Four dimensions, each derived from the one before
A framed picture is a stack of four rectangles and the confusion in framing comes from people quoting one of them when they mean another. Work outwards and the ambiguity disappears.
The mat opening is the hole cut in the board. It is smaller than the artwork, because the mat has to overlap the edges of the sheet to hold it and hide the tape or the deckle. A quarter of an inch per side is the standard overlap, so an 8 × 10 in print gets a 7.5 × 9.5 in opening.
The mat outer size is opening plus borders. It is also the glass size and the backing size, because all three are cut identically and stack face to back. When someone says "an 11 × 14 frame", this is the dimension they mean.
The rabbet size is the rectangle the frame's inner lip encloses. It is the mat outer size plus a small allowance — 1/8 in is the trade standard — so the package slides in without being forced. Frames are cut to the rabbet, which is why a "16 × 20 frame" actually measures 16 1/8 by 20 1/8 inside.
The outside size is the rabbet plus twice the moulding face width, and it is the only figure that matters for the wall. A 16 × 20 image in a 2 in moulding occupies 20 1/8 × 24 1/8 in of wall, which is a very different object from the one you pictured.
The arithmetic, and where the traditions come from
Opening. opening = artwork − 2 × overlap, in both directions. The overlap is not decoration — it is structural. Paper moves with humidity, and a mat resting on 1/8 in of margin will eventually show a white edge as the sheet contracts. Choose 0 only when you are float-mounting, where the art sits on top of a backing board with its edges visible and the mat opening is deliberately larger than the sheet.
Mat outer size. Width = n × opening + (n − 1) × gap + 2 × border. Height = opening height + top border + bottom border. For a single opening the width term collapses to opening + 2 × border.
The weighted bottom. Traditionally the bottom border is cut 10–20% deeper than the sides and top. The reason is optical, not structural: a picture is usually hung with its centre above eye level, so a geometrically even mat appears bottom-light. Adding roughly a fifth to the bottom restores visual balance. A 2 in border with a 1.2 multiplier gives a 2.4 in bottom, which is barely noticeable as a measurement and clearly noticeable as an effect.
Rabbet and outside. rabbet = mat outer + allowance; outside = rabbet + 2 × face width. The face width here is measured across the front of the moulding from its outer edge to the rabbet lip, which is exactly the definition that makes this addition exact. If you measure the full moulding width including the rabbet cut-out instead, your outside dimension will come out a quarter-inch too large per side.
Moulding to buy. The four rails together measure 2 × (outside width + outside height), because each rail's long point equals the rabbet dimension plus two face widths. Add the saw kerf: a frame takes eight mitre cuts, so at a 1/8 in blade that is 1 in of stick turned into sawdust. Buy in whole feet and round up — a frame short by half an inch is a frame you cannot make.
Worked example: an 8 × 10 print with a 1.75 in mat
You have an 8 × 10 in print, a 1/4 in overlap, a 1.75 in border all round, a 1.5 in moulding face, and the standard 1/8 in rabbet allowance.
- Opening. 8 − (2 × 0.25) = 7.5 in wide. 10 − 0.5 = 9.5 in high.
- Mat outer, and therefore glass and backing. 7.5 + (2 × 1.75) = 11 in wide. 9.5 + 1.75 + 1.75 = 13 in high. Cut the glass and the foam-core backing at 11 × 13 in.
- Rabbet. 11 + 0.125 = 11.125 in; 13 + 0.125 = 13.125 in. This is the size the frame is chopped to.
- Outside. 11.125 + (2 × 1.5) = 14.125 in wide; 13.125 + 3 = 16.125 in high. That rectangle is what the wall sees.
- Moulding. 2 × (14.125 + 16.125) = 60.5 in, plus 8 mitre kerfs at 1/8 in = 1 in, so 61.5 in — buy a 6 ft stick.
Notice that 11 × 13 in is not a stock frame size. This is the moment most framing projects branch. If you want a ready-made 11 × 14 frame instead, the mat outer size is fixed at 11 × 14 and the borders have to absorb the difference: side borders stay at (11 − 7.5) ÷ 2 = 1.75 in, but the vertical space becomes 14 − 9.5 = 4.5 in, split as 2.25 in top and bottom. That is precisely why every commercially cut 11 × 14 mat for an 8 × 10 print has borders that are wider top-and-bottom than at the sides — and it looks perfectly good, because the eye reads a taller mat as intentional.
Choosing a border that works
Start at two to three inches for prints up to about 16 × 20 in, and scale up from there; a 24 × 36 in poster carries a 4 in border comfortably. These are working proportions rather than rules. The single most common mistake is a border that is too narrow: below about 1.5 in the mat stops reading as a resting space and starts reading as a white line.
Compare the border against the moulding face. If the moulding is wider than the mat border, the frame dominates and the artwork sits in a tunnel. A wide mat with a narrow moulding is the gallery look; a narrow mat with a wide ornate moulding is the traditional look. Both work; a wide mat with a wide moulding usually does not, because the artwork becomes a small event in a very large object.
Watch the 32 × 40 in ceiling. A full sheet of matboard is 32 × 40 in, and glass, backing board and moulding all get markedly more expensive above it. If your mat outer size crosses that boundary, check whether trimming the border by half an inch brings it back inside. Above roughly 24 × 36 in, switch from glass to acrylic anyway: it weighs about half as much and it does not turn into a hazard if the frame comes off the wall.
Glazing choice does not change any dimension, but it changes the money. Standard glass, UV-filtering glass and museum anti-reflective glass are all cut to the same mat outer size and differ by a factor of several in price. For anything with sentimental or monetary value, UV filtering is the one upgrade that measurably slows fading.
Once the outside dimension is known, you can plan the wall. Hang the centre of the picture at 57–60 in from the floor, the standard gallery eye line, and check the composition against the other fixed heights in the room — the TV mount height calculator gives the screen centre, and the rug size calculator sets the footprint underneath.
Standard print and frame pairings with a 1/4 in overlap
| Frame (mat outer) | Mat opening | Side border | Top / bottom border | |
|---|---|---|---|---|
| 5 × 7 in | 8 × 10 in | 4.5 × 6.5 in | 1.75 in | 1.75 in |
| 8 × 10 in | 11 × 14 in | 7.5 × 9.5 in | 1.75 in | 2.25 in |
| 8 × 10 in | 12 × 16 in | 7.5 × 9.5 in | 2.25 in | 3.25 in |
| 11 × 14 in | 16 × 20 in | 10.5 × 13.5 in | 2.75 in | 3.25 in |
| 12 × 16 in | 18 × 24 in | 11.5 × 15.5 in | 3.25 in | 4.25 in |
| 16 × 20 in | 20 × 24 in | 15.5 × 19.5 in | 2.25 in | 2.25 in |
| 18 × 24 in | 24 × 30 in | 17.5 × 23.5 in | 3.25 in | 3.25 in |
| 24 × 36 in | 30 × 40 in | 23.5 × 35.5 in | 3.25 in | 2.25 in |
Side border = (frame width − opening width) ÷ 2; top/bottom = (frame height − opening height) ÷ 2. The last row is the case where the ready-made proportions run the other way and the side border is the wider one — worth knowing before you assume every mat is taller than it is wide.
Conservation matters more than millimetres
The dimensions here decide whether a frame fits. What decides whether the artwork survives is the material: use acid-free, lignin-free ("conservation" or "museum") matboard and backing, hinge the art with Japanese tissue and reversible starch paste or acid-free linen tape, and never dry-mount anything you might one day want to sell. A buffered board sitting directly against a photograph can damage it; unbuffered board is the safer default for photographic material. None of this changes a single number in the calculation, and all of it changes the outcome in twenty years.
Mistakes that make the parts not fit
- Cutting the glass to the rabbet size. Glass, mat and backing are all cut to the mat outer size. The rabbet is deliberately 1/8 in larger so the package moves; glass cut to the rabbet has nowhere to go and will crack when the frame expands.
- Measuring the moulding face wrongly. The face width in this calculator runs from the outside edge to the rabbet lip. Measuring the full moulding width instead adds about a quarter inch per side to the outside dimension.
- Sizing a mat to the image rather than the sheet. If your print has a white margin you intend to show, the opening should be measured from the paper edge, not the printed area, and the overlap works from the sheet.
- Ignoring the rabbet depth. Width arithmetic is only half the fit. Glass (2.5 mm) plus mat (4-ply, about 1/16 in) plus backing (3/16 in foam core) needs roughly 3/8 in of rabbet depth; a shallow moulding will not close.
- Ordering moulding without kerf allowance. Eight mitre cuts at 1/8 in remove a full inch. Add it, then round up to the next whole foot.
- Forgetting the multi-opening gap. With three openings there are two gaps, not three. The formula uses (n − 1), and a mat cut on the wrong count is a mat you cannot reuse.
Ready-made, custom, or cut your own
Three routes lead from these numbers to a hanging picture. Ready-made is cheapest and fastest: pick a frame from the reference table, accept the borders the size dictates, and buy a pre-cut mat or cut one with a hand-held bevel cutter and a straightedge. Custom gives you the exact borders this calculator produced and any moulding in the shop's catalogue, at several times the price; take the mat opening, mat outer and rabbet figures to the counter and the conversation takes two minutes instead of twenty. Cutting your own from a 32 × 40 in sheet is worth it from about the third frame onwards, and the arithmetic above is the whole of the setup.
For a gallery wall, work in outside dimensions only. Lay every frame's outside rectangle out on the floor, keep 2–3 in between frames, and treat the group as one shape whose centre lands at the 57–60 in eye line. The wallpaper roll calculator covers the wall behind it, and the shelf spacing calculator handles the picture ledge if you decide to lean the frames instead of hanging them. If the frames are going into a fitted alcove, check the opening against the available-versus-required logic before you cut anything.