Why is 30 the default length for VARCHAR when using CAST?

In SQL server 2005 this query

select len(cast('the quick brown fox jumped over the lazy dog' as varchar))

returns 30 as length while the supplied string has more characters. This seems to be the default. Why 30, and not 32 or any other power of 2?

[EDIT] I am aware that I should always specifiy the length when casting to varchar but this was a quick let's-check-something query. Questions remains, why 30?

Solution 1:

Why don't you specify the varchar length? ie:

SELECT CAST('the quick brown fox jumped over the lazy dog' AS VARCHAR(45))

As far as why 30, that's the default length in SQL Server for that type.

From char and varchar (Transact-SQL):

When n is not specified in a data definition or variable declaration statement, the default length is 1. When n is not specified when using the CAST and CONVERT functions, the default length is 30.

Solution 2:

On your question as to why 30 and not 32 or any other power of 2, the storage size is n + 2 bytes for varchar(n), which makes the byte storage size 32 for a string of length 30. Might be that this is what they looked at?

Then just a point of clarity on some of the comments: The default length for an unspecified length varchar field is n=1. The default string length that CAST or CONVERT returns for a conversion of this data type is 30.

Very cool question!