages. Treat it as a standard part of your on-page SEO workflow at every content update, not as a one-time project. The compounding effect of consistently well-labelled images across a large site is measurable in both traditional search traffic and AI citation frequency.
