A 15 x 18 frame is measured by its inside rabbet, while the mat opening is cut from the artwork itself — about 1/2 inch smaller so the mat overlaps the image.
Two numbers matter in framing, and mixing them up is the reason prints slip through openings or mats refuse to sit flat. The frame’s own size is the inside rabbet — the recessed lip at the back that holds the glass, mat, and backing. The mat opening is a completely separate measurement built around the artwork. A 15 x 18 frame is treated as a standard size in most frame-size charts, but that label describes the opening the assembled stack has to fill, not the art it can hold.
What Size Mat Fits A 15 x 18 Frame?
The mat’s outer dimensions must match the frame’s inside rabbet, which is usually 15 x 18 inches on a frame sold at that size. The window inside that mat is derived from your artwork instead.
Logan Graphic Products, a framing supplier that publishes sizing documentation, defines the outer mat size as the piece measured from the back, inside the rabbet. In most retail frames, the 15 x 18 label refers to that opening. If the frame lists its size by glazing rather than by the outer frame edge, the rabbet should measure the same. Measure the back of your specific frame rather than trusting the label — an opening that reads 14 7/8 by 17 7/8 will reject a mat cut to a full 15 x 18.
Border width is your design choice. Logan’s guidance recommends borders between 2 and 4 inches unless you want a deliberately different look, and the same width on all four sides keeps the art visually centered. Wider borders suit small artwork; narrow borders work on large pieces where you want the image to dominate.
| Sizing Element | How To Determine It | Typical Value |
|---|---|---|
| Mat outer size | Measure the inside rabbet from the back of the frame | Matches the frame opening, about 15 x 18 inches |
| Mat window opening | Measure the artwork, then cut the opening smaller | About 1/2 inch smaller overall |
| Overlap per side | Split the difference between opening and artwork | About 1/4 inch on each side |
| Minimum overlap | Never go below this on any edge | 1/8 inch on all four sides |
| Border width | Your design preference after the opening is set | 2 to 4 inches |
| Artwork measurement | Measure the flattened print, not the packaging | Whatever the actual piece reads |
| Stack fit | Rabbet must clear mat, glass, and backing together | Rabbet slightly larger than the full stack |
How To Measure The Artwork And Set The Mat Opening
The mat window is built from the artwork, never from the frame size, and it should be roughly half an inch smaller than the print in both directions.
- Flatten the print first. Rolled or curled paper measures long in one direction and short in the other, so lay it under a book for a few hours or weight the corners before you touch a ruler.
- Measure the actual artwork’s width and height. Logan Graphic Products recommends measuring the piece itself, not the packaging or a listed print size, because trimmed prints and hand-cut paper often differ from the stated dimensions.
- Decide your overlap. A quarter inch per side is the common starting point, giving a total reduction of about half an inch. The mat must cover at least 1/8 inch of the image on every side — less than that, and edges peek through the window or the art drops through it entirely.
- Subtract to get the window. An 11 x 14 print with a quarter-inch overlap yields a mat opening of 10 1/2 x 13 1/2 inches. That window then sits inside a 15 x 18 mat outer, leaving your border width around the art.
- Add borders to confirm the outer size. Take the window dimensions and add twice your chosen border width to each direction. If the result doesn’t land on your frame’s rabbet measurement, adjust the border, not the opening.
Fragile prints with torn or delicate edges can take a larger overlap for more protection, but 1/8 inch is the floor. If your piece needs an exact visible amount — a signed print where every millimeter of the signature matters — Level Frames’ framing help documentation suggests requesting a specific mat window rather than relying on generic size charts. Once the numbers are settled, comparing ready-made tested 15 x 18 frame options is faster than ordering a custom build.
Where 15 x 18 Measurements Usually Go Wrong
Most framing mistakes trace back to using the frame’s label instead of the rabbet, or sizing the mat opening from the frame rather than the art.
The most common error is trusting the nominal 15 x 18 label without checking the inside rabbet from the back. Frames sold by glazing size and frames sold by outer dimensions can differ by fractions of an inch, and that gap is enough to throw off a pre-cut mat. Logan Graphic Products lists this mismatch as a top sizing problem, and the fix is a tape measure against the rabbet lip, not the frame’s outer edge.
Cutting the mat opening to the artwork’s exact size is the second frequent slip. Without overlap, the image slides through the window or shows raw, uneven edges. The third is forgetting the stack — the rabbet has to clear the mat, glass, and backing together, not just the mat alone, or the assembly won’t seat. Unflattened prints round out the list: a curled sheet measures deceptively on both axes.
Standard frame-size charts list 15 x 18 as a common size, but the chart’s opening is derived from artwork dimensions in every case. Measure the rabbet, measure the art, subtract the overlap, add the border. Those four numbers settle any 15 x 18 build.
FAQs
How much smaller should a mat opening be than the artwork?
About 1/2 inch smaller overall, which works out to roughly 1/4 inch of overlap on each side. This holds the print in place so it can’t shift or fall through the window. Never go below 1/8 inch of overlap on any edge, since thinner coverage lets raw print edges show through the opening.
Is a 15 x 18 frame the same as a 15 x 18 mat?
Not always. The frame’s 15 x 18 label usually describes the inside rabbet or glazing opening, while a mat’s 15 x 18 refers to its outer dimensions. Those match when the label reflects the rabbet. Measure the back of your frame inside the rabbet lip to confirm before ordering a pre-cut mat.
Can I use a pre-cut mat in a 15 x 18 frame?
Yes, if the mat’s outer size matches your frame’s rabbet and its window fits your specific artwork with the proper overlap. Pre-cut mats built for standard print sizes work when your art is that exact size. For trimmed or unusual prints, request a custom window instead of forcing a stock mat to fit.
References & Sources
- Logan Graphic Products. “Measuring and Sizing” Explains frame openings, rabbet measurement, and mat outer size.
- Logan Graphic Products. “Measuring Mats” Covers mat window sizing, overlap, and border widths.
- Level Frames. “Measuring Your Artwork for Framing” Supports measuring the actual artwork and requesting specific mat windows.
