While the total service time includes all the time to service transactions within a given interval, service time is specifically related to a single arrival service. The unit of time should be in the numerator, and the unit of work should be in the denominator. Depending on your data source, the information may be provided as work over time. Make sure to switch it to time over work. If you forget to do this, any other calculation based on the service time (for example, utilization and response time) will likely be incorrect.

Besides the general utilization formula presented in the previous sections, the classic utilization formula is as follows:

It is important to understand the service time and arrival rate are independent and also have a direct and linear relationship with utilization. Theoretically, when the arrival rate increases, service time does not increase. What may increase, if the workload increases enough, is the queue time. More practically, if it takes 10 ms to process one user call when the system is lightly loaded, then it will continue to take 10 ms to process one user call when the system is heavily loaded. This is why response-time curves are more or less flat until queuing sets in. Remember that the users do not experience only service time, but the combination of service and queue time.