Measure your piece of artwork or photograph. Allow for one-eighth inch of the print, on all sides, to be covered by the border.
Place your print inside several different mats, to determine which border size is the most visually pleasing to you. While there’s no rule, the framers at Metroframe recommend at least a 2-inch border for smaller prints and 2.5 to 5 inches for prints larger than 16 by 20 inches.
Calculate the sides and top of the border to be equal dimensions, such as 2 inches. Make the bottom border one-half to 1 inch larger than the other three sides. When all four sides are the same, the bottom border can actually appear to be smaller, but this trick can help make the borders look even.