Image Accessibility: Writing Effective Alt Text
Alt text makes images accessible to screen reader users and appears when images fail to load. It also contributes to image SEO. Writing useful alt text requires understanding context, purpose, and conciseness.
Key Takeaways
- Approximately 2.2 billion people globally have a vision impairment.
- Describe the function of the image, not just its appearance.
- Images that convey information need descriptive alt text:
- Keep alt text under 125 characters when possible.
- Include relevant keywords naturally — alt text is a ranking signal for image search.
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Why Alt Text Matters
Approximately 2.2 billion people globally have a vision impairment. Screen readers convert alt text to speech, making images accessible to blind and low-vision users. Beyond accessibility, alt text serves as fallback content when images fail to load (slow connections, CDN outages) and helps search engines understand image content for image search results.
The Golden Rule
Describe the function of the image, not just its appearance. Ask yourself: if I replaced this image with text, what would that text say?
Writing Effective Alt Text
Informational Images
Images that convey information need descriptive alt text:
- Poor:
alt="chart"— provides no information - Poor:
alt="image of a bar chart showing quarterly revenue data for 2025 with Q1 at 2.3M Q2 at 2.8M..."— too verbose - Good:
alt="Quarterly revenue chart — steady growth from $2.3M (Q1) to $3.1M (Q4) in 2025"— conveys the key insight
Decorative Images
Images that serve only as visual decoration (background patterns, spacers, purely aesthetic photos) should have an empty alt attribute: alt="". This tells screen readers to skip the image entirely. An image without an alt attribute at all is an accessibility error — the screen reader will read the filename instead.
Functional Images
Images used as buttons or links need alt text describing the action, not the image:
- Poor:
alt="magnifying glass"(on a search button) - Good:
alt="Search"— describes what happens when clicked - Poor:
alt="red arrow pointing right"(on a next-page button) - Good:
alt="Next page"— describes the action
Complex Images
For complex images like infographics, charts, or diagrams, provide a brief alt text summarizing the key takeaway, then offer the full data in a text alternative nearby (a data table, expandable section, or linked page).
Length Guidelines
Keep alt text under 125 characters when possible. Screen readers pause at the end of alt text, so excessively long descriptions create an awkward listening experience. If the image requires more than 125 characters to describe, use the longdesc attribute or an adjacent text description.
SEO Considerations
Include relevant keywords naturally — alt text is a ranking signal for image search. However, keyword stuffing in alt text is a poor practice that hurts both accessibility and SEO. Write for humans first; the SEO benefit follows from genuinely descriptive text.