Alt text — short for alternative text — is one of the most overlooked quick wins in on-page SEO. It describes images to search engines (which cannot see images) and to screen readers used by visually impaired users. Writing effective alt text takes seconds per image, improves accessibility for all users and contributes to both image search rankings and overall page relevance. This guide covers exactly how to write alt text that works for both SEO and users.
What Is Alt Text?
Alt text is an HTML attribute added to the <img> tag that provides a text description of the image. It appears in the HTML source as:
<img src="keyword-density-chart.jpg" alt="Bar chart showing keyword density percentage by content type" />
Browsers display alt text visually when an image fails to load — it appears in the image's place. Screen readers (used by visually impaired users) read alt text aloud when they encounter an image. Search engine crawlers use alt text to understand what the image depicts, since they cannot process visual image content the way humans do.
Alt text is required for accessibility compliance under WCAG (Web Content Accessibility Guidelines) standards and is referenced in Google's image SEO best practices as a key signal for image search ranking.
How Alt Text Benefits SEO
Alt text benefits SEO in three ways:
1. Image search rankings — Google Images receives billions of searches every day. Images with descriptive alt text that includes relevant keywords can rank in Google Images, driving additional traffic to your site beyond regular organic search.
2. Page relevance signals — the alt text on all images on a page contributes to Google's understanding of the page's overall topic. A blog post about keyword density that includes images with alt text like "keyword density checker screenshot" and "keyword density chart example" reinforces the page's topic to Google.
3. Accessibility-driven engagement — meaningful alt text improves the experience for all users, including those using screen readers, those on slow connections where images fail to load, and those using text-only browsers. Better accessibility reduces bounce rate and improves engagement signals that indirectly benefit rankings.
How to Write Good Alt Text
Good alt text is concise, specific and descriptive. The goal is to convey what the image shows to someone who cannot see it. Write as if describing the image to someone over the phone:
- Be specific — "a woman using a laptop" is better than "a person at a computer," which is better than "photo"
- Include relevant details — context that matters for the image's purpose: colours, actions, subjects, text visible in the image
- Be concise — aim for 50–125 characters (8–15 words)
- Write in plain language — no jargon unless the image specifically requires technical terminology
- Describe the image content, not the image itself — "a bar chart showing keyword density data" not "an image of a bar chart showing keyword density data." The word "image" is redundant — screen readers already announce that it is an image.
Examples of alt text quality levels:
- Bad: "image1.jpg" (filename as alt text — provides no information)
- Poor: "chart" (too vague)
- Better: "keyword density chart" (includes a keyword but lacks specificity)
- Best: "bar chart comparing keyword density across five content categories with SEO, technology and health topping 2%" (specific, descriptive, keyword-natural)
When to Leave Alt Text Empty
Not every image needs descriptive alt text. Decorative images — images used purely for visual design that add no informational content to the page — should have empty alt text (alt=""), not omitted alt attributes.
Decorative images that should have alt="":
- Background images, patterns, gradients and purely decorative borders
- Spacer images or layout images that serve no content purpose
- Icons that are accompanied by visible text labels (the text label already describes the icon's function)
- Repetitive images where the same visual appears multiple times with the same meaning (e.g., star rating icons where the rating number is also displayed as text)
The distinction between alt="" (empty) and omitting the alt attribute entirely matters: an image without any alt attribute at all is an accessibility error — screen readers will read the filename instead. An image with alt="" tells screen readers to ignore the image intentionally.
Alt Text Length
Alt text should be as long as necessary to describe the image accurately, but no longer. Most guidelines recommend keeping alt text under 125 characters because some older screen readers truncate at that length. However, for complex images like charts, diagrams, screenshots or infographics, longer descriptions may be necessary.
For complex images that require more than 125 characters to describe adequately, two approaches work well:
- Alt text + figcaption — use a brief alt text for the most essential description and add a
<figcaption>element below the image for additional detail - Alt text + nearby body text — ensure the image's key information is also described in the surrounding body text, so the alt text can be a brief summary
For SEO purposes, Google can process longer alt text, but concise and accurate descriptions work better than lengthy keyword-stuffed paragraphs masquerading as alt text.
Using Keywords in Alt Text
Including relevant keywords in alt text is appropriate and beneficial when the keyword genuinely describes the image content. If your page is about keyword density and you have a screenshot of your keyword density checker tool in use, alt text like "screenshot of keyword density checker showing 2.5% density for target keyword" is both descriptive and keyword-relevant.
However, do not force keywords into alt text when they do not describe the image. Adding "keyword density checker free tool SEO" to the alt text of a decorative header image is keyword stuffing — it provides no value to users and can be detected as manipulative by Google's algorithms.
The test for alt text keyword usage: would you naturally say this word when describing the image to someone who cannot see it? If yes, include it. If no, leave it out.
Common Alt Text Mistakes
The most common alt text mistakes to avoid:
- Missing alt text — the most common mistake. Images without the alt attribute at all are an accessibility violation and a missed SEO opportunity.
- Using the filename — "IMG_20240315_123456.jpg" or "screenshot-1.png" as alt text provides no useful description and is a sign of auto-generated or unconsidered alt text
- Starting with "Image of" or "Photo of" — redundant, since screen readers already identify the element as an image
- Keyword stuffing — "free keyword density checker SEO tool keyword analysis" crammed into alt text as a keyword insertion method is manipulative and unhelpful
- Same alt text on multiple images — if you have multiple images on a page, each should have unique, specific alt text unless they are genuinely depicting the same thing
- Using alt text on logo links without brand context — a logo image linking to the homepage should have alt text like "SearchRankTool homepage" not just "logo"
Adding Alt Text in WordPress and Other CMS
Most CMS platforms provide built-in fields for alt text:
WordPress: When uploading an image in the Media Library, fill in the "Alt Text" field. You can also add or edit alt text from the block editor by clicking an image and finding the "Alt Text" field in the right sidebar.
Shopify: Go to the product or collection page, click the image and look for the "Add alt text" option.
Squarespace: Click an image block and look for "Image Settings" → "Alt Text."
Custom HTML: Add the alt attribute directly to the img tag: <img src="image.jpg" alt="Your descriptive text here">
For sites with many images lacking alt text, a site crawl using Screaming Frog (free up to 500 URLs) will identify all images missing alt text. The "Images" tab in Screaming Frog shows every image on your site alongside its current alt text — or lack thereof. Systematically working through this report to add descriptive alt text is a high-value, low-effort SEO improvement.
Frequently Asked Questions
Does alt text directly affect organic search rankings?
Alt text contributes to the overall relevance signals on a page and directly affects image search rankings. It is not a dominant ranking factor for regular organic results, but it is one of many on-page signals that collectively determine how well Google understands your page. Well-written alt text also improves accessibility, which reduces bounce rate and improves engagement — indirectly benefiting rankings.
Should I add alt text to images in my blog posts?
Yes, always. Every meaningful image in a blog post should have descriptive alt text. The only exception is purely decorative images (background patterns, dividers) which should have empty alt text. Adding alt text to existing blog posts that are missing it is a quick way to improve both SEO and accessibility across your site.
What is the difference between alt text and the image title attribute?
The alt attribute describes the image content for accessibility and SEO. The title attribute provides advisory information that typically appears as a tooltip when users hover over the image. The title attribute has no SEO value and is not required — if used, it should contain different text from the alt attribute. Alt text is the priority; title is optional supplementary information.
Can Google actually read image content without alt text?
Google's computer vision technology (Google Lens) can analyse image content to some degree. Google has confirmed it uses machine-generated image descriptions when alt text is missing. However, these machine descriptions are less accurate than human-written descriptions and may not capture the specific context of the image. Providing explicit, well-written alt text remains best practice — do not rely on Google to infer it correctly.