How do I calculate a mat opening?

Check aspect ratio before ordering the print

Common print sizes use different ratios. Crop geometry should be evaluated before resolution because cropping changes the pixels that remain for the final print.

Apply the answer to this specific question

A mat opening is normally slightly smaller than the artwork so the mat overlaps its edges and retains it. Subtract twice the chosen overlap from the artwork dimensions. The practical consequence should be checked with Mat Opening Calculator when your own dimensions, page count, cost or production values differ from the example implied by the question.

What commonly changes the result

For “How do I calculate a mat opening?”, the most important variables are the ones represented by the linked calculator and any provider-controlled specification. A correct arithmetic result can still be unsuitable if the source dimensions, category, page yield, trim, bleed, margin or production method were assumed incorrectly.

A better verification workflow

For How do I calculate a mat opening?, record the source measurement or controlling specification first, then run the real job values through mat opening. Keep the result with the job notes, compare it with the physical file or material, and recheck any provider-controlled rule immediately before production or upload.

Practical answer

A mat opening is normally slightly smaller than the artwork so the mat overlaps its edges and retains it. Subtract twice the chosen overlap from the artwork dimensions.

When the decision affects a paid print run, book upload, client order or expensive material, keep the calculation with the job notes and verify any external specification again immediately before production.