How much smaller should the mat opening be?

Per edge is different from overall.

A reduction in the total width is shared between the left and right edges when the opening is centered. Reducing width by 1/4 inch overall means 1/8 inch per edge, not 1/4 inch per edge. Height has its own top and bottom reduction. This language difference can double the amount covered.

The general equation is window width = paper width − left coverage − right coverage. Use the equivalent height equation for top and bottom. Unequal coverage is possible in a geometric plan; whether it suits the object and mounting must be confirmed separately.

Check the supplier’s own policy.

As checked on November 6, 2026, Frame Destination states that its standard mats for artwork from 4 × 6 through 18 × 24 inches use an exact opening 1/4 inch smaller overall, covering 1/8 inch on each edge. It states a different deduction for larger standard sizes, and asks for exact dimensions for custom openings.

Frame It Easy’s help center states that its mat covers 1/4 inch of the entered art size on each edge. These examples establish that order language and policies differ. Neither policy is a universal rule for every artwork, material, supplier or mounting method.

Avoid subtracting twice.

Suppose you want an exact 7 3/4 × 9 3/4-inch window. Entering that into an art-size field that automatically subtracts another 1/2 inch overall would produce 7 1/4 × 9 1/4 inches instead. The arithmetic is correct for that extra deduction, but the opening would differ from your intention.

Ask: ‘Will you cut this exact window, or do you reduce the dimensions I enter?’ Keep the final exact window in your written confirmation. If you are ordering a labeled premade product, read its actual window specification rather than assuming its label is the hole size.

Visibility and support are separate checks.

Less coverage can expose a signature, but it can also change how the paper is retained. Zero coverage is allowed as a geometry input in our tools; it is not an instruction for securing the piece. A float presentation, visible paper edges or unusual media may need a different design and mounting approach.

Before settling on coverage, mark all image details, signatures and edition numbers that need to remain visible. If the intended mounting and the visible area conflict, ask a framer to resolve the assembly. The calculator cannot decide a suitable overlap or cutting tolerance for your object.

Use an explicit plan.

Measure the physical sheet, choose coverage with the relevant supplier or framer, calculate the exact window, and inspect important details. Then choose the board outside size and check its borders. Never replace those separate decisions with an assumed standard deduction.