Frame Mat (Mount Board) Calculator
A picture frame mat (also called a mount board) serves two purposes: it protects the artwork by keeping it from touching the glass, and it provides a visual border that enhances presentation. Getting the mat dimensions right is essential before ordering custom framing or cutting your own mat board. The mat window opening must be slightly smaller than the artwork so the mat overlaps the edges by a small amount (typically 0.25 inches per side), holding the piece in place and hiding any rough edges. The mat outer dimensions determine the frame size you need: the frame inner (rabbet) opening must match the mat outer width and height exactly. A weighted mat with a slightly wider bottom border (typically 0.25 to 0.5 inches more than the sides) creates visual balance and is the standard in professional framing. This calculator takes your artwork dimensions, border widths for top, bottom, and sides, and the overlap amount, and instantly returns the window opening size, frame inner dimensions, and total mat area for pricing.
Window opening: -- x -- in | Frame inner: -- x -- in | Mat area: -- sq in
How it is calculated
window_width = artwork_width - (2 x overlap)
window_height = artwork_height - (2 x overlap)
frame_inner_width = artwork_width + border_left + border_right
frame_inner_height = artwork_height + border_top + border_bottom
mat_area = frame_inner_width x frame_inner_height
Worked example
- Artwork: 8 x 10 inches. Overlap: 0.25 inches.
- Window width: 8 - (2 x 0.25) = 8 - 0.5 = 7.5 inches.
- Window height: 10 - (2 x 0.25) = 10 - 0.5 = 9.5 inches.
- Borders: top 2 in, bottom 2.5 in, sides 2 in each.
- Frame inner width: 8 + 2 + 2 = 12 inches.
- Frame inner height: 10 + 2 + 2.5 = 14.5 inches.
- Mat area: 12 x 14.5 = 174 square inches.
Standard mat border widths by artwork size
| Artwork size | Side borders | Top border | Bottom border | Style |
|---|---|---|---|---|
| 4 x 6 in | 1.5 in | 1.5 in | 1.75 in | Narrow |
| 5 x 7 in | 1.75 in | 1.75 in | 2 in | Narrow |
| 8 x 10 in | 2 in | 2 in | 2.5 in | Standard |
| 11 x 14 in | 2.5 in | 2.5 in | 3 in | Standard |
| 16 x 20 in | 3 in | 3 in | 3.5 in | Wide |
| 18 x 24 in | 3.5 in | 3.5 in | 4 in | Wide |
| 24 x 36 in | 4 in | 4 in | 4.5 in | Extra wide |
Overlap is typically 0.25 inches for prints and photographs. For artwork on canvas paper, 0.125 to 0.25 inches is standard. These border sizes are guidelines; personal preference and artwork style may call for different proportions.
Frequently asked questions
What is the overlap on a picture frame mat?
The overlap is the amount of mat board that covers the edges of the artwork. A typical overlap is 0.25 inches on each side. This holds the artwork in place and hides any uneven edges. The mat window opening is therefore smaller than the artwork by twice the overlap on each dimension.
Why is the bottom mat border usually wider?
A wider bottom border, sometimes called a weighted mat, is a traditional design convention in picture framing. When all four borders are equal, the bottom border can appear visually narrower due to an optical illusion. Adding 0.25 to 0.5 inches to the bottom border creates a more balanced and pleasing appearance.
What do mat outer dimensions mean?
The mat outer dimensions are the same as the frame inner (rabbet) dimensions. This is the size of frame you need to order. The mat must fit precisely inside the frame, so the mat outer width and height must match the frame inner opening exactly.
How do I calculate mat area for pricing?
Mat area in square inches is calculated by multiplying the mat outer width by the mat outer height. Custom framers often price mat board by the square inch or use the total mat area to determine how much board to cut. Subtracting the window opening area from the outer area gives you the actual board area used.
What is the standard mat border width?
For most artwork, a mat border of 2 to 3 inches on the sides and top, with a slightly wider bottom border of 2.5 to 3.5 inches, is considered standard. For very small artwork (4x6 inches or less), narrower borders of 1.5 inches may be more proportionate. For large artwork, borders of 3 to 4 inches are common.
Official sources
- Professional Picture Framers Association: ppfa.com - industry framing standards and guidelines.
- General framing conventions for mat border widths and overlap are based on industry-standard practice as documented by professional framing associations.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.