Frame Size Calculator
Build outward from your artwork through the mat border and the molding width to the finished outer frame size — plus how much molding stock to buy to cut it.
Frame Dimensions
“Molding to buy” adds a 10% allowance to the outer perimeter to cover waste from mitring the four 45° corners — buy stock at least that long.
From artwork to finished frame, one layer at a time
A frame's outer size is built up in layers: the artwork itself, a mat border around it (optional), and the frame's molding around that. Each layer adds its width twice — once on each side — so small changes to the mat border or the molding profile compound quickly into the final dimensions.
Whether you're ordering a custom frame, buying stock molding to cut yourself, or checking that a finished piece will fit a specific spot on the wall, working through the layers in order keeps the numbers honest and avoids a frame that comes back the wrong size.
Frequently Asked Questions
How is the outer frame size calculated?
Starting from the artwork, the mat border is added twice (once per side) to get the rabbet opening — the size the frame's inner lip has to clear. Then the molding's face width (the visible width of the frame profile, from the rabbet edge to the outer edge) is added twice more to get the finished outer frame size. So outer size = artwork + 2×(mat border) + 2×(molding width).
What if I'm not using a mat?
Set the mat border width to 0. The calculator will add only the molding width to the artwork size, giving you the outer frame dimensions for framing directly against the artwork with no mat in between.
How much molding should I actually buy?
The calculator reports the frame's outer perimeter (twice the sum of the outer width and height) and then adds a 10% allowance on top of that for waste from cutting the four 45° miter corners — buy at least that length of molding stock. If you're new to mitring, buying a bit extra beyond even that is cheap insurance against a miscut.
Why does molding width matter so much for the final size?
Molding width is applied on both sides of both dimensions, so it has an outsized effect — a chunky 3-inch gallery molding adds 6 inches to both the width and height, while a slim 3/4-inch molding adds only 1.5 inches. If you're ordering a custom frame or buying stock molding, always confirm the finished outer size against the wall space or shipping constraints before cutting.
Does the rabbet opening need to be exactly the mat's outer size?
In practice, framers cut the rabbet very slightly larger than the mat's outer size (often by a millimeter or two) so the mat, glazing, and backing board slide in without binding. This calculator gives the nominal rabbet opening; leave a small manufacturing allowance if you're cutting the frame yourself.
Educational estimate — always confirm the finished outer size against your wall space, and add a small manufacturing allowance if you're cutting the frame yourself.