That’s a good question. Nowadays tools like Google Analytics can offer real time monitoring which will give you the best insight into your peak levels of traffic and how that traffic may spike for a few minutes. If you have this information it already provides a good estimate for many peak users you should test for in LoadStorm.

Most commonly though Google Analytics will have data on the number of visits per hour to your site as well as the average duration of those visits. This makes it a bit harder to estimate what volume of users you should test for. With this information we can estimate the average concurrent visitors that were on your site using the following equation:

U = V / (60/D)

Where:
U is the number of load test virtual users (that’s what we are trying to figure out)
V is the average number of visitors per hour
D is the average duration of a visitor
60 is the number of minutes in an hour

Let’s state this formula again in English like a math word problem:

Load Test Virtual Users is equal to the Average Visitors per Hour divided by the User Turnover Rate per Hour.

Ideally you should test for an amount that is a bit greater than this estimate just to make sure you have a cushion for unusual spikes in traffic. For a more lengthy explanation please visit our blog post about virtual user calculations.