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Data URI images

Data URIs encode binary files (images, icons) as base64 strings inside URLs: <code>data:image/png;base64,...</code>. Saves an HTTP request but inflates file size 33% and loses browser caching. Best for tiny critical-path icons (under 5KB); avoid for larger images.

Updated May 2026 · 4 min read
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Definition

Data URIs encode binary files (images, icons) as base64 strings inside URLs: <code>data:image/png;base64,...</code>. Saves an HTTP request but inflates file size 33% and loses browser caching. Best for tiny critical-path icons (under 5KB); avoid for larger images.

What it means

Format: <code>data:[mime/type];base64,[base64-encoded-data]</code>. Browsers decode and render inline. <strong>Pros</strong>: no extra HTTP request (saves round-trip latency, useful for above-the-fold rendering); image is bundled with parent file (no broken-link risk). <strong>Cons</strong>: 33% size inflation (base64 overhead), no browser caching (every page reload re-fetches the parent file with embedded image), bloats CSS/HTML files, hard to debug. <strong>Modern alternatives</strong>: SVG inline (XML-based, often smaller than PNG), CSS sprites (multiple icons in one PNG, single HTTP request), HTTP/2 multiplexing (multiple HTTP requests can be done in parallel without overhead, partially solving the request-per-asset problem).

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Why it matters

Performance optimization. For very small icons (1-3KB), data URIs reduce time-to-first-render by skipping the additional HTTP request. For larger images (10KB+), data URIs hurt: cumulative file-size inflation slows initial page load, lost caching slows subsequent loads. Modern web-perf wisdom: data URIs only for tiny critical-path icons; use img tags + caching for larger images; consider SVG for icons that scale.

Example

Tiny SVG icon (300 bytes original) as data URI: <code>data:image/svg+xml;base64,PHN2ZyB4bWxucz0...</code> &mdash; saves an HTTP request, manageable inline size. 50KB photo as data URI: 67KB inflated, blocks initial parse of CSS/HTML, useless for caching. Always use img tag + browser cache.

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Frequently asked questions

Should I data-URI my logo?

Logos are often loaded on every page. Browser caching works in your favor; img tag + caching is more efficient than re-loading the data URI on every page. Exception: above-the-fold tiny logo (under 1KB) might benefit.

Are data URIs good for SVG?

SVG inline (without data URI encoding) is more efficient: <code>&lt;svg&gt;...&lt;/svg&gt;</code> in HTML. If you must inline via CSS, data URI works but URL-encoded SVG (without base64) is smaller for SVG specifically.

Do data URIs affect SEO?

Search engines can index inline images via data URI. The performance impact (page-load speed) is more important to SEO than the technique itself. Slow pages hurt rankings; data URIs that slow pages hurt rankings.

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