Chargeable number of active contacts is computed using the rolling average number of active contacts for the month. In another words, this is the average number of active contacts in your account throughout the month. To estimate monthly usage, you can go to active contacts page and check current number of active contacts for your account.
After subscribing, you can view chargeable usage for number of active contacts via usages page. 2 key reasons why rolling average computation is used.
Instead of using the number of the active contacts at the end of the month, it is generally more accurate to compute usage based on the average numbers throughout the month.
Another key reason for this approach is to prevent users from abusing our system and avoid paying for their actual usage. For example, bulk updating contacts to inactive at the end of month to avoid paying for active contacts.