What are pro-rata billing charges in Vtiger?

If you add new users between your billing cycles, you will be charged pro-rata for that month, which will be reflected in the next month’s invoice.

Let us consider an example to understand the concept
Your billing date is on the 5th of every month. If you add a new user on May 7th, you will be charged 1.96 in the next month's billing. This will be reflected in the invoice generated on June 5th.
Wondering what is 1.96 and how it is calculated? Read on...
May 5th to June 4th is 31 days. A user is added on May 7th, which is the 29th day. Therefore, the user is billed for 29 days on a pro-rata basis along with the charge of the upcoming month, which makes 0.96+1 = 1.96

Here is the formula for calculation pro-rata charge - No. of days the user is active / No. of days in the month

To know more about Billing in Vtiger, click here.