What is transfer size and why should you measure it? For that matter, how do you measure your website’s transfer size?
Transfer size refers to the amount of data that needs to be transmitted from a web server to a browser (or a robot) when loading a web page.
Naturally, the smaller you can make the transfer size, the better. Reducing transfer size will help to improve nearly every other speed metric you are trying to optimize.
One way to measure this is by using Chrome’s Dev Tools. Before opening a webpage, right click in Chrome and click “Inspect”. Once the Dev Tools area opens, click “Network” in the top bar. (Note that this may be hidden from view and you’ll find Network by clicking “>>”.)
After opening up Network, load the webpage in Chrome. You will see all of the resources needed for your page to load listed in the Network area. Once everything is loaded, at the bottom of the Network panel you will see the transferred size.
In the screenshot, that is 1.0MB. That means, there are 1MB worth of files (such as HTML, CSS, JavaScript, images, videos, etc.) required to be downloaded from the server in order to render my website’s homepage properly in a browser or for a robot.
The average transfer size is around 2.9MB on mobile devices as of November 2025 according to HTTPArchive. That means a 1MB transfer size is better than the average transfer size. There are probably some ways I could further optimize my website though to make the transfer size even smaller.
The other number listed is the uncompressed size of all the resources that have been loaded. When files like HTML, CSS, JavaScript, images, or other assets are sent over the internet, they are often compressed using techniques such as Gzip or Brotli compression. This compression reduces the file sizes, making them faster to transfer over the network but the browser has to uncompress those. In the screenshot, that is 1.6MB.
As you optimize your website’s speed, pay attention to these two numbers. You should work to reduce the transfer size and the uncompressed resources file size by making the files you need to load that much smaller.