Calculating DB backup size

Question

Is there a way to know the size of a database full backup file before we actually do the backup? I did some analyses and figured sp_spaceused could give a close figure but thats not all, there seem(s) to be one/more factors other than the used space that go into determining the size?

Has anyone been down this path before? Any ideas?

Answer 1

sp_spaceused should be pretty close if you look at the reserved space.

Estimating the Size of your Database Backups

Paul Randal blogged recently about how to know how large your TLOG backup might be:

How much data will the next log backup include

A combination of the two values, seems to be really close to the actual backup size in some brief testing.

Answer 2

It's pretty much the size of the mdf and ldf files.