Calculating total "on row" bytes for each row ... the easy way We want to calculate the total "on row" storage bytes for each row in the table.  As we understand it, we must add up the DATALENGTH() of each column while also accounting for NULLs and things like VARCHAR(MAX) which only have a 24-byte pointer "on row".  We are aware there is also some overhead for each row which is not accounted for in the query below.

SELECT   ROW_ID,

         CASE
              WHEN COLUMNPROPERTY(OBJECT_ID('EXAMPLE_TABLE'),'COL1','PRECISION') = -1 THEN 24
              ELSE ISNULL(DATALENGTH(COL1), 1)
         END
          +
         CASE
              WHEN COLUMNPROPERTY(OBJECT_ID('EXAMPLE_TABLE'),'COL2','PRECISION') = -1 THEN 24
              ELSE ISNULL(DATALENGTH(COL2), 1)
         END
          +
         CASE
              WHEN COLUMNPROPERTY(OBJECT_ID('EXAMPLE_TABLE'),'COL3','PRECISION') = -1 THEN 24
              ELSE ISNULL(DATALENGTH(COL3), 1)
         END
          +
         ...
         ...
         AS ROW_SIZE

FROM     EXAMPLE_TABLE

ORDER BY ROW_SIZE DESC
;


What a beast!  And it's only an approximation.

Then we discovered

DBCC SHOWCONTIG ('EXAMPLE_TABLE') WITH TABLERESULTS


which returns MaximumRecordSize.  This reveals that there is already an algorithm buried somewhere within SQL Server which is capable of calculating the exact size of a row.

How can we access that algorithm directly?
How can we access that algorithm directly?


In so far as the answer to this question, specifically, you can't access it directly. There is nothing where you can say SELECT GetMeMaxRowSize(MyTable, MyPartition, MyIndex).

However, as LowlyDBA has pointed out, you can use sys.dm_db_index_physical_stats to give you more information than ShowContig. The quite interesting thing is that if you run DBCC SHOWCONTIG(), capturing deprecated information, you should see a message to use the aforementioned DMV in place of the ShowContig command.