Choose a mat opening roughly 1/8 to 1/4 inch smaller than your artwork on each side, so the print stays centered and secure.
Getting the mat size right for an 18×24 frame comes down to one measurement decision: the window opening, not the outer mat dimensions. Cut that opening slightly smaller than your art, and the print holds itself in place. Set it too large, and the piece slips through the frame. This guide walks through the exact numbers for common print sizes and the steps to measure correctly before you order.
The Core Rule: Mat Opening vs. Artwork Size
The mat opening is the window cut into the mat board where your artwork sits. It must be smaller than the artwork itself, typically by 1/8 to 1/4 inch on each dimension.
For an 18×24 print specifically, the mat window should measure roughly 17.75 x 23.75 inches for a 1/4 inch overlap, or 17.875 x 23.875 inches for a 1/8 inch overlap. The overlap is what holds the print in place behind the mat, so never cut the opening to the exact artwork size.
If you’re ready to buy rather than measure, our tested roundup of the best 18 x 24 frame with mat options compares ready-made setups with proper openings.
Common Print Sizes and Their Mat Openings
Different artwork dimensions need different mat window sizes inside the same 18×24 outer frame. The table below shows the standard openings for common print sizes, based on the 1/4-inch overlap convention most off-the-shelf frames use.
| Artwork Size | Recommended Mat Opening | Border Width in 18×24 Frame |
|---|---|---|
| 11×17 inches | 10.75 x 16.75 inches | ~3.5 inches on all sides |
| 12×16 inches | 11.75 x 15.75 inches | ~3 to 4 inches depending on centering |
| 12×18 inches | 11.75 x 17.75 inches | ~3.25 inches on all sides |
| 13×19 inches | 12.75 x 18.75 inches | ~2.5 to 3 inches |
| 16×20 inches | 15.75 x 19.75 inches | ~2 to 3 inches |
| 18×24 inches (no mat) | No mat needed | Frame lip reduces visible area to ~17.5 x 23.5 inches |
If you prefer a wider bottom border for visual weight, adjust accordingly but keep the opening dimensions unchanged.
How to Measure and Calculate the Right Opening
Measuring your artwork correctly prevents the most common framing mistakes. Use these steps with a ruler or tape measure, and write down every number before ordering.
- Measure the artwork height and width in inches at the outermost edges of the print.
- Decide how much overlap you want. Standard options are 1/8 inch for custom framers or 1/4 inch for off-the-shelf frames, per the Patchogue Arts matting guidance.
- Subtract twice the overlap from each dimension. For a 12×18 print with 1/4 inch overlap, the opening is 11.5 x 17.5 inches.
- Choose an outer mat size that leaves a balanced border, typically 2 to 4 inches per side depending on the artwork.
The most reliable approach is to confirm the mat opening size directly with the seller before purchasing, since product listings often specify different openings for the same nominal frame size.
Common Mistakes to Avoid
Framing mistakes usually trace back to a few recurring oversights. Avoid these pitfalls and your 18×24 frame will look right the first time.
- Confusing the outer mat size with the window opening size, which leads to ordering the wrong mat entirely.
- Cutting the opening exactly to artwork size, which lets the print slip through or buckle.
- Choosing a mat border too narrow for the frame, which makes the presentation look unbalanced and cramped.
References & Sources
FAQs
Should I cut the mat opening exactly to the artwork size?
No, cut it smaller by 1/8 to 1/4 inch per side. An exact-size opening lets the print shift, slide through, or buckle over time. The overlap is what holds the artwork flat and centered behind the mat, and framers use this undersizing as a standard practice.
What mat opening do I need for a 12×18 print in an 18×24 frame?
Use an opening of 11.75 x 17.75 inches, which gives a 1/4-inch overlap on each side. That leaves roughly 3.25 inches of mat border on all sides, creating a balanced presentation with enough space between the art and the frame edge.
Does an 18×24 frame show the full 18×24 area of the artwork?
No, the frame’s rabbet and lip overlap covers part of the art’s edge. Most 18×24 frames show roughly 17.5 x 23.5 inches of the print, so if the full image must stay visible, choose a larger frame with a mat instead.