Art and Craft Supplies
Mat borders and window.
Tell it the artwork and the frame, and get the opening to cut in the mat and how wide each border will be. Or pick the border you like and get the frame size.
Window = artwork size - 2 x overlap. An 8 x 10 in print under a 16 x 20 in mat with 1/4 in overlap gets a 7.5 x 9.5 in window and 4.25 in side borders.
Artwork and frame
Mat dimensions
Fill the form and press Calculate.
Worked example
An 8 x 10 in print going in a 16 x 20 in frame, with a 1/4 in overlap. The window is 8 - 0.5 = 7.5 in wide and 10 - 0.5 = 9.5 in high. Side borders are (16 - 7.5) / 2 = 4.25 in, and the top and bottom are (20 - 9.5) / 2 = 5.25 in. Adding 1/2 in of bottom weighting gives a 5.00 in top border and a 5.50 in bottom border. In the other direction, 2.5 in borders around the same window need a mat 12.5 x 14.5 in.
How it works
A mat window is cut a little smaller than the artwork so the mat edge overlaps the paper. The window size is the artwork size minus twice the overlap. The border on each side is then half of the space left between the window and the outside of the mat (which is the frame opening size).
Bottom weighting moves a small amount of border from the top to the bottom so a picture hung at eye level looks centred. The overlap and the bottom amount are common framing conventions and personal taste, not a published standard, so adjust them for your piece and your framer's advice.
Sources and basis (opened 2026-10-08): 1 in = 2.54 cm from Inch (Wikipedia). The overlap and bottom-weight defaults are common framing practice, not from a published standard.
FAQ
What size mat do I need for an 8x10 picture in a 16x20 frame?
A 16 x 20 in mat with a 7.5 x 9.5 in window (1/4 in overlap). The borders are 4.25 in at the sides and 5.25 in at the top and bottom.
How much should the mat overlap the artwork?
About 1/4 in (6 mm) per side is a common choice. It is a convention rather than a standard, and a narrower overlap shows more of the art.
What is a bottom-weighted mat?
A mat whose bottom border is a little wider than the top. With 1/2 in of weighting the example becomes 5.00 in on top and 5.50 in on the bottom.
How do I find the frame size from the border I want?
Choose "the border width I want". 2.5 in borders around an 8 x 10 in print with 1/4 in overlap need a 12.5 x 14.5 in mat and frame.
Does the mat thickness change the size?
No. The mat sits inside the frame opening, so the outer size is the frame size. Check the frame's rabbet depth for the glass, mat, art and backing stack.
Disclaimer
Results are arithmetic from the numbers you enter. Framing conventions vary. For valuable or archival pieces, ask a professional framer.