Updated in August, 2021, MatboardCalc is a free calculator created by math-phobic photographer Ken Schuster.
MatboardCalc gives you the dimensions of your mat, board and frame, as well as where to cut the "window" opening in your mat. Try different combinations of image dimensions, border sizes, mat window openings and board / frame sizes, and MatboardCalc will update the results automatically.
It works beautifully with Microsoft Excel for Mac and Windows. The Apple Numbers and LibreOffice-Calc versions work well, but kinda clunky in comparison.
You can enter measurements in inches with fractions or decimals, or both, or metric.
The files are so small, and download so quickly, you might not realize it happened. Check your downloads folder.
Please feel free to pass around MatboardCalc to everyone on the planet, but do not modify it, and never charge for it.