Frame & Mat Size Calculator

The formula for the Frame & Mat Size Calculator is: Outer Frame Width = Artwork Width + 2 × (Mat Border Width) + 2 × (Frame Rabbet Depth); Outer Frame Height = Artwork Height + 2 × (Mat Border Width) + 2 × (Frame Rabbet Depth). Variables: Artwork Width and Height are the dimensions of the artwork itself in inches; Mat Border Width is the desired width of the mat border around all four sides (typically 1.5 to 4 inches); Frame Rabbet Depth is the depth of the groove in the frame that holds the mat and artwork (usually 0.25 to 0.5 inches). The glass size matches the Outer Frame dimensions, as the glass sits behind the frame and covers the entire opening. This formula accounts for the mat border providing visual breathing room and the rabbet depth ensuring the artwork is securely held without being obscured. It is calculated by adding twice the mat border (for both left/right and top/bottom) and twice the rabbet depth (for both sides) to each artwork dimension, because the frame overlaps the mat on all sides.

A good mat border width typically ranges from 1.5 to 4 inches, with 2 to 3 inches being common for most artworks. The border should be wide enough to separate the artwork from the frame visually, but not so wide that it overwhelms the piece. For small artworks (under 8 inches), a border of 1.5 to 2 inches works well; for larger pieces, 3 to 4 inches can provide balance. The frame rabbet depth is usually between 0.25 and 0.5 inches for standard frames, but always check your specific frame's specifications. The outer frame size determines the glass size, so ensure the glass is cut to match exactly. If the artwork is not perfectly square or rectangular, measure the widest and tallest points. This calculation assumes a single mat; for double mats, add the border width of both mats together. Always measure in inches for consistency, and double-check your measurements before ordering.

A common mistake is forgetting to include the frame rabbet depth, leading to an outer frame that is too small and the artwork being pinched or the mat not fitting. Another error is measuring the artwork with its frame or mat already attached, which skews the dimensions. Some people assume the mat border is only added once per side, but it must be doubled for both width and height. Edge cases include artworks with irregular shapes—here, use the bounding box dimensions. Also, when using a double mat, remember to add both border widths. Finally, not accounting for the glass thickness (usually negligible) is fine, but ensure the frame groove is deep enough for the stack of glass, mat, and artwork.

Mat Border

The visible margin of mat board surrounding the artwork, typically measured from the edge of the artwork to the inner edge of the frame.

Frame Rabbet Depth

The depth of the groove cut into the back of the frame that holds the glass, mat, and artwork in place.

Outer Frame Dimensions

The total width and height of the frame from its outer edges, which determines the size of the glass needed.

Artwork Bounding Box

The smallest rectangle that encloses an irregularly shaped artwork, used for measurement in framing.