Picture frame mat calculator
Outer mat dimensions and border area from a visible image opening.
Fill in the fields and the result will appear here automatically.
Calculates the outer mat rectangle from the visible image opening, equal top and side borders, and an optional extra bottom border. Bottom weighting is a composition choice: equal borders are valid and there is no mandatory 1–2 cm addition. The result is the mat size, not the outside of the moulding.
How it works
Formula and logic
Width = w +2 b; height = h +2 b +e; bottom border = b +e. Border area =2 b(w+h)+4 b²+e(w+2 b), the outer rectangle minus the opening. This expanded form preserves narrow borders around large openings.
Example
A 20×30 cm opening, 5 cm border and 1 cm bottom addition give a 30×41 cm mat, 6 cm bottom border and 630 cm² of borders. With zero addition the height is 40 cm.
Fields and units
- Visible opening width — cm
- Visible opening height — cm
- Top and side border — cm
- Bottom weighting — cm
How to use
- — Enter the visible opening width and height in centimetres after choosing the overlap on the image.
- — Set a positive top-and-side border for your design, rather than a fixed fraction of every photograph.
- — Enter the bottom addition separately; zero makes all borders equal.
- — Compare the outer mat with the frame fitting size; moulding, rebate, glass and assembly tolerances are not modelled.
Method and limitations
- Calculation method
- Formula and logic
- Limitation
- Rectangular mat geometry. Composition, fitting allowances and materials are project choices; moulding outside dimensions and conservation outcomes are not calculated.
FAQ
Why is the bottom border wider?
A wider bottom is an optional composition technique, not a mandatory optical rule. Choose the addition for the image and framing; zero is also valid.
How wide should the borders be?
Choose borders for the image, frame and intended appearance. A fraction of the shorter side is not a required rule or a calculator constraint.
Should I allow for the mat overlap?
Yes: the inputs describe the visible opening. Choose image overlap separately for the mounting method; the physical print can be larger than the opening.
Does this work for canvas?
The formula describes rectangular spacing. Canvas frames, stretchers and rebates need a separate construction plan; this number is not a finished moulding size.