This picture frame calculator turns one measurement — the size of the artwork — into every other number you need to build or order a frame. Framing is a chain of dependent dimensions, and each one is derived from the last: the mat opening comes from the artwork, the mat outer size comes from the opening, the glass matches the mat, the frame opening is cut from the glass, and the moulding length comes from the perimeter. Get one link wrong and everything downstream is wrong with it.
Arb Digital publishes this in a free tools library covering the arithmetic behind practical projects. The calculator handles the four measurements that get confused most often: the overlap of mat onto artwork, the weighted bottom border, the rebate that hides the glass edge, and the mitre waste allowance that decides how much moulding to actually buy.
What This Picture Frame Calculator Does
It produces four dimensions from your inputs. The mat opening is what you cut into the mat board, slightly smaller than the artwork so the mat holds it. The mat outer size is the mat board itself, and it is also the size of the glass and the backing board, because all three are cut identically. The frame outside size is what the finished piece measures on the wall. The moulding length is the raw material to buy, including the waste consumed by cutting four mitres.
It also handles the asymmetric bottom border properly. Enter a single border figure for the sides and top, then an extra amount for the bottom, and the tool adds it to the height only — which is what weighting the bottom actually means.
The site's nearest tools do adjacent jobs. The aspect ratio calculator works out proportional resizing of an image but knows nothing about mats or mouldings. The rectangle area calculator gives you the area of a shape without the framing chain. If you are planning where several frames go on a wall, the wall area calculator covers the surface they have to fit on.
How to Use It
- Measure the artwork itself. Measure the printed or painted area you intend to show, not the paper edge, if there is an unprinted margin you plan to cover.
- Set the mat overlap. A quarter inch each side is standard. Less risks the artwork slipping through; more starts hiding the image.
- Choose the border width. Wider borders read as more formal and give the eye somewhere to rest. Add bottom weighting if the piece will hang at or above eye level.
- Enter the moulding face width and rebate. Both are on the moulding supplier's specification, and the rebate is the number people forget.
- Buy the moulding length shown, not the perimeter. Mitre cuts consume material at every corner, which is what the waste allowance covers.
The Formula: Every Dimension in the Chain
Working outward from the artwork:
- Mat opening = artwork dimension − (2 × overlap), on both width and height.
- Mat, glass and backing size = opening + (2 × border) on the width, and opening + (2 × border) + bottom weight on the height.
- Frame outside size = glass size + 2 × (moulding face width − rebate), on both dimensions.
- Moulding length = 2 × (glass width + glass height) + (waste multiplier × moulding face width).
Worked example, which is the default on this page. An 8 by 10 inch artwork with a quarter-inch overlap gives a mat opening of 7.5 by 9.5 inches. A three-inch border all round with an extra half inch at the bottom gives a mat, glass and backing size of 7.5 + 6 = 13.5 wide by 9.5 + 6 + 0.5 = 16 inches high. With a 1.5 inch moulding and a quarter-inch rebate, the frame adds 2 × (1.5 − 0.25) = 2.5 inches to each dimension, so the finished frame measures 16 by 18.5 inches on the wall.
The moulding needed is 2 × (13.5 + 16) = 59 inches of frame perimeter, plus the standard trade allowance of 8 × 1.5 = 12 inches of mitre waste, giving 71 inches — just under six feet. At six units per foot that is about 35.50 in moulding cost. Buying only 59 inches would leave you a corner short, which is exactly the mistake the waste allowance exists to prevent.
Why the Frame Is Smaller Than the Glass It Holds
The single most common framing error is confusing the frame's outside size with the size of the glass. A moulding has a groove cut along its inner edge called the rebate, or rabbet, and the glass, mat and backing all sit inside it. The lip of that rebate covers the outer few millimetres of the glass, which is what holds the whole sandwich in place.
So the frame is cut to an opening that matches the glass, and the visible aperture is smaller than the glass by twice the rebate overlap. The outside size is then the glass size plus twice the moulding width, minus twice the rebate — which is why a "16 by 20 frame" and a "16 by 20 mat" are not the same measurement, and why ordering by outside dimension without knowing the rebate gets you a frame that does not fit anything you own.
Rebate depth is a separate concern from rebate overlap. Depth is how much thickness the groove can swallow — glass plus mat plus artwork plus backing plus the retaining hardware. A shallow moulding will not close over a double mat and a foam backing board, and this is not something a calculation catches. Add up the thicknesses of everything going in and compare it against the moulding's stated rebate depth before ordering.
Choosing a Mat Border That Looks Right
There is no rule that produces a correct border width, but there are patterns worth knowing. Narrow borders of an inch or so make a piece feel casual and let it sit in a group without dominating. Borders of three inches and more read as gallery framing and give a small work presence on a large wall. A border narrower than the moulding face tends to look squeezed, so as a starting point make the mat border at least as wide as the frame itself.
Bottom weighting is the convention this tool implements explicitly. Because most pictures hang at or above eye level, a mat with mathematically equal borders appears to have a thinner bottom — the viewing angle foreshortens it. Framers compensate by adding a small amount to the base, commonly between a quarter and one inch depending on the size of the piece. The effect is that the mat reads as even when it is not.
Double mats add a second board with a slightly larger opening beneath the first, revealing a thin line of the under-mat colour. If you are planning one, cut the under-mat opening a quarter inch larger than the top mat on each side and calculate the top mat here as normal; the glass and frame dimensions do not change, because both boards are the same outside size.
Conservation Considerations Before You Cut Anything
If the piece has any value — a print you cannot replace, an original work on paper, a document — the materials matter more than the measurements. Ordinary mat board contains lignin and acidic compounds that migrate into artwork over time and produce the brown mat burn line you see on old framing. The Library of Congress's preservation guidelines for matting and framing set out what preservation-grade framing requires: cotton rag or purified, lignin-free board, at least four ply, and glazing that keeps the object from touching the glass.
Attachment is the other half of it. Adhesive tapes fail, stain and are difficult to remove, and current conservation practice favours non-adhesive methods — corners, edge strips and channels — or reversible hinges made with Japanese tissue and wheat starch paste. The Library of Congress guidance on matting and hinging of works of art on paper covers the techniques in detail. The same source notes that permanent display of paper is not recommended at all, because light exposure causes irreversible fading and embrittlement.
None of that changes the arithmetic on this page. The mat opening is the same size whether the board is rag or cardboard. It changes what you should buy, and it is worth knowing before you cut a mat for something irreplaceable.
Buying Moulding and Cutting Mitres
Moulding is sold by length, and the length you need is not the perimeter. Each of the four mitre cuts removes material, and each piece has to be long enough on its outer edge to reach the corner — the outer edge of a mitred piece is longer than the inner edge by twice the moulding width. The standard trade allowance of eight times the moulding width covers all four corners with a little to spare.
That allowance scales with the moulding, which is why it is a multiplier rather than a fixed figure. A slim half-inch moulding needs only four inches of waste; a chunky three-inch moulding needs twenty-four. If it is your first frame, take the generous option in the selector — a spoiled mitre on the last corner is an expensive way to learn.
Finally, cut the glass and backing to the mat size, not to the frame opening, and check the diagonals of the assembled frame before the glue sets. A frame that measures correctly on both sides can still be a parallelogram, and the diagonals are the only measurement that catches it.
Arb Digital's free tools library covers the pricing, sizing and shipping maths behind selling physical products, and our team is happy to talk through anything the tools cannot answer.
Browse Free Tools Talk to Arb DigitalCommon Mistakes to Avoid
- Cutting the mat opening the same size as the artwork — without an overlap on every edge the artwork has nothing to sit on and drops through.
- Confusing frame outside size with glass size — the rebate makes them different, and ordering by the wrong one gets a frame that fits nothing.
- Buying moulding equal to the perimeter — four mitre cuts consume material, and you will finish one corner short.
- Ignoring rebate depth — a shallow moulding will not close over a double mat, glass and foam backing however well the widths work out.
- Using ordinary card for anything valuable — acidic board leaves a permanent burn line on artwork, and the damage is not reversible.
Related Free Tools From Arb Digital
Use the aspect ratio calculator to resize an image proportionally before printing, the paper size converter for standard print formats, the rectangle area calculator for plain area arithmetic, the wall area calculator when planning a hanging arrangement, and the square footage calculator for larger surfaces. Everything else is in the free online tools hub.
Frequently Asked Questions
Usually a quarter of an inch on each edge, so half an inch off each overall dimension. That overlap is what holds the artwork behind the mat. Less risks it slipping through; more starts cropping the image.
Because a piece hung at or above eye level is viewed at an angle that foreshortens the bottom border. Adding a small amount at the base makes the mat read as visually even, which is why framers weight it.
The same size as the mat board and the backing — all three are cut identically. The frame is then cut to hold that size, with its rebate lip covering the outer edge on each side.
The perimeter of the glass size plus a waste allowance for the four mitre cuts. The standard trade allowance is eight times the moulding face width, which is what this calculator adds by default.
It is the groove along the inner edge of the moulding that the glass, mat and backing sit inside. Its lip covers the outer few millimetres of the glass, which is why the visible aperture is smaller than the glass and the frame outside size is not the glass size.
Calculate the top mat here as normal. The under-mat has the same outside size, so the glass and frame dimensions are unchanged, and its opening is simply cut slightly larger to reveal a line of colour.
For anything you care about, yes. Acidic board causes a permanent brown burn line over time. Preservation guidance calls for cotton rag or purified lignin-free board and glazing that keeps the artwork from touching the glass.
Conservation practice avoids it. Adhesives stain and fail, and current guidance favours non-adhesive corners, edge strips or channels, or reversible hinges made with Japanese tissue and wheat starch paste.
This tool performs dimensional arithmetic from the measurements you enter. It is not conservation advice, and it cannot assess the condition or value of any artwork. Anything irreplaceable should be framed by a qualified conservator or a framer working to preservation standards.
