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, and all are defined as NOT NULL.
Formula 


4 
(Overhead) 

+ 
Sum of bytes in all fixedlength columns 

= Data row size 
Use this formula if the table contains any variablelength columns or columns that allow null values.
The table in the example contains variablelength columns, so the computations are shown in the right column.
Formula 
Example 


4 
(Overhead) 
4 

+ 
Sum of bytes in all fixedlength columns 
+ 
100 

+ 
Sum of bytes in all variablelength columns 
+ 
50 

= Subtotal 
154 




+ 

(Subtotal / 256) + 1 (Overhead) 
1 

+ 

Number of variablelength columns + 1 
3 

+ 
2 
(Overhead) 
2 

= Data row size 
160 