According to this https://sites.google.com/a/webpagetest.org/docs/using-webpagetest/metrics/speed-index "The Speed Index is the average time at which visible parts of the page are displayed. It is expressed in milliseconds and dependent on size of the view port." Reading the explanation, I doubted that.

Essentially you're making an integral of the % visually incomplete. That means taking the area under the curve % v Time. Which to me suggest the metric has something like units of %/s.

Any thoughts on this?


Learn something new today..! if you will read the complete article of Speed Index, it's calculated in the millisecond(ms). if you will observe the area plot there, you have two fields Visibility Complete(which don't have any unit) w.r.t Time(ms) based on the formula(integration) & its mention in an article as well.

The Speed Index is the "area above the curve" calculated in ms

enter image description here

Speed Index will be directly proportional to time after integration when you will integrate for any visually complete equation(based on the plot) for the limit from 0 to end where "end"(Time)field after integration, Speed index will be proportional to time.


Speed index has no unit, it is just a number, or as description says:

value calculated from the area above the graph

explained in the page you linked: https://sites.google.com/a/webpagetest.org/docs/using-webpagetest/metrics/speed-index

You can see there are 2 graphics - one explains low speed index - which is good, graph goes up to approx 90%, then after some time 100% is reached. In such case the area above the graph will be small, so the index will be small. The other one shows big speed index value, where area above graph is bigger. The area is calculated using integral.

  • I know how to calculate the speed index. My point is that from their example, how can they then say that the unit is milliseconds? I mean, you're integrating a % over time. If I integrate velocity (m/s)over time(s) I will get the distance(m) as my result. When you report a speed index do you use ms as the unit? Do you use no units? Do you use %/ms ? If I understand you correctly @moro you say they are wrong and that SpeedIndex has no units. Jun 19 '19 at 13:47
  • I went through the article again - I cannot find the place where they assign units to the index. Both graphics have index values displayed without any units.
    – Moro
    Jun 19 '19 at 19:27
  • I quoted them using it. Jun 19 '19 at 19:41

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