How to Choose the Right Mat for a 24×30 Frame? | Sizes, Openings, and Rules

The correct mat for a 24×30 frame has outer dimensions of exactly 24×30 inches, with a window (opening) cut about ¼ inch smaller than your artwork on each side to hold it securely in place.

Finding the right mat for a 24×30 frame means matching three numbers: the outer board dimension, the print size, and the window opening. Pick the wrong opening and your art slips, buckles, or looks off-center. The guide below covers the standard measurements, how to calculate your opening, which pre-cut options save you a cutting step, and the one margin rule that makes a framed piece look professionally done.

What Size Mat Fits a 24×30 Frame?

The mat board itself must measure exactly 24 inches wide by 30 inches tall on its outer edges. That’s the only board size that will sit flush inside the frame’s rabbet without cutting or shimming. The inner opening (the window the artwork shows through) depends entirely on your print size.

How to Calculate the Correct Mat Opening

The standard rule is simple: subtract ¼ inch from both the width and height of your artwork. That ¼-inch reduction creates a ⅛-inch overlap on all four sides, giving the mat enough lip to hold the print flat against the mounting board without covering any of the image.

Formula: Opening Width = Print Width – 0.25 inch | Opening Height = Print Height – 0.25 inch

For an 18×24 print, that means a 17.5×23.5 opening. For a 20×24 print, it’s 19.5×23.5. The outer board stays 24×30 no matter what.

Pre-Cut vs. Uncut Mats: Which Should You Buy?

Pre-cut mats save time but lock you into one opening size. Uncut blank boards give you flexibility but require a mat cutter and steady hands.

Pre-Cut Mats

Frame Amo sells a 24×30 mat with a 17.5×23.5 opening, specifically designed for 18×24 prints. It’s beveled and acid-free, priced around $15–$25 per sheet. Excellent option if your art is exactly 18×24.

Uncut Blank Boards

Matboard Plus offers a 6-ply blank sheet up to 24×30 inches, sold uncut for around $20–$30. You mark your calculated opening and cut it yourself using a mat cutter with stop-start marks at each corner to prevent over-cutting. This is the route if your print is an unusual size or you want a custom border width.

Cutting the Mat Yourself: Step-by-Step

If you chose an uncut blank board, follow this exact sequence:

1. Measure your artwork width and height precisely.
2. Subtract 0.25 inch from each to get the opening dimensions.
3. Mark the opening on the back of the blank 24×30 board.
4. Use a mat cutter with a stop-start mark at each corner — cut each side so the blade retracts at the corners to prevent tearing the bevel.
5. Apply transfer adhesive to the outer edges of the mat to secure it to your mounting board.

If a 24×30 mat with that specific opening sounds harder to find than it should be, our roundup of tested 24×30 frames with included matting can save you the legwork.

The One Margin Rule That Makes It Look Professional

Industry-standard framing almost always gives the bottom margin an extra ½ inch compared to the top and sides. On an 18×24 print in a 24×30 frame, the top and side borders would be about 2.5 inches, while the bottom border would be 3 inches. That extra bottom weight stops the art from looking like it’s sinking visually — a subtle trick that separates a framed piece from a DIY project.

Common Mistakes That Ruin a Framing Job

Cutting the Opening Too Large

If you cut the window exactly the same size as the print, there’s no overhang to hold the art in place. The print can shift behind the glass, and the mat won’t secure it to the back board. Always cut the window smaller.

Ignoring the Bottom Margin Rule

Same-size margins all around make the image look bottom-heavy. Warmer, balanced to the eye comes from that extra ½ inch at the bottom. The Redimat framing guide demonstrates the difference clearly.

Using Non-Archival Materials

For valuable prints or artwork you intend to keep, use a mat that is alpha-cellulose, pH neutral, acid-free, and buffered. Standard cardboard mats contain lignin, which can yellow and discolor the art over time. Acid-free mats prevent that migration.

Not Accounting for Frame Rabbet Depth

The frame’s inner lip (the rabbet) is typically about ¼ inch deep. If your mat is too thick (like a 6-ply board), it may not fit behind the glass without bending the mat or breaking the frame. Check the frame’s rabbet depth before buying a 6-ply mat.

Choosing the Mat Color and Style

The mat’s color should be slightly darker or grayer than the lightest tone in your artwork. For double matting (two layers), the top mat opening should be smaller than the bottom mat to create depth. A white or off-white beveled mat remains the safest default for most posters and prints.

Mat Board Options at a Glance

Final Checklist for Buying a 24×30 Mat

• Confirm the outer dimensions are exactly 24×30 inches.
• Calculate the opening by subtracting 0.25 inch from both print dimensions.
• Choose pre-cut if your print is 18×24, 22×26, or another standard size with available pre-cut boards.
• Choose uncut if your print is an unusual size or you want custom border widths.
• Apply the bottom-margin rule: add 0.5 inch to the bottom border.
• Verify material safety: acid-free, buffered, alpha-cellulose for archival work.
• Check frame rabbet depth fits the mat thickness.