adding 3 columns to a table - how to figure out space required

DB: Oracle 11.2.0.3

RAC / ASM

OS = Linux

I need to figure out how many additional space will I need for 8 of my huge fact/transaction tables.

My Steps for calculation:

I end up loading 1 million rows with these 3 columns  in a separate table and gathered stats. so, I came up with the following:

Per the DBA_TABLES, this table uses 2,858 blocks.

1. 2858(used blocks) * 16384(block size) =  46,825,472 (bytes) / 1000000 rows in table = 46.83 bytes per row.

    2. if I look at the dba_tables, after gathering stats, it will show Average Row Length = 36

Question: what’s the difference between the Average Row Length =36 from the DBA_Tables  vs the 46.83 bytes per row from my calculation?  If it’s row overhead, wouldn’t we want to leave that out since the row overhead is already there in the existing rows?