Worked example
A 16 × 20 in mat with 2 in side and top borders and a 2.5 in bottom border leaves a 12 × 15.5 in opening. At 300 pixels/in, the exposed image needs at least 3,600 × 4,650 pixels.
Measure the part that will show
The opening is the visible rectangle, not necessarily the cut size of the photo. A mat may overlap the photo edges, so allow extra image area for mounting and cropping. These calculations use the border widths you enter; they do not select an aesthetic border for you.
Pixels refer to the exposed image
Multiplying the opening dimension by a target pixels-per-inch value gives the minimum whole-pixel dimensions for that opening. It does not improve a low-resolution source or account for printer margins, trimming or bleed.
Use actual measurements
Mat sizes and openings can vary by manufacturer. Measure the physical outside and intended opening in inches before committing to a print. The Library of Congress matting guide discusses planning an opening around the object; this page handles only the rectangle arithmetic.
Explore the related calculation
For a different starting point, try the poster tiling calculator or return to the main calculator. Browse the complete tool directory for this site.
Reference and corrections
The definitions and assumptions above follow these sources; the examples and calculator arithmetic are original to this page.
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