The usual way folks determine the number of employees to hire (or the number of FTEs) is to make the sum of all workloads in hours over one week and divide by the number of hours a full-time person would do. For example, if our store needs a total of 200 hours of clerks per week, this would give us an equivalent of 5 full time employees to cover the work each week. But that’s not counting on absences.
Since the employees are human, they will go on vacation, they will be sick, and they’ll want to be with their families on any statutory holiday. So you need to add the absences in order know the right number of FTEs to hire or at least measure against. Typically, you multiply by an absenteeism rate that is usually known. If you don’t have one, you can count how many days off an employee gets over a year and get a ratio. In our example, if we have 2 weeks of vacation, 2 weeks of sick leave, 2 weeks of statutory holidays, you get 6 weeks out of 52 that are not worked by each employee. That equates to roughly 12% which you need to multiply to the 5 FTEs we had earlier. This would mean that we need to hire 5.6 FTEs. So right off the bat, you could say I need 5 full-time employees and 1 part-time at 24 hours a week (0.6 multiplied by 40 hours).
But your workload has a certain weekly and daily profile. If you are managing a store, chances are that you will have your load of customers on Friday evenings and weekends. So if most of your workload needs to be performed then, you can’t hire full time folks. A 40-hour employee will need to work at least 4 days a week and therefore will not be in sync with the workload. So it is insufficient to simply determine what to hire based only on the math of a sum of hours. The workload profile has to be taken into account.
The ideal way to take all that in is to create a schedule using fictitious names. By completing a schedule, you can measure and analyze the results before even hiring and also compare between different types of positions (part-time vs. full-time, change of minimum rest constraints, etc). So next time a new department opens or a new store is built, have a planner complete a fictitious schedule so that you have a better sense of what will be needed at the hiring level.