Optimizing Images with Base64: When and How to Use

Optimizing Images with Base64: When and How to Use

Optimizing images is a critical step in boosting web performance. Inlining images by converting them to Base64 and embedding them directly into HTML, CSS, or JavaScript files can eliminate extra network requests for small images, speeding up page load times. However, this technique can have the opposite effect on large images, hurting performance. In this article, we'll explore the pros and cons of using Base64, practical thresholds, and its impact on caching.

Rules of Thumb and Comparisons

The main advantage of converting an image to Base64 is that it eliminates an HTTP request, which can significantly reduce page load time for small assets like icons or background images in CSS. However, remember that Base64 encoding increases the file size by approximately 33% compared to its binary equivalent. This can bloat your main file (HTML or CSS). Therefore, knowing when to use this method is key.

• Use Base64 for icons and small assets under approximately **2 KB**. This threshold is a good balance between the network latency saved and the file size increase.
• For anything larger, especially **raster formats** like JPEG and PNG, serving the image as a separate external file is almost always a better option.
• Modern protocols like **HTTP/2** and **HTTP/3** are highly efficient at handling multiple small requests, which further reduces the performance benefit of inlining images.

Caching and Implementation

Inlined Base64 images are cached along with the main file (HTML or CSS). This means that every time the main file changes, the image has to be re-downloaded. External images, on the other hand, are cached independently, which prevents them from being re-downloaded when the main file changes. Therefore, it might be better to avoid inlining assets that change frequently. To experiment, you can use the Image → Base64 encoder tool to compare the payload sizes of different images and make an informed decision.