How to Choose the Right Mat for a 20 X 20 Frame?

Choose the mat by measuring your artwork first, and use a 1/8- to 1/4-inch overlap on all four sides.

For the full breakdown, see our best 20 x 20 Frame guide.

Picking a mat for a 20 x 20 frame sounds easy until the art arrives and slips through the opening. The process is straightforward once you measure the artwork and the frame’s inner lip, not the label on the box. Here’s the order that works.

Measure the Artwork and Frame First

Start with the frame. The critical number is the inner rabbet size, where the mat and glass actually sit. A frame labeled 20 x 20 often measures slightly larger inside the rabbet to allow clearance. Logan Graphic, a framing supplier, notes that a nominal 8 x 10 frame may actually measure about 8 1/8 x 10 1/8 inside the rabbet.

Next, measure the artwork. Write down both dimensions. These two numbers drive every decision below.

Calculate the Mat Opening Size

The mat opening should be slightly smaller than the artwork on every side. A 1/8-inch overlap per edge is the minimum to hold the art securely; 1/4-inch is a safer, more common choice. This lip hides rough edges and keeps the print from falling through.

The math for a square frame works like this:

(Artwork size + 2 x overlap) gives the opening size. For art that is 19.5 inches square with a 1/4-inch overlap, you would cut the opening at 19 inches — the art’s 19.5 inches minus a half-inch total for the two sides.

Figure Out the Border Width

Using the rabbet size of the framing space, subtract the mat opening to get the total mat surface, then divide by two for each border. If the rabbet is 20 inches and your opening is 14 inches, the leftover is 6 inches; split evenly, you get a 3-inch border on every side.

Borders of 2 to 3 inches are common for visual balance and give the art room to breathe. A border that is too narrow can make the piece feel cramped inside a large frame.

Avoid These Common Mat Mistakes

Use the rabbet measurement, not the frame’s outside label. The outside size tells you little about where the mat will sit.

• Cutting flush to the artwork edge: A zero-overlap opening lets edges show or the art slip through. Always keep at least 1/8 inch of the mat covering the art.
• Choosing a nonstandard outer size: If a standard mat size fits your frame, use it. Custom sizes cost more and take longer to source.
• Ignoring manufacturer tolerances: Most mat cutters have about a 1/16-inch tolerance, so account for that small variation when you cut.

For a ready-made 20 x 20 frame, the outside mat size should match the frame’s inner rabbet, not the nominal label. Standard retail mats are sold to fit common frames, but tolerances vary by manufacturer, so always measure the actual rabbet before you buy.

Convenience Sizing: A Quick Reference

Safety, Fit, and Material Choices

A mat does three jobs: it holds the artwork, creates visual space, and keeps the glass from touching the print. The mat, glass, and backing must fit the rabbet with slight clearance so they can expand with heat and humidity without buckling.

For archival quality, use acid-free or archival mat board. Standard 4-ply board runs about 0.040 to 0.052 inch thick; 8-ply board is roughly 0.125 inch and offers more protection. If your mat cutter or frame depth is limited, factor that thickness into your choice.

Different manufacturers size their mats differently, so treat sizing charts as guidelines, not universal standards. When in doubt, measure the rabbet and the artwork, then add the overlap and border math. That process never fails.