Required amount of storage for vault stores

When an item is archived, it is first compressed and then metadata is added to it. As a general rule, the item is compressed to half its original size and the metadata comprises approximately 5 KB. When an item is shared, only the metadata is added.

The following general rules can be used for estimating the amount of storage needed:

Take the total size of items to be archived and halve it.
For email items, divide by the average number of recipients.
Add 5 KB multiplied by the total number of items.

The compression ratio may vary considerably. Office documents tend to compress well. Other document types, such as ZIP files or JPG files, are already compressed and cannot be compressed further. For this reason, you should always overestimate the amount of storage needed.

The above general rule applies to most types of archiving, but care needs to be taken with File System Archiving (FSA). For example, if compressed image or map files are archived, then there is no space saving.

For email archiving, growth in the number of mailboxes and the number and size of messages must also be taken into consideration. Because of these extra factors, a more conservative method of estimating storage is to assume that space used by archiving will equal the space used by Exchange Server or Domino Server in storing items.