Picture Frame Mat

Turning Artwork Dimensions Into a Mat Opening and Outer Frame Size

A mat’s cut opening is slightly smaller than the artwork it frames, and its outer size is that opening plus a border on every side. Enter your artwork’s width and height, how wide a mat border you want, and (optionally) extra width for a wider bottom border, and this calculator returns the exact opening to cut and the outer mat/frame size you need.

Two long-standing framing conventions are built in. First, the mat’s opening overlaps the artwork’s own edge by a small amount — commonly about 1/8 in (3 mm) — so the mat physically covers the edge and no gap of backing board shows through. Second, many framers make the bottom border a little wider than the top and sides (a “weighted” mat), since an exactly uniform mat can make the art look like it’s floating slightly too high once it’s hung on a wall — this calculator’s optional extra-bottom-border field adds that automatically.

The Formula

Mat opening (smaller than the artwork by the overlap allowance on every edge):

$$\vD{\text{Opening Width}} = \vA{\text{Artwork Width}} - 2 \times \vC{\text{Overlap}}$$ $$\vE{\text{Opening Height}} = \vB{\text{Artwork Height}} - 2 \times \vC{\text{Overlap}}$$

Border widths — top and both sides always use the plain border width; the bottom adds any optional extra amount for a weighted mat:

$$\text{Bottom Border} = \vF{\text{Border Width}} + \text{Extra Bottom Border}$$

Outer mat/frame size — the opening plus a border on every side:

$$\vG{\text{Outer Width}} = \vD{\text{Opening Width}} + 2 \times \vF{\text{Border Width}}$$ $$\vH{\text{Outer Height}} = \vE{\text{Opening Height}} + \vF{\text{Border Width}} + \text{Bottom Border}$$

Worked Example

An 8 x 10 in (20.32 x 25.4 cm) photo, a 5 cm mat border, and the default 0.3 cm overlap allowance, with a plain uniform mat (no extra bottom border):

1. Opening width: $\vD{\text{Opening Width}} = \vA{20.32} - 2 \times \vC{0.3} = \vD{19.72} \text{ cm}$.
2. Opening height: $\vE{\text{Opening Height}} = \vB{25.4} - 2 \times \vC{0.3} = \vE{24.8} \text{ cm}$.
3. Outer width: $\vG{\text{Outer Width}} = \vD{19.72} + 2 \times \vF{5} = \vG{29.72} \text{ cm}$.
4. Outer height: $\vH{\text{Outer Height}} = \vE{24.8} + \vF{5} + \vF{5} = \vH{34.8} \text{ cm}$.
5. The mat cutter needs a 19.72 x 24.8 cm opening; the outer mat (and frame) size needed is 29.72 x 34.8 cm.

Key Factors to Consider

• A wider mat border generally makes a piece of art feel more prominent and gallery-like. Framers commonly use proportionally wider mats for larger pieces or gallery-style presentations — a narrow border can make a piece feel cramped against its frame, while a generous border gives visual breathing room around the image.
• Acid-free (archival) mat board matters for protecting artwork or photos over the long term. Standard mat board can yellow and degrade over time, and acids from lower-quality board can actually damage the artwork it touches — archival mat board is specifically designed to avoid this, which matters most for valuable or irreplaceable pieces.
• Double or multiple mats (layering two different-colored mats with a small reveal of the bottom color) add visual depth but require more precise, sequential cutting. Each additional mat layer needs its own slightly larger opening than the layer beneath it, cut to leave a consistent narrow reveal — a more advanced technique than a single mat, but a common way to add a complementary accent color.
• Custom-cut mat sizes rarely match standard off-the-shelf frame sizes exactly. Once the outer mat size is known, a custom frame (or a standard frame combined with a custom-cut mat sized to fit it) is often needed rather than assuming a common frame size will match — worth checking frame availability before finalizing the exact mat dimensions.

Common Mistakes

• Forgetting the overlap allowance entirely and cutting the opening to the artwork’s exact size. Without the small overlap, the mat board doesn’t cover the artwork’s own edge, and a hairline gap of backing board can show around the image once it’s framed.
• Making every border the same width when the mat will be hung on a wall. A perfectly uniform mat can make the art look like it’s floating slightly too high — adding a wider bottom border (a “weighted” mat) is a simple fix most framers use by default.
• Ordering a frame before confirming it matches the calculated outer mat size. Custom mat dimensions rarely land on a common off-the-shelf frame size exactly — checking frame availability against the outer size first avoids an expensive mismatch.

Useful to Know

• Building a wood frame from scratch instead of buying one? Woodworking Board Feet Calculator converts lumber dimensions into board feet for ordering material.
• Adding a fabric mat or fabric-wrapped border to the piece? Fabric Yardage Calculator estimates the fabric yardage needed for a project.