I need an advice on what is the industry to calculate the monthly required headcount of a single service at contact center. I'm using erlang C calculator on Excel and one of the inputs required for this formula is the per interval (1 hour) number of calls. But this formula does not define the period required for this number of calls at this interval. In other words, I can input the total weekly volume at this specific interval OR I can use the monthly volume at this specific interval. However, erlang C formula will two different answers. Using monthly volumes will result in lower number of man hours required compared to using the weekly volumes. Having said that is there an industry practice the defines what is the period of volume I should input for each interval?