Speed Index
What Speed Index Measures
Speed Index is a page speed metric that shows how quickly the visible part of a web page is filled with content while it loads. It is measured in milliseconds, and a lower number means the screen was populated sooner.
Most load metrics point to a single moment, such as the first pixel on the screen or the point when the load event fires. Speed Index looks at the whole visible area over time instead, which is a different question from the one asked in the page load time article. A page that shows its header at one second, its article text at two seconds and its images at four seconds scores differently from a page that shows everything at once at four seconds, even though both finish at the same time.
The metric originated in WebPageTest, a website testing service, and is also reported by Lighthouse, the audit tool built into Chrome DevTools. Both record how the viewport changes during a load, so the result reflects what a visitor sees rather than what the server sent.
How the Speed Index Score Is Calculated
The test first measures visual completeness at each point in time. This is the share of the viewport that is painted, from 0 percent for a blank screen to 100 percent when everything visible has appeared. Testing tools estimate it from screenshots or video frames taken during the load.
Speed Index is then the area above that completeness curve. Every moment in which part of the screen is still blank adds to the score, and the emptier the screen, the more it adds. Once the screen is almost full, the remaining time adds little.
Consider a simple example. The screen is blank for the first second. From one to two seconds, half of the viewport is painted. At two seconds, the page is complete. The blank first second adds 1 second. The half-painted second adds another 0.5 seconds, because half of the screen is still missing. The total is 1.5 seconds, which is a Speed Index of 1,500. Real calculations use many small time slices, but the principle is the same.
Lighthouse also uses the Speed Index value as one part of its overall performance score. How that score is weighted is covered in the Lighthouse performance score article.
Frequently asked questions
Is a lower Speed Index always better?
Generally, yes. A lower Speed Index means the visible part of the page was filled sooner. However, the number alone does not prove that a page is useful. A page can fill the screen quickly with placeholders, banners or cookie notices while the real content is still loading. Read Speed Index together with other measurements, such as First Contentful Paint.
Why does my Speed Index change from one test to the next?
Speed Index changes because each load depends on conditions that vary between runs. These include network speed, server response time, the device and browser used and the timing of third-party requests. Small differences can shift when content appears on screen. Running a test several times and comparing the median result gives a more reliable picture than any single run.