How much size "Null" value takes in SQL Server

If the field is fixed width storing NULL takes the same space as any other value - the width of the field.

If the field is variable width the NULL value takes up no space.

In addition to the space required to store a null value there is also an overhead for having a nullable column. For each row one bit is used per nullable column to mark whether the value for that column is null or not. This is true whether the column is fixed or variable length.

The reason for the discrepancies that you have observed in information from other sources:

• The start of the first article is a bit misleading. The article is not talking about the cost of storing a NULL value, but the cost of having the ability to store a NULL (i.e the cost of making a column nullable). It's true that it costs something in storage space to make a column nullable, but once you have done that it takes less space to store a NULL than it takes to store a value (for variable width columns).

• The second link seems to be a question about Microsoft Access. I don't know the details of how Access stores NULLs but I wouldn't be surprised if it is different to SQL Server.

The following link claims that if the column is variable length, i.e. varchar then NULL takes 0 bytes (plus 1 byte is used to flag whether value is NULL or not):

• How does SQL Server really store NULL-s

The above link, as well as the below link, claim that for fixed length columns, i.e. char(10) or int, a value of NULL occupies the length of the column (plus 1 byte to flag whether it's NULL or not):

• Data Type Performance Tuning Tips for Microsoft SQL Server

Examples:

1. If you set a char(10) to NULL, it occupies 10 bytes (zeroed out)
2. An int takes 4 bytes (also zeroed out).
3. A varchar(1 million) set to NULL takes 0 bytes (+ 2 bytes)

Note: on a slight tangent, the storage size of varchar is the length of data entered + 2 bytes.

From this link:

Each row has a null bitmap for columns that allow nulls. If the row in that column is null then a bit in the bitmap is 1 else it's 0.

For variable size datatypes the acctual size is 0 bytes.

For fixed size datatype the acctual size is the default datatype size in bytes set to default value (0 for numbers, '' for chars).