Sizes and the stack
Three calculators: the window, mat and frame sizes derived from your sheet; the nearest ready-made size above and below it; and how deep the whole package will be once everything is in it.
Each box does arithmetic on the figures named under it. None of them has seen your sheet: measure to the sixteenth of an inch or to the millimeter, twice, and on both diagonals, and write the result in Your pictures before you order anything. Measuring and the parts of a frame explains every number these boxes ask for.
1. The window, the mat and the frame
Enter the sheet, choose how far the window overlaps it, and set the three border widths. The box returns the window opening, the mat outside size (which is also the glazing and backing size), and the frame to order.
Assumptions
- The window is cut smaller than the sheet so the mat holds the edge down. The overlap default of 1/8 inch is our convention, not a published rule. One conservation institute says the window must overlap the artifact by at least 2 to 4 mm to hold the work down; one US maker cuts its openings 1/4 inch smaller for artwork between 4 by 6 and 18 by 24 inches, which is 1/8 inch of overlap per side, and 1/2 inch smaller above that size, which is 1/4 inch per side, in its own published note on mat openings. The UK trade body's specification gives 5 to 8 mm for the hinge overlap on the other face. Convention
- Mat outside size = window + the side borders, and + the top and bottom borders. The glazing, the boards and the backing are all cut to that size; the frame's rabbet is about 1/8 inch larger so the package can move. A preservation center asks for 1/16 to 1/8 inch of total play between the boards and the frame, so the boards can move with humidity without pressing on the rabbet. One practitioner's supplement, hosted by a national library, reports board expanding 1/8 inch over a 40 inch length with a ten percent rise in relative humidity. Physical principle
- Border widths are a judgment. A federal leaflet gives margins of 2 to 7 inches, with 4 to 5 inches suiting a 24 by 30 inch object and 2 to 3 inches a 5 by 7, and says either all four equal or equal top and sides with the bottom about 30 percent wider. The box reports the 30 percent figure beside whatever you chose. Convention
- The double-mat reveal is a design choice with no published figure. The one published constraint on a double mat is the trade specification's: where double-sided tape joins the two boards it must be kept no less than 12 mm from the bevel edge. Preference
- Not modeled: a sheet that is out of square, a cockled or buckled support, an old mat whose window has already marked the paper, a signature below the image you want to leave visible, or a float mount, where the window is larger than the sheet rather than smaller. When a mat is wrong covers the last of those.
The values above are Priya Raman's letterpress print: an 18 by 24 inch sheet, a 1/8 inch overlap, 3 inch borders with 3 1/2 at the bottom. The window comes out at 17 3/4 by 23 3/4 and the mat outside at 23 3/4 by 30 1/4, which is not a size anyone stocks. The next box is where she finds that out.
2. The nearest ready-made size
Takes the outside size from the first box and finds the nearest stocked frame above and below it, then works out what the borders would be if you cut a mat to suit that frame. Ready-made frames explains the route.
Assumptions
- The size list is a framing tool maker's published trade list: 4x5, 4x6, 5x7, 6x8, 8x10, 8x12, 8.5x11, 9x12, 11x14, 12x16, 14x18, 16x20, 18x24, 20x24, 22x28, 24x30, 24x36, 26x32, 30x40, 32x40, 36x48, 40x60, 48x60 and 48x96 inches, from Logan's guide to frame types. Read 2026-11-03. Product-specific
- Why those sizes: the same maker says they come from the photography industry, from sheet sizes of watercolor paper, and from the common sizes of stretched canvas, in its note on designing mats for standard sizes. They are not a standards body's sizes. A photographic lab's crop ratio table shows why a camera file fits some of them and not others: cameras default to 2:3, which gives 4x6, 8x12 and 12x18, while 5x7 and 8x10 are different ratios and crop. Product-specific
- A Canadian conservation institute gives a slightly different pre-cut list in its matting note: 8x10, 10x12, 14x18, 16x22, 22x28 and 28x36 inches. If you are buying there, check both.
- Nominal against actual: a frame sold as 16 by 20 has a rabbet about 16 1/8 by 20 1/8 inches, which is the clearance the glazing and boards need. Cut the glazing and boards to 16 by 20, not to the rabbet. Product-specific
- Both paper-size standards exist and neither can be reproduced here: ISO 216 is current at its 2007 edition, confirmed in 2021, and excludes posters from its own scope; ASME Y14.1-2012 is superseded by the 2020 edition. Both paywall their dimension tables, so this manual uses makers' size lists as dated product facts and states the date. Ask the authority
These lists are what two makers published on 2026-11-03, and a shop's own stock is the real answer. The box tells you how far your piece sits from a size anyone keeps, which is most of what makes a custom frame cost what it does; it does not tell you what a frame costs, and this manual gives no price figures because none were published anywhere in the research.
3. How deep the package is
Adds up everything that goes behind the glazing and compares the total with the one published figure for a complete package on paper. It also reports the gap between the art and the glazing, which is the number that matters most. Frames explains why depth is the first thing to check.
Assumptions
- The comparison figure is a conservation institute's: the frame's rabbet must be deep enough for mat, backing board and glazing together, about 2 cm in total, and it adds that metal clip-together frames are often not deep enough for all three. Convention
- Board thickness is derived, not published. We take a 4-ply board as 1/16 inch (about 1.6 mm), which is what the practitioner supplement hosted by a national library implies when it says a four-ply window over a four-ply back mat gives a space of 1/16 inch in front of the art. Boards vary between makers; measure yours with a caliper if the margin is close. The UK trade body's minimums are stated differently again, at 1100 microns for a mount and 1500 microns for a backboard. Test it
- Glazing thicknesses are makers' published figures: picture framing glass at 2.5 mm, conservation acrylic at 3.0, 4.5 and 6.0 mm, laminated glass at 4.4 and 8.8 mm. Product-specific
- The gap figure is the thing to watch. Every publisher in the research says nothing inside the frame may touch the glazing, and that the gap is made by the mat's thickness or by a spacer. For pastel, chalk and charcoal the trade specification asks for at least 5 to 6 mm and the conservators' own wiki asks for at least an eight-ply mat's worth of space. Physical principle
- Not modeled: the thickness of the art itself, a sink mat built up to the height of a cockled sheet, a float mount's spacer walls, a stretched canvas in the rabbet, or the frame's own back-securing hardware.
Hal Brewer's grandmother's watercolor, 11 by 14 inches, is being reframed. The package he is quoted is 2.5 mm glass, one 4-ply window mat, a 4-ply back mat, a 1/8 inch acid-free corrugated backing and a dust cover: about 11 mm in total, comfortably inside a 1/2 inch rabbet, with about 1.6 mm of space in front of the paper. Known: the sizes and the materials. Assumed: that a 4-ply board is a sixteenth of an inch. Open: whether the sheet is flat enough for 1.6 mm to be enough, which the framer settles with the sheet in front of them.
This box returns sizes and depths, not a cut list and not a specification. It does not know whether your sheet is out of square, whether an old mat has already marked the paper, or whether the frame you are holding is strong enough for what will be in it. A sheet that will not lie flat is a case for the framer, and trimming art to fit a frame is never this manual's answer.