How do I calculate the number of concurrent users to use in a load test?

Running a load test requires that you specify how many concurrent users should be simulated during testing. In other words, how many simulated users will be active, loading things or interacting with your site/app at the same time. Unfortunately, when looking at Google Analytics for example, we only see how many visits a website has per day or per month. A site can have a million visits per month, but still only ever experience max 100 concurrent visitors.

To convert the "visits per X" metric from Google Analytics, or some other analytics system, into a "concurrent users" metric that you can use for load testing, you can use the following method.

First, find out two things:

1. You need the total number of visits for a short time period when your site/app is at peak traffic levels. This can easily be found via e.g. Google Analytics by seeing what the highest number of visits was for a single hour in the course of e.g. a month. Look at the day that has seen the highest number of visits, and drill down to see what hour of that day was the busiest and how many visits you had during that hour. Note this value down. I will call this value "peak_hourly_visits" in this text.
2. You need to know the average time a user spends interacting with your site/app. In Google Analytics this is called "Average session duration" and I will call it that in this text also, but sometimes it is called "Average time on site". If this value changes a lot for your site/app depending on which time period you look at you might want to use one of the larger values you find, to be on the safe side. We want all times in seconds, so if e.g. Google Analytics tells you "00:03:19" (3 minutes, 19 seconds) you should note down 199 as the average session duration.

When you have those two values you use this formula to calculate the number of concurrent users to use in your load test:

concurrent_users = (peak_hourly_visits * average_session_duration) / 3600

Provided that each simulated user (VU) in your load test behaves realistically (i.e. simulates a real user well), you will now be able to stress your site/app with the same kind of traffic that it normally only sees during peak traffic hours.