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What a mat does, and why its size matters
A mat, also called a mount or passepartout, is the board with a window cut in it that sits between a print and the frame glass. It serves three jobs at once. It holds the print flat and away from the glass, so the print cannot stick to the glass over time. It gives the eye breathing room around the image, so the print looks finished rather than crammed into the frame. And it lets a small print fill a large frame gracefully, which is often cheaper and better looking than buying a custom frame for every print size. Getting the mat window and borders right is what separates a professional looking frame job from one that looks slightly off, even when nobody can say why.
The window is cut slightly smaller than the print, so the mat overlaps the print edge by a small amount, usually about a quarter of an inch on each side. That overlap hides the print’s cut edge and stops the print from slipping out of the window. The borders are the strips of mat board left around the window, and they come from simple subtraction: take the frame size, subtract the window size, and split what remains between the two opposite sides. A 16 by 20 inch frame holding an 11 by 14 inch print with a quarter inch overlap gives a window of 10.5 by 13.5 inches, side borders of 2.75 inches, and vertical borders totalling 6.5 inches, split between top and bottom.
How to use this picture frame mat calculator
Enter the frame width and height, the print width and height, the overlap per side, and any extra bottom weight you want. The tool shows the window size, the side borders, and the top and bottom borders. Bottom weighting means giving the bottom border a little extra width, often half an inch, and taking that amount from the top border’s share, so the print sits slightly high in the frame. This looks balanced because frames usually hang above eye level, and the eye reads the lower border as shorter than it is. With the example above and half an inch of bottom weight, the top border becomes 3 inches and the bottom becomes 3.5 inches, while the sides stay at 2.75 inches.
As a second example, a 20 by 16 inch landscape frame holding a 14 by 11 inch print with the same quarter inch overlap gives a window of 13.5 by 10.5 inches and even borders of 3.25 inches on the sides and 2.75 inches top and bottom. Even borders suit prints that hang at eye level or in a grid, where consistency matters more than optical correction. Very small prints in large frames get very wide borders, which can look elegant, but borders wider than about 4 inches on a small print sometimes look empty, so consider a smaller frame or a double mat, where a thin inner mat in an accent colour adds structure inside a wide outer mat.
Choosing borders that flatter the print
Border width changes how a print feels. Narrow borders, around 2 inches, feel modern and tight, and they suit large bold prints and contemporary rooms. Wide borders, around 3 to 4 inches, feel classic and calm, and they suit small detailed prints that need room to breathe. Very wide borders can also rescue a print with busy edges, because the calm mat gives the eye a resting place before the frame. Whatever width you choose, keep the two side borders equal and decide the top and bottom split deliberately, because unequal sides look like a mistake while a deliberate bottom weight looks like a choice.
Practical details matter as much as the numbers. Cut or order the window with a bevel, the angled edge that shows the board’s core, because a bevel catches light and looks far more finished than a straight cut. Use acid free mat board for any print you value, since ordinary board can yellow and damage prints over decades. Attach the print with archival hinges at the top only, so the print can expand and contract with humidity without buckling. Measure twice before cutting, because a window cut too large cannot be uncut, and use this calculator to confirm every number before the blade touches the board.
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