What is a weighted bottom border on a mat?

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.

How much overlap should the mat cover?

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.

What is a double mat and when should I use it?

A double mat uses two mat boards stacked together, with the inner mat visible as a thin border (reveal) around the window. It adds depth and a second colour accent. Double mats are popular for watercolours, photographs and limited edition prints. A 5 mm reveal is the most common width.

What standard mat board sizes are available in the UK?

The most common sizes are 508 x 711 mm (20 x 28 inches) and 812 x 1016 mm (32 x 40 inches). Mounting board is also sold in A1 and A2 sheets. Daler-Rowney, Canson and Colourmount are popular brands available from UK framing suppliers like Wessex Pictures and Lion Picture Framing.

ArtCalcs

Mat Window Calculator — Opening & Border Dimensions

Calculate mat board window opening and outer dimensions with border ratios for professional picture framing.

Artwork Width

Width of the artwork being framed

Artwork Height

Height of the artwork being framed

Border Width

Top and side border width (65mm is standard)

Weighted BottomProfessional framing uses a weighted bottom

How We Calculate This

Calculates the window opening and outer mat dimensions based on artwork size, border width and overlap.

Formulas

Window opening = Artwork size - (Overlap x 2)
Outer width = Artwork width + (Side border x 2)
Outer height = Artwork height + Top border + Bottom border
Bottom border = Side border + Weighted extra (if enabled)

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Always verify quantities before purchasing materials.