is a free calculator created by math-phobic photographer Ken Schuster.
MatboardCalc will give you the dimensions of your mat, board and frame, as well as where to cut the "window" opening in your mat board. Try different combinations of image dimensions, border sizes, mat window openings and board / frame sizes and MatboardCalc will update the results automatically.
It works best with Microsoft Excel (Windows) and LibreOffice Calc (free for Mac, Windows and Linux), and may work with similar software such as Numbers (Mac).
You can enter measurements in inches with fractions or decimals, or both, or use metric measurement.
It's only about 26 kb and downloads so quickly you might not realize it happened.