Sum entire column

Excel supports "full column" and "full row" references like this:

=SUM(A:A) // sum all of column A
=SUM(3:3) // sum all of row 3

You can see how this works yourself by typing "A:A", "3:3", etc. into the name box (left of the formula bar) and hitting return — Excel will select the entire column or row.

Be careful

Full column and row references are an easy way to reference data that may change in size, but you need to be sure that you aren't unintentionally including extra data. For example, if you use =SUM(A:A:) to sum all of column A, and column A also includes a date somewhere (anywhere), this date will be included in the sum.