I checked SQL Server database size by sp_spaceused. Even though there seems to be enough unallocated space, some database issued the file expanding error.

-- sp_spaceused result

use db_01;
EXEC sp_spaceused; 

database_name    database_size      unallocated space  
--------------- ------------------ ------------------ 
DM_01              868463.31 MB       55504.24 MB        
reserved      data          index_size    unused        
------------- ------------- ------------- ------------- 
760088904 KB  435604016 KB  321346232 KB  3138656 KB   

EXEC sp_spaceused N'dbo.target_talbe'; 

name                rows        reserved        data        index_size      unused
-------------------------------------------------------------------------------------------
dbo.target_talbe    744509531  386101616 KB 170471256 KB    215097944 KB    532416 KB

Error Message

2021-12-27 00:38:49.04 spid52 Could not allocate a new page for database 'DM01' because of insufficient disk space in filegroup 'PRIMARY'. Create the necessary space by dropping objects in the filegroup, adding additional files to the filegroup, or setting autogrowth on for existing files in the filegroup.

This error was cleared when I added the new data file (ndf) or expanded the max size of datafile.

I'm wondering why DB unallocated space couldn't be reused. Target table had 519MB as unused space and new data was about 455MB, I know I should consider the data size by page. But 'Unallocated space' in DB can not be reusable space for other table's new data?

If so, is there any better idea to check the exact table size and avoid any file expanding error?

Any advice would be appreciated. Thank you.