Alt text is one of those small SEO details that delivers an outsized impact when done well. It makes your images accessible to people using screen readers, helps search engines understand your visual content, and gives your images a chance to rank in image search. Yet many website owners either skip alt text entirely or stuff it with keywords in ways that help no one. Writing good alt text is a skill, and once you understand the principles, it becomes quick and natural. This guide explains exactly how to write alt text for SEO that serves both real users and search engines.
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What Alt Text Actually Is
Alt text, short for alternative text, is an HTML attribute added to image tags that describes the content and function of an image. Its original purpose is accessibility: screen readers read alt text aloud so visually impaired users understand what an image shows. It also appears when an image fails to load, giving users context in place of the missing visual. Search engines rely on alt text to interpret images, since they cannot truly see pictures the way humans do. Good alt text therefore serves two audiences at once, real people and search crawlers.
Describe the Image Clearly and Concisely
The golden rule of alt text is to describe what the image actually shows in a clear, concise way. Imagine you are explaining the image to someone who cannot see it. Instead of a vague description like image or photo, write something specific such as a description of the subject, action, and relevant context. Aim for roughly one short sentence, generally under 125 characters, since some screen readers cut off longer descriptions. Being accurate and specific matters far more than being clever, because the goal is genuine understanding, not decoration.
Incorporate Keywords Naturally
Because search engines use alt text to understand images, including a relevant keyword can help your image rank for related searches. However, this must be done naturally. If your target keyword fits the description of the image, include it; if it does not, leave it out. For example, if an image genuinely shows a product in use, describing it with the product name is both accurate and keyword-relevant. Never cram unrelated keywords into alt text, as this is considered keyword stuffing, harms accessibility, and can trigger search engine penalties. Relevance and honesty always win.
Avoid Common Alt Text Mistakes
Several mistakes appear again and again. Do not begin alt text with phrases like image of or picture of, since screen readers already announce that an element is an image. Do not stuff multiple keywords separated by commas, and never repeat the exact same alt text on every image. Avoid leaving alt text blank on meaningful images, but do use empty alt attributes for purely decorative images so screen readers can skip them. Steering clear of these errors keeps your alt text useful and compliant with accessibility standards.
Handle Decorative and Functional Images Differently
Not every image needs descriptive alt text. Decorative images, such as background flourishes or dividers that add no information, should have an empty alt attribute so assistive technology ignores them. Functional images, like a magnifying glass icon that triggers search, should describe the function rather than the appearance, for example search rather than a description of the icon's shape. Informative images that convey content, such as charts or product photos, need full descriptive alt text. Matching your alt text approach to the image's purpose is a hallmark of thoughtful optimization.
Write Alt Text for Complex Images
Some images, such as infographics, charts, and diagrams, contain more information than a short sentence can capture. For these, provide a brief alt text summary of the main point, then include the detailed information in nearby body text or a longer description on the page. This ensures the key message is accessible to everyone and available to search engines in crawlable text form. Trying to compress an entire infographic into a single alt attribute fails both users and search crawlers, so pair concise alt text with fuller on-page context.
Test and Maintain Your Alt Text
Once your alt text is in place, verify it works. Use a screen reader or an accessibility auditing tool to hear how descriptions sound and to catch missing attributes. As you add new images over time, make alt text part of your standard publishing checklist so nothing slips through. Periodically audit older content, since sites accumulate untagged images as they grow. Consistent maintenance keeps your entire image library accessible and optimized, supporting both your users and your ongoing digital marketing goals.
Conclusion
Writing alt text for SEO comes down to a simple principle: describe images clearly and honestly for people who cannot see them, and let relevant keywords appear naturally when they genuinely fit. Avoid stuffing, handle decorative and functional images appropriately, and give complex visuals the fuller context they need. When you do this consistently, your images become more accessible, more likely to rank in image search, and a stronger contributor to your overall SEO. It is a small effort with lasting rewards, and one worth building into every page you publish.
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