To calculate the mat opening size: subtract the total border width from the frame's outer dimensions. For example, if your frame is 20" × 16" and you want 3" borders on all sides, the opening will be 14" wide (20 − 3 − 3) and 10" tall (16 − 3 − 3). Use our calculator above — simply switch to "Find Opening" mode, enter your frame size and desired borders, and the opening dimensions are computed instantly.

Bottom-weighted matting is a traditional framing technique where the bottom border is slightly wider than the top border — typically by ¼" to ⅜". This compensates for an optical illusion that makes the bottom border appear narrower when the framed piece hangs on a wall at eye level. The extra weight at the bottom creates visual balance and is considered a mark of professional framing. Our calculator includes a bottom-weight toggle to automatically apply this adjustment.

Standard mat border widths typically range from 2" to 4" for most frame sizes. For small frames (8"×10" or smaller), 1.5"–2.5" borders are common. For medium frames (11"×14" to 16"×20"), 2.5"–3.5" works well. For large frames (20"×24" and above), 3"–5" borders create a substantial, gallery-quality look. The border width should complement both the artwork and the frame — larger pieces generally benefit from wider mats.

Measure the visible area of your artwork that you want to show through the mat opening. For photos and prints, measure the image area precisely. For float-mounted artwork, measure the entire piece. Always use a reliable ruler or tape measure, and double-check your measurements. The mat opening should be slightly smaller than the artwork (by about ¼" on each side) to securely hold it in place behind the mat board.

The frame size refers to the outer dimensions of the entire frame. The mat opening (also called the window) is the cutout in the mat board through which the artwork is visible. The difference between the two determines the mat border width. For example, a 20"×16" frame with a 14"×10" mat opening has 3" mat borders on each side. The frame holds the glass, mat board, artwork, and backing board together as one assembled unit.

Yes — while equal borders are the most common choice, you can absolutely customize each border independently. This is useful for creative layouts, off-center compositions, or when working with oddly sized frames. Simply uncheck the "Link All Borders" option in our calculator and enter your desired width for each side. Asymmetrical matting can create striking visual effects when done intentionally.

Our calculator supports three measurement units: inches (in), centimeters (cm), and millimeters (mm). Inches are the standard for picture framing in the United States, while centimeters and millimeters are commonly used internationally. Switch between units at any time using the unit selector — all values update automatically while preserving the proportions of your design.

The calculations are mathematically precise based on the dimensions you enter. However, when cutting a real mat board, always account for the blade kerf (the thickness of the cut) and leave a tiny margin — typically 1/32" to 1/16" — for a proper fit. Professional framers often cut the opening slightly smaller than calculated and then fine-tune the fit. Use our calculator as your planning tool, then confirm all measurements physically before cutting.