To find out the space used by a table we can use sp_spaceused procedure. Most of the times sp_spaceused will give correct information. Why do I say most of the time? Well consider this example. I just inserted a large amount of data in Orders table in Northwind database and ran sp_spaceused.

EXEC sp_spaceused 'Orders'

which returns this result

But if I run sp_spaceused with true for its second parameter which is updateusage

EXEC sp_spaceused 'Orders', TRUE

I get the following result

Notice the difference in reserved space and index size. What has happened here is that by passing in true for updateusage SQL Server automatically runs DBCC UPDATEUSAGE which corrects any inaccuracies in catalog views. So it is a good idea to run sp_spaceused by supplying true for updateusage parameter if you absolutely must get accurate information. And especially after inserting large amounts of data.