Calculation of data row size is slightly different depending on whether the row stores any variable-length columns. Use the first formula if all the columns are fixed-length and defined as NOT NULL. Use the second formula if the row contains variable-length columns or if the columns are defined as NULL.
For fixed-length columns, calculate:
2 + (Sum of bytes in all fixed-length columns) = Data Row Size
For fixed- and variable-length columns, calculate:
2 + (Sum of bytes in all fixed-length columns) + (Sum of bytes in all variable- length columns) = Subtotal
Subtotal ((Subtotal / 256) + 1) + (Number of variable-length columns + 1) + 2 = Data Row Size
The values 1 and 2 in the previous equations are overhead that SQL Server uses for internal storage.