Rows that store variablelength data require more overhead than rows that contain only fixedlength data, so there are two separate formulas for computing the size of a data row.
Use this formula if the table contains only fixedlength columns defined as NOT NULL:
6 
(Overhead) 

+ 
Sum of bytes in all fixedlength columns 

Data row size 
Dataonly locked tables must allow room for each row to store a 6byte forwarded row ID. If a dataonlylocked table has rows shorter than 10 bytes, each row is padded to 10 bytes when it is inserted. This affects only data pages, and not indexes, and does not affect allpageslocked tables.
Use this formula if the table contains variablelength columns or columns that allow null values:
Formula 
Example 


8 
(Overhead) 
8 

+ 
Sum of bytes in all fixedlength columns 
+ 
100 

+ 
Sum of bytes in all variablelength columns 
+ 
50 

+ 
Number of variablelength columns * 2 
+ 
4 

Data row size 
162 