How to Calculate size of AD (Active Directory Databse)

Before you can install Active Directory, you will also need to know the potential size of the Active Directory database. Active Directory space requirements per object are much smaller than you might think. The approximate sizes of objects and attributes in Active Directory are as follows:
• Security principal (User, Group, Computer) = 3,600 bytes
• Organizational unit (OU) = 1,100 bytes
• Security certificates mapped to a user = 1,500 bytes
• Object attributes = ^100 bytes
• Access Control Entry (ACE) = 70 bytes per ACE