Picture Frame Mat Calculator (Window Size and Border Widths)

What this tool does

The mat's outer size is the frame opening (cut it a millimetre or so under so it drops in) and its window is the picture minus the overlap on each side, usually 3–6 mm (about ⅛–¼ in), enough to hide the picture's edge and hold it in place. Whatever is left is the border, shared equally on the sides and the top. Framers often make the bottom border a little wider than the top — commonly 5–12 mm more — because a picture centred exactly looks as if it is sliding down; put that in the bottom-weighting field. If a border comes out negative, the picture is too large for the frame.

Formula

window = picture − 2 × overlap · side border = (frame − window width) ÷ 2 · top border = (frame − window height − extra) ÷ 2 · bottom = top + extra

Variables

Worked example

Limitations

• Mixing units is the most common source of error. Convert every input to the units shown next to each field before calculating.
• The calculation runs at full precision and only the display is rounded. If you copy an intermediate value and retype it, small differences can appear.