Problem: Often we would like to check how much size backup file will take on the disk? You can follow below simple steps to estimate the backup file size?

Solution:
Execute the SP_SPACEUSED procedure to get the database size statistics.
USE [sqldbpool]
GO
EXEC sp_spaceused @updateusage = ‘true’

Now you have to subtract the ununsed pages from reserved pages and divided by 1024 to get the output in MB.

In our scenario we have 2592 (KB) – 96 (KB) = 2496/1024 = 2.4374(MB) will be the backup size.