Image Alt Text SEO: How to Write Alt Tags That Help Rankings
What Is Alt Text and Why It Matters for SEO
Alt text (alternative text) is an HTML attribute on img tags that describes the image in words. Google cannot reliably see images the way humans do — it reads alt text to understand what an image shows, what it is about, and how it relates to the surrounding content.
Images with good alt text can rank in Google Images and contribute to the page's topical relevance for text searches. A page about sitemap errors with images that have descriptive alt text signals stronger relevance for that topic than the same page with blank or missing alt attributes.
Alt text is also critical for accessibility. Screen readers read alt text aloud for visually impaired users. Writing good alt text serves both search engines and people — the two goals are almost always aligned.
The HTML Syntax
The alt attribute sits inside the img tag, immediately after the source. The value should describe what is actually in the image, not what the page is about in general.
<img src="sitemap-checker-screenshot.png" alt="SitemapFixer sitemap checker showing 5 crawl errors on example.com">
Always include the alt attribute on every img tag — even when you want it empty. An image tag without any alt attribute is different from one with alt="": missing means undefined, empty means deliberately decorative.
How Google Uses Alt Text
Google uses alt text to understand image content and match images to search queries in Google Images. It also contributes to the page's overall relevance signals for web search — a page whose images all describe the topic reinforces that topic to Google's ranking algorithm.
Pages ranking in Google Images almost always have descriptive alt text. Google's vision AI (Google Lens) is getting better at identifying image content without alt text, but alt text remains the primary signal Google publicly acknowledges using. Alt text also appears in Google's accessibility audit in Lighthouse — images without alt text fail the accessibility check and can affect your overall Lighthouse score.
Writing Good Alt Text for SEO
The core rule: describe what is actually in the image, naturally including relevant keywords where they fit. Good alt text reads like a caption a sighted person would write for someone who cannot see the image.
- Good: "screenshot of XML sitemap with 47 URLs and one error highlighted in red"
- Bad: "image1.jpg" or "photo" — these tell Google nothing about the image content
- Bad (stuffed): "sitemap fixer sitemap checker sitemap tool sitemap validator" — Google ignores keyword-stuffed alt text and may treat it as a spam signal
Keep alt text under 125 characters. This limit matters for both SEO and accessibility — screen readers typically cut off at around 125 characters. If the image needs more description, add a visible caption below it instead.
When to Leave Alt Text Empty
Decorative images that add no information — dividers, background patterns, purely visual icons — should have alt="" (empty alt, not missing alt). An empty alt attribute explicitly tells screen readers to skip the image, which is correct behavior for decorative images.
Missing alt (no attribute at all) means Google and screen readers see the filename instead — divider-line.png gets read aloud, which creates a confusing experience for screen reader users. Always include the alt attribute even when it is intentionally empty: alt="".
Alt Text for Different Image Types
Different image types require different approaches to alt text:
- Product image: "blue men's running shoe size 10, white sole" — be specific about color, material, and distinguishing features
- Screenshot: "Google Search Console coverage report showing 247 indexed pages" — name the tool and the key data shown
- Chart: "bar chart showing organic traffic growing from 1,200 to 3,400 visitors between January and June 2026" — describe the trend, not just "chart"
- Infographic: describe the main insight, not every data point — if the infographic has too much content, use a visible caption or linked transcript
- Logo: "SitemapFixer logo" — simple and accurate is enough for logos
Common Alt Text Mistakes
These are the most frequently seen alt text errors, all of which are easy to fix once identified:
- Missing alt attribute entirely — very common, easy to audit; Google and screen readers fall back to the filename
- Keyword stuffing — repeating your target keyword five times in alt text is a spam signal, not an SEO tactic
- Using the filename as alt —
IMG_4529.jpgtells Google nothing about the image content - Generic text — "image" or "photo" conveys no information; treat it the same as missing alt
- Not updating alt text when images change — if you replace an image but keep the old alt text, the description no longer matches the content
- Same alt text on multiple images — if ten images on a page all have the same alt text, Google learns nothing unique from any of them
Finding and Auditing Alt Text Issues
Google Search Console does not report missing alt text directly, so you need dedicated tools for this audit. Screaming Frog's Images tab shows all images crawled with and without alt text — export the list and filter for empty or missing alt values to get a prioritized fix list.
Chrome's Lighthouse Accessibility audit flags images without alt text and gives you a score impact estimate. For a quick manual check, right-click any image, choose Inspect, and look for the alt attribute inside the img tag in the HTML panel. Ahrefs Site Audit also checks for images without alt text as part of its on-page SEO issue report.
Alt Text and Google Image Search
Google Images drives significant traffic for visual topics — recipes, products, travel destinations, design inspiration, technical diagrams. Pages ranking in Google Images for valuable queries get a meaningful stream of visitors who are often closer to taking action than a general web search visitor.
Ranking in Google Images requires several signals working together: a descriptive filename (not IMG_4529.jpg), descriptive alt text, surrounding text context that matches the image topic, image quality and appropriate file size, and the page's overall PageRank. Product pages especially benefit from Google Image traffic for product-name searches — a well-optimized product image can appear in both web results and image results simultaneously.
Alt Text vs Title Text
Alt text and title text are two different HTML attributes on img tags, and they serve entirely different purposes. Alt text is the SEO signal — Google reads it to understand image content. Title text is the tooltip that appears when a user hovers over an image in a browser — it is a minor accessibility feature with very low SEO value.
Never skip alt text and use only title text instead. If you are writing a single attribute, write a good alt text and skip title text entirely — you get the SEO and accessibility value without the overhead. They serve different purposes: alt text is for when the image fails to load or a screen reader is reading the page; title text is optional additional description for sighted hover interactions.