How do you calculate absence rate?

Two schools of thought on this:

No. 1: % of workforce absent at any given time

i.e. (Number of absent employees/total workforce headcount) x 100.

No. 2: Periodic Absence rate: (Working time lost/Working time available) x 100

i.e. If you have 10 employees who work a standard 8 hours a day, 9 to 5, 40 hour week and 3 of them are absent over the course of a week, then the absence rate for that week would be 6%, as in (3 [Working days lost]/50 [Working days available, 10 x 5]) x 100 = 6%. You could break it down further into hours also.

Answer done by Jimbob.