Today we received an excellent question from a customer of ours:

We were wondering if you all have any information that says “X Unique visitors per day translates into Y simultaneous users at any given time.”

Essentially, we’re looking for a way to determine how many simultaneous users we should load test with if we know the sites normal daily traffic.

While every site is different, we recommend following this line of reasoning to help you find the answer. Suppose your site gets 100K unique visitors per day, with peak traffic in the mornings and afternoons. Assume that 40% of the traffic comes between 7AM and 11AM, 40% at 4PM to 9PM, and 20% at other times. This means during your peak hours (7AM to 11AM and 4PM to 9PM) you’ll get ~10% of your unique visitors per hour, or 10K uniques in our example.

Now that you know how many unique visitors you’ll get in an hour, you can start turning that in to concurrent users. To do that, it’s important to understand that a unique visitor is roughly equivalent of a transaction. So really you want to figure out how many users you need to reach 10K transactions in an hour.

Suppose your script (or scripts) take an average of 2 minutes to complete. That means a single user will execute 30 transactions in an hour. So to reach 10K transactions, you’d need 334 users (10K divided by 30). If you decide you want to create realistic scripts that include human think time, then the scripts will take that much longer and you’ll need that many more concurrent users. For example, if the script gets 5X think time added and now takes 10 minutes to run, then you’ll need 1,667 users (10K divided by 6).

Of course, this calculation will only get you the load on a typical day (assuming a single hour sees 10% of traffic). Your traffic patterns may vary, or you may want to prepare for a larger surge. For example, if you want to test what happens when 60% of the daily traffic visits in an hour, then you’d need 2,000 users (60K divided by 30).

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