On Tue, 30 Dec 2008 06:34:06 -0800 (PST), Seanie
wrote:

I have 7 rows of data in one column, the SUM of which adds up to Zero,
as expected, except it doesn't. In the SUM cell formatting to 13
decimal place shows as 0000etc13. I've even retyped the 7 rows of data
(max of 2 decimal places) and my result is still the same. I even
pasted-special values ROUND after that and same result

How can that be?


The reason is that Excel uses binary numbers with a finit number of
digits to store numbers. For decimal numbers that means they are often
rounded, i.e. the stored number is not exactly correct.
And when you do calculations, these small errors propagate and can
give results as you have experienced.

Here is a link to an article describing this in more detail
http://support.microsoft.com/kb/78113

Hope this helps / Lars-Åke