Mat Window Calculator — Opening & Border Dimensions

A weighted bottom means the bottom border is slightly wider than the top and sides. This corrects an optical illusion where equal borders make the artwork look like it is dropping. Adding 5-10 mm to the bottom creates a balanced, professional appearance. Most framers in the UK use this technique by default.

A 5 mm overlap on each side is standard for most artwork. This hides the edge of the paper or canvas and prevents it slipping through the window. For valuable original artwork, conservation framers may use a smaller 3 mm overlap to show more of the image.

A double mat stacks two boards. The bottom (inner) mat has the smaller opening — it sits against the artwork. The top (outer) mat opening is cut larger by twice the reveal, so a thin strip of the bottom mat shows through as a second colour accent. So if the bottom window is 290 x 390 mm with a 5 mm reveal, the top window is cut at 300 x 400 mm. Double mats add depth and are popular for watercolours, photographs and limited-edition prints; a 5 mm (about 1/4 inch) reveal is the most common width.

UK mountboard is sold in the ISO A-series — A2 (420 x 594 mm), A1 (594 x 841 mm) and A0 (841 x 1189 mm) — plus the picture-framing trade full sheet of 1016 x 813 mm (sold as 32 x 40 inches). Daler-Rowney, Canson and Colourmount are popular Fine Art Trade Guild brands available from UK framing suppliers like Lion Picture Framing and Wessex Pictures. There is no common 20 x 28 inch board in the UK; for a tidier border, cut down the next size up rather than forcing your artwork to a fixed sheet.