Picture Frame Calculator — Moulding, Glass & Mat

Allow 40-60mm extra per piece for mitre waste. Each 45-degree mitre cut removes a triangle of material equal to the moulding width, and you lose a saw kerf width at each cut. The calculator defaults to 50mm total waste per piece, which is sufficient for most moulding widths.

Standard mat width is 50-65mm. For smaller artworks (under A4), 40-50mm is appropriate. For larger pieces, 65-80mm provides better visual framing. Some framers make the bottom mat slightly wider (10-15mm more) than the other three sides for visual balance — this is called a weighted mat.

The glass is cut to the frame's internal (rebate) opening, which equals the outer size of the mat board — the glass, mat, artwork and backing all stack inside the rebate. The calculator gives you the exact glass dimensions. For valuable artwork, consider UV-filtering glass or museum glass. Standard picture framing glass is 2mm thick.

The rebate (also called a rabbet in the US) is the L-shaped step cut into the back of the moulding that holds the glass, mat, artwork and backing. A typical rebate is 5-7mm wide and 10-12mm deep. Make sure the combined thickness of your stack (glass 2mm + mat 1.5mm + artwork ~1mm + backing 3mm ≈ 7.5mm) does not exceed the rebate depth — the calculator flags this for you.

The mat opening is slightly smaller than the artwork — typically 3mm smaller on each side. This 3mm overlap holds the artwork in position behind the mat and prevents the edges from showing. So for a 200 × 300mm artwork, the mat opening would be 194 × 294mm.