Serving desktop-sized images to mobile devices can use 2–4x more data than needed. Follow the guidelines on this page to improve the user experience by serving different image sizes to different devices.
Responsive images and Core Web Vitals
When you serve responsive images, your page evaluates the display capabilities of the user's device and choosing one of a set of image candidates that are optimal for display based on those criteria. This saves data for your users, mainly by serving smaller images to devices with smaller screens.
The effects of faster image loading can also extend to your page's Largest Contentful Paint (LCP). For example, if your page's LCP element is an image, serving it responsively can reduce its resource load duration.
Lower resource load durations lower the load time for an LCP image, improving your page's LCP score. A lower LCP means users will see your site as loading faster, and especially as loading its most important content (the LCP element) faster. Serving responsive images can also reduce bandwidth contention for other resources on the page, which can improve how fast your page loads in general.
Resize images
Two of the most popular image resizing tools are the sharp npm package and the ImageMagick CLI tool.
The sharp package is a good choice for automating image resizing (for example, generating multiple sizes of thumbnails for all the videos on your website). It is fast and integrates well with build scripts and tools. ImageMagick is more convenient for one-time image resizing because it runs entirely from the command line.
sharp
To use sharp as a Node script, save this code as a separate script in your project, then run it to convert your images:
const sharp = require('sharp');
const fs = require('fs');
const directory = './images';
fs.readdirSync(directory).forEach(file => {
sharp(`${directory}/${file}`)
.resize(200, 100) // width, height
.toFile(`${directory}/${file}-small.jpg`);
});
ImageMagick
To resize an image to 33% of its original size, run the following command in your terminal:
convert -resize 33% flower.jpg flower-small.jpg
To resize an image to fit within a 300px wide by 200px high space, run the following command:
# macOS/Linux
convert flower.jpg -resize 300x200 flower-small.jpg
# Windows
magick convert flower.jpg -resize 300x200 flower-small.jpg
How many image versions should you create?
There is no single "correct" answer to this question. However, it's common to serve 3-5 different sizes of an image. Serving more image sizes is better for performance, but takes up more space on your servers and requires you to write slightly more HTML.
Other options
Image services like Thumbor (open-source) and Cloudinary are also worth checking out. Both are straightforward ways to create responsive images that also provide image manipulation on demand. To set up Thumbor, install it on your server. Cloudinary takes care of the details for you and requires no server setup.
Serve multiple image versions
When you specify multiple image versions, the browser chooses the best one to use:
| Before | After |
|---|---|
| <img src="flower-large.jpg"> | <img src="flower-large.jpg" srcset="flower-small.jpg 480w, flower-large.jpg 1080w" sizes="50vw"> |
The <img> tag's
src,
srcset,
and sizes
attributes all interact to achieve this end result.
The "src" attribute
The src attribute makes this code work for browsers that don't
support the srcset and sizes
attributes. Those browsers fall back to loading the resource specified in the
src attribute.
The "srcset" attribute
The srcset attribute is a comma-separated list of image filenames and their
width or density descriptors.
This example uses
width descriptors,
which tell the browser how wide an image is so it doesn't have to download the
image to find out. 480w is a width descriptor tells the browser that
flower-small.jpg is 480px wide. 1080w is a width descriptor tells the
browser that flower-large.jpg is 1080px wide.
Extra Credit: You don't need to know about density descriptors to serve different image sizes. However, if you're curious about how density descriptors work, check out the Resolution Switching code lab. Density descriptors are used to serve different images based on the device's pixel density.
The "sizes" attribute
The sizes attribute tells the browser how wide the image will be when it's displayed, though it doesn't affect the image's display size, so you still need CSS for that.
The browser uses this information, along with what it knows about the user's device (including its dimensions and pixel density), to determine which image to load.
If a browser doesn't recognize the "sizes" attribute, it falls back to
loading the image specified by the "src" attribute. (sizes and srcset
were introduced at the same time, so every browser supports either both
attributes or neither.)
Extra Credit: If you want to be fancy, you can also use the sizes attribute to specify multiple slot sizes. This accommodates websites that use different layouts for different viewport sizes. Check out this multiple slot code sample to learn how to do this.
(Even more) Extra credit
In addition to all the extra credit already listed (images are complex!), you can also use these same concepts for art direction. Art direction is the practice of serving completely different-looking images (instead of different versions of the same image) to different viewports. You can learn more in the Art Direction code lab.
Verify
After you implement responsive images, you can use Lighthouse to make sure you didn't miss any images. Run the Lighthouse Performance Audit (Lighthouse > Options > Performance) and look for the results of the Properly size images audit. These results list the images you still need to resize.