I would generally not blog about this kind of topic, except that my customer recently talked to me about it. According to his tests, he seemed to find that the total size limitation across all databases was 10GB. That did not sound correct to me, because it is quite clearly documented that the 10GB limit is per database.

I quickly tested and found out that the documentation is correct. It is only when an individual database is attempted to be created with a data file beyond the size limit of 10GB, that you get the error:

Msg 1827, Level 16, State 4, Server SOMECOMPUTERSQLEXPRESS, Line 1
CREATE DATABASE or ALTER DATABASE failed because the resulting cumulative database size would exceed your licensed limit of 10240 MB per database.

To summarize:

1. 10GB is the per-database, data-file size limit
2. If you have more than one data file in your SQL Express database, the size limit is applied to the total size across all such data files
3. Log files are not subject to this limit