Picture Frame Calculator (mat window, glass & outside frame size, moulding length)

Understanding the picture frame calculator — concept and principle

This calculator works out the mat window, the glass and backing size, the outside size of the frame and the length of moulding needed to frame a picture with a mat. Framers and DIY makers use it to cut mats and order moulding and glass.

From artwork outward: with the default inputs the result is 45.8 × 55.8 cm (outside frame · 203 cm moulding).

How it is calculated

Working from the inside out:

Mat window = artwork − 2 × overlap (so the mat holds the art's edges).
Mat outer size = window + 2 × mat border; this is also the glass, backing board and frame opening (rabbet) size.
Outside frame size = opening + 2 × (moulding width − rabbet).
Moulding length (outer edges) = 2 × (outside width + outside height).
Glass area = opening width × opening height.

The principle

A frame is a set of nested rectangles. The mat overlaps the artwork slightly so the art cannot slip through the window. The glass and backing sit in the rabbet, the lip at the back of the moulding, so they are the same size as the mat's outer edge. The moulding covers the glass edge by the rabbet width and extends beyond it by the rest of its width, which sets the outside size. Mitred corners mean each piece is measured along its outer edge, so the four pieces together equal the outside perimeter.

Worked example

Default inputs: artwork 30 × 40 cm, mat border 6 cm, overlap 0.5 cm, moulding 3 cm, rabbet 0.6 cm.

Mat window = 30 − 1 = 29 cm by 40 − 1 = 39 cm.
Opening / glass = 29 + 12 = 41 cm by 39 + 12 = 51 cm; area 0.209 m².
Outside frame = 41 + 2 × 2.4 = 45.8 cm by 51 + 4.8 = 55.8 cm.
Moulding = 2 × (45.8 + 55.8) = 203.2, shown as 203 cm.

Each point is the calculator re-run with only “Artwork width (cm)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Artwork width (cm)	Outside frame width (cm)
7.5	23.3
15	30.8
22.5	38.3
30	45.8
37.5	53.3
45	60.8
60	75.8

How to read the result

Buy extra moulding for saw kerfs and trimming the mitres; how much depends on the saw and the moulding profile. Cut the glass a millimetre or two smaller than the opening so it drops in easily. Many framers make the bottom mat border slightly wider so the picture does not look as if it is sliding down.

Common mistakes

Cutting the mat window the same size as the artwork, so it falls through.
Ordering glass at the outside frame size.
Measuring moulding along the inner edge instead of the outer edge.
Forgetting the saw blade width when cutting mitres.

Keep in mind: Handle glass with care and use protective gloves. For valuable art, use conservation-grade mats and backing.

Frequently asked questions

How big should a mat window be?

Slightly smaller than the artwork, typically by a few millimetres on each side, so the mat holds the edges.

How much moulding do I need?

Twice the sum of the outside width and height, plus extra for cuts.

What size glass do I need?

The size of the frame opening, which equals the mat's outer size.

Sources: Rectangle geometry of a matted frame (mat window, border, rabbet and moulding widths); mitred pieces measured along their outer edges

Checked 9 Nov 2026. Rectangle geometry of a matted frame (window, mat border, rabbet and moulding widths); mitred pieces measured along their outer (long) edges. Overlap and rabbet defaults are example values.

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