Matthew Pennel's Grid Calculator is a tool to calcuate a grid based on easy to use slider controls and forms.

[A]llows you to choose the font size you are going to use, and then select the number of columns, column width, and gutter width based on that base font size. As you fiddle with the dimensions, the total width is dynamically updated, so you can check that you haven’t gone over that magic 1024px!

If you’re not bothered about the column and gutter widths having an exact proportional relationship to the font size, you can edit the values directly (they are plain text input boxes). The total width will reflect changes as you type.