How to Measure Artwork for Custom Framing (and Build a Cut-Sheet)

Most framing mistakes are not glue mistakes or finishing mistakes. They are measuring mistakes that happened twenty minutes earlier, at the bench or behind the counter, and only revealed themselves at the saw or under the glass. A mat opening cut an eighth too wide leaves a gap of unframed paper. A piece of glass cut to exact size binds in the rabbet and pops a corner. A moulding length cut without enough allowance for the rabbet leaves a frame that will not close. None of these are difficult problems. They are arithmetic problems, and arithmetic done by hand under a queue of customers is where errors live.

This guide walks through the full chain of measurements a custom frame requires: from the artwork, to the mat opening, to the glass and backing, to the frame outer size, and finally to the moulding length you cut at the chop saw. The numbers here are conventions, not laws — shops and mouldings differ, and your own standards should be written down and applied consistently. What does not differ is the order in which the sizes depend on one another, and getting that order right is the whole job.

Start with the right size: artwork, sight, and image

Before you measure anything, decide which size you are measuring. The three that matter are easy to confuse.

• Artwork size is the full physical dimension of the piece — the whole sheet of paper, the whole canvas, the whole print, edge to edge. This is what you would measure with a steel rule across the actual object.
• Image size is the printed or painted area only, ignoring any border of blank paper around it. A 16×20 sheet might carry a 14×18 image with a one-inch white margin.
• Sight size is what the viewer will actually see once the piece is framed — the area left visible inside the mat opening or, in a matless frame, inside the rabbet. Sight size is always smaller than the artwork size by however much the mat or rabbet covers.

You frame to one of these intentionally. If you want a clean printed image to “float” with its paper margin showing, you mat to the artwork size and let the margin breathe. If you want the mat to crop into the image — common with photographs where the edge is uneven — you mat to a chosen sight size inside the image area. Either way, the number you carry forward into the mat-opening calculation is a deliberate decision, not whatever the rule happened to read.

Measure carefully. Use a steel rule, measure to the nearest sixteenth, and measure both dimensions in two places — paper is rarely square, prints get trimmed crooked, and canvases bow. Take the larger reading if a discrepancy will not resolve, because a frame that is slightly too big can be packed out while one that is too small will not accept the work at all. Record the size width-first or height-first consistently, and never mix the convention within a shop.

The mat opening: where overlap lives

If the piece is matted, the mat opening is the single most error-prone number in the whole job, because it is the only one where you deliberately make the cut smaller than the thing it surrounds.

The principle: the mat opening must be cut slightly smaller than the artwork (or smaller than your chosen sight size) so that the mat board overlaps the edges of the art and holds it from showing a gap. If you cut the opening exactly to the artwork size, the smallest shift in mounting, the slightest paper expansion, or any imprecision in the cut will leave a sliver of bare mounting board or a white edge peeking through. That sliver is glaring, and it is the classic giveaway of a rushed job.

The conventional overlap is small — roughly 1/8” to 1/4” of the mat lapping over each edge of the art, with 1/8” being typical for prints with a clean printed-to-edge image and a slightly larger lap used when you want to be sure no border shows. To get that overlap, you subtract twice the overlap from each dimension when you set the opening:

• Opening width = sight (or image) width − (2 × overlap)
• Opening height = sight (or image) height − (2 × overlap)

So for a 14×18 image with a 1/8” overlap on all sides, the opening would be cut at 13¾ × 17¾. The art shows 13¾ × 17¾ of itself; the remaining 1/8” all around hides behind the mat bevel.

Two errors dominate here:

1. Opening too tight (too small). Over-crop the image and you lose part of what the customer wanted to see, sometimes lopping a signature or a horizon line. Worse, an opening cut very tight to a piece you intend to float can clip detail nobody agreed to lose.
2. Opening too loose (too large). Cut the opening larger than the art and the overlap vanishes — now the mat does not cover the edges, and you see bare paper border, mounting tape, or hinge. This is the failure that forces a fresh sheet of mat board and a re-cut.

Decide your overlap, apply it the same way every time, and write the opening dimension down before you go near the mat cutter. If you are double-matting, remember the bottom mat’s opening is cut smaller than the top mat’s so a reveal of the bottom mat shows — that reveal is typically 1/4” but is a deliberate, separate choice. The full mechanics of single versus double mats, and how the reveal changes the opening math, are covered in the single vs double mat guide.

Mat outer size and the frame rabbet

The mat’s outer size is set by how wide a border you want around the opening — the visible mat margin. A common choice for a print this size is a 2” to 3” border, but it is entirely an aesthetic decision driven by the room, the art, and the customer’s eye.

• Mat outer width = opening width + (2 × border)
• Mat outer height = opening height + (2 × border)

The mat outer size matters because it defines the size the frame must hold. The frame does not wrap around the mat’s visible face; it grips the mat (and glass and backing) inside its rabbet — the L-shaped recess cut into the back inner edge of the moulding. The rabbet is what the package sits in and what the moulding’s lip hides.

This is where the first quiet error appears: people compute the mat outer size and then cut the frame to exactly that, forgetting that the glass, mat, and backing all have to drop into the rabbet, and the rabbet itself eats a little of every edge. The frame’s interior — measured rabbet to rabbet — must be very slightly larger than the package it holds, so the contents seat without binding.

Glass and backing: cut slightly under, never over

Glass and backing board are sized to the same nominal dimension as the mat package — but here is a rule that saves more re-cuts than any other:

Cut glass and backing slightly under the rabbet size, not to it, and never over it.

The reasons compound:

• Glass cut to the exact interior dimension has nowhere to go. Wood moves with humidity, the rabbet is never perfectly square to the thousandth, and a piece of glass under even slight compression can bind, bow, or crack — sometimes weeks later when the season changes. A clearance of roughly 1/16” under the rabbet size on each dimension lets the package seat cleanly and allows for expansion.
• Backing board (foam board, acid-free backing) wants the same small clearance so it lies flat and does not buckle against a tight frame.
• Glass cut even slightly over size simply will not enter the rabbet, and there is no recovery — it goes back to be re-cut, wasting material and time.

So if the rabbet interior is, say, 21 × 25, you would cut glass and backing closer to 20⅞ × 24⅞. The exact under-cut is a shop convention — somewhere in the 1/16” to 1/8” total range is common — but the direction is not negotiable: under, never over.

Note also the rabbet depth. The rabbet has to be deep enough to swallow the whole stack — glass, mats, art, backing, and any spacers — and still leave a lip. If you are building a deep package (multiple mats, a thick backing, a shadow-box spacer) on a shallow moulding, the contents will stand proud of the rabbet and the points or fitting will not hold them. Check the depth against your package, not just the width.

Frame outer size and why the moulding’s face matters

The frame’s outer size — the overall footprint of the finished frame — is the rabbet interior size plus the width the moulding adds on each side. The moulding has a face (the visible front width) and a rabbet set back from it; the outer size is driven by the full face width, since each stick adds its face on both the left/right and top/bottom.

For most purposes you do not even need the outer size to build the frame — you need it to quote the footprint to the customer, to size a shipping box, and to sanity-check that the piece fits where it is going. What you genuinely need to cut the frame is the next number, and it depends on the moulding’s geometry directly.

Chop length and corner allowance

A picture frame is four sticks of moulding cut at 45° and joined into mitred corners. The length you cut each stick to is not the frame’s outer dimension and not the rabbet dimension — it is the rabbet (sight) dimension plus an allowance that accounts for the moulding’s own rabbet and face geometry at the mitre.

Think about a single corner. The 45° cut runs across the full face of the moulding. The rabbet sits behind the face. For the rabbet opening of the assembled frame to equal the size your glass package needs, each length of moulding has to be longer than that rabbet opening by enough to cover the rabbet width at both ends. That extra is the allowance (sometimes called the corner or chop allowance).

The standard way framers express it:

• Chop length per side = rabbet (sight) dimension + (2 × allowance)

where the allowance is tied to the moulding’s rabbet width — effectively, how far the rabbet sits in from the outer edge of the face. A narrow moulding with a small rabbet needs a small allowance; a wide moulding with a deep face needs a larger one. Because the allowance depends on the specific moulding, it is not a single universal number; it comes from the moulding’s profile and is something you should record per profile in your shop.

The consequences of getting the allowance wrong are immediate and physical:

• Undercut (too short). The mitres meet but the rabbet opening comes out smaller than your glass package — the contents will not drop in, or the corners gap because the sticks were forced. You cannot lengthen a cut stick. That stick is scrap, and on an expensive moulding an undercut is a costly mistake.
• Overcut (too long). The rabbet opening is larger than the package, so the glass, mat, and backing rattle inside the frame. The corners may also fail to close tightly because the joint geometry no longer matches, leaving visible gaps at the mitres. Overcutting wastes the extra length and produces a loose, sloppy frame.

This is why the chop allowance, more than almost any other number, rewards consistency and punishes mental arithmetic. It is also why many shops buy chop service from a distributor — ordering the four legs cut to length — precisely to push the allowance calculation onto someone whose only job is to get it right. When you cut your own, the allowance has to be computed from the moulding in hand, every time.

If your business depends on getting these numbers and the resulting price right, the companion piece on how to price a custom picture frame and the breakdown of united-inch pricing explain how the same measurements feed the quote.

A worked example: 16×20 print, 2.5” mat

Walk a real job through the whole chain. Assume a 16×20 print that we will frame to its full artwork size (no cropping into the image), a 2.5” mat border, a 1/8” mat overlap, and a moulding whose rabbet/allowance we will state explicitly. Every number below is illustrative; your shop’s conventions may differ, but the method is the point.

1. Artwork size: 16 × 20 (we treat this as the sight basis — the full sheet shows up to the mat).

2. Mat opening (overlap 1/8” each edge, so subtract 1/4” from each dimension):

• Opening width = 16 − 0.25 = 15¾”
• Opening height = 20 − 0.25 = 19¾”

The mat overlaps 1/8” onto each edge of the print, hiding the very margin and any uneven trim.

3. Mat outer size (2.5” border on all four sides, so add 5” to each opening dimension):

• Mat outer width = 15¾ + 5 = 20¾”
• Mat outer height = 19¾ + 5 = 24¾”

This 20¾ × 24¾ is the size of the mat package — the dimension the frame’s rabbet must hold.

4. Glass and backing (cut ~1/16” under the package on each side, so subtract ~1/8” total per dimension):

• Glass / backing width ≈ 20¾ − 1/8 = 20⅝”
• Glass / backing height ≈ 24¾ − 1/8 = 24⅝”

Cut to these, the glass and backing seat into the rabbet with room to breathe. Cut to 20¾ × 24¾ exactly and you risk binding; cut over and they will not go in at all.

5. Frame rabbet (sight) size. The rabbet opening of the assembled frame should equal the mat package plus a hair of clearance so it drops in — call it 20¾ × 24¾ as the nominal rabbet size to plan around (some shops add ~1/16” overall clearance here too; record your convention).

6. Chop length. Suppose the moulding’s profile calls for an allowance of 1/4” per end (i.e., its rabbet sits 1/4” in from where the cut matters). Then:

• Long sides chop length = 24¾ + (2 × 0.25) = 24¾ + 0.5 = 25¼”
• Short sides chop length = 20¾ + (2 × 0.25) = 20¾ + 0.5 = 21¼”

You cut two sticks at 25¼” and two at 21¼”, each mitred at 45° on both ends. Assembled, the rabbet opening comes back to roughly 20¾ × 24¾, which holds the mat package, which holds the 15¾ × 19¾ opening, which shows the print.

7. Frame outer size (for the quote and the box) is the rabbet size plus twice the moulding face width. If this moulding has a 1.5” face, the outer footprint is roughly 20¾ + 3 = 23¾ by 24¾ + 3 = 27¾ — a 23¾ × 27¾ finished frame.

Notice how every number traces back to two inputs: the artwork size and the mat choices. Change the overlap and the opening shifts. Change the border and the package shifts, which shifts the glass, the rabbet, and the chop. Change the moulding and the allowance shifts the chop again. Get any one of them wrong and the error rides downstream to the saw.

The cut-sheet: what the bench actually needs on paper

A cut-sheet is the single piece of paper that travels with the job to the bench. Its purpose is narrow and important: to hand the person at the mat cutter and the saw the exact numbers, so nobody re-derives them under pressure and nobody re-cuts an expensive stick.

A clear cut-sheet for the example above carries, at minimum:

• Job and customer identifier — so the right sheet meets the right materials.
• Artwork size — 16 × 20, and a note of how it is framed (to sheet, floated, cropped to sight).
• Mat: colour/material, opening 15¾ × 19¾, outer 20¾ × 24¾, overlap 1/8”. For a double mat, the bottom-mat opening and reveal too.
• Glass: type (regular, conservation, non-glare) and cut size 20⅝ × 24⅝.
• Backing: material and cut size 20⅝ × 24⅝.
• Moulding: profile/SKU, chop 25¼ (×2) and 21¼ (×2), allowance used, face width, rabbet depth check.
• Frame outer size for the quote/box: 23¾ × 27¾.
• United inches for pricing reference, and any notes (spacers, fillet, mounting method).

Why this discipline pays off:

• It prevents re-cuts. Every dimension is decided once, written once, and read off — not recalculated at the saw with a customer waiting.
• It separates decisions from execution. Whoever measures and prices makes the choices; whoever cuts simply executes the sheet. That hand-off is where consistency comes from.
• It is a record. If the customer reorders, or a corner fails, the sheet tells you exactly what was built.
• It reduces waste. Material — especially moulding and conservation glass — is the expensive part of framing. A clean sheet is the cheapest insurance against scrapping a stick.

The format barely matters as long as it is consistent. What matters is that the numbers on it are right and that they are produced the same way every job. Keeping these sheets and quotes organised digitally fits naturally into a paperless office workflow, and the broader habits in running a small framing business from a phone and a Mac apply directly.

Where Mitre fits

Everything above is correct, mechanical, and exactly the kind of work that goes wrong when it is done by hand on a busy counter. The chain of dependencies — artwork to opening to package to glass to rabbet to chop — is short enough to do in your head and long enough to make an error in. The cost of one undercut stick of premium moulding can exceed a day’s profit on a small frame.

Mitre — Picture Framing Studio is a native app for iPhone, iPad, and Mac built to remove that arithmetic from the bench. You enter the artwork size and your mat and reveal choices, and it derives the professional measurements automatically: united inches, glass and backing size, mat openings, frame outer size, and the moulding chop length with corner allowance. It shows a live, to-scale proof so you can see the layout before you cut, works in inches or centimetres, and prints the numbers on a shop cut-sheet alongside the customer quote PDF — the exact paper the bench needs. It runs fully offline, with no account, no cloud, and no subscription — a one-time purchase of around $14.99.

Mitre — Picture framing quotes & proofs Download

The point is not that the math is hard; it is that doing it repeatedly, by hand, under time pressure, is where re-cuts come from. Letting the app derive and print the cut-sheet means the only thing you do at the saw is cut to the number on the page.

For the wider toolkit a frame shop runs on, these pair naturally with Mitre. If you produce templates, layout guides, or oversized proofs, XLPrinter (Print Posters & Blueprints Tiled) tiles a large cut-sheet across ordinary letter pages so you can print at full size without a wide-format printer. Photo to PDF turns a photographed job sheet or reference image into a clean PDF for the file, and PDF Compressor keeps emailed quotes small enough to send. When you want the numbers out of the app and into a spreadsheet — a price list, a materials order, a year-end tally — CalXport exports tabular data cleanly. For the full picture, see the hub: the best apps for picture framing and frame shops.

A short checklist before you cut

Confirm which size you are framing to and write it down. Set the mat opening by subtracting twice your overlap (typically 1/8”–1/4”) from each dimension, and never cut the opening larger than the art. Add your border to get the package size. Cut glass and backing slightly under the package — never over. Check the rabbet depth against the full thickness of your stack. Compute chop length as the rabbet dimension plus twice the moulding’s allowance, recorded per profile. Then put every number on the cut-sheet and recalculate nothing at the saw.

Do those things the same way every time and the re-cut pile shrinks to almost nothing. The measurements are not difficult. They are only unforgiving — and the discipline of a clean, consistent cut-sheet is what makes them reliable.

Frequently Asked Questions

How much should a mat opening overlap the artwork?

A common convention is 1/8” to 1/4” of mat lapping onto each edge of the art, with 1/8” typical for clean prints and a slightly larger lap when you want to be certain no bare border shows. You achieve it by cutting the opening smaller than the art — subtract twice the overlap from each dimension. The exact figure is a shop convention; pick one and apply it consistently so jobs match.

Why cut glass slightly smaller than the frame instead of exactly to size?

Wood moulding moves with humidity, rabbets are never perfectly square, and glass cut to the exact interior dimension can bind, bow, or crack as conditions change. A clearance of roughly 1/16” under the rabbet size on each dimension lets the package seat cleanly and allows for expansion. Glass cut over size simply will not enter the rabbet and must be re-cut, so the rule is always under, never over.

What is corner (chop) allowance and why does it matter?

Chop allowance is the extra length added to each moulding stick so the assembled rabbet opening equals the size your glass package needs. It accounts for the moulding’s own rabbet and face geometry at the 45° mitre. The allowance depends on the specific profile — wider mouldings with deeper faces need more — so it is not a single universal number. Undercutting scraps the stick; overcutting leaves a loose frame with gapping corners.

What is the difference between sight size and artwork size?

Artwork size is the full physical dimension of the piece, edge to edge. Sight size is what stays visible once it is framed — the area inside the mat opening or rabbet, always smaller because the mat or rabbet covers some of the edges. You decide which one drives your mat opening: frame to the artwork size to show a paper margin, or to a chosen sight size to crop into the image.

What should a framing cut-sheet include?

At minimum: a job/customer identifier, the artwork size and how it is framed, the mat material with opening and outer sizes and overlap, the glass type and cut size, the backing material and cut size, the moulding profile with chop lengths and allowance, the frame outer size for the quote, and united inches for pricing. The goal is that whoever cuts simply reads the numbers and recalculates nothing.

Can an app calculate all of this for me?

Yes. Mitre — Picture Framing Studio takes the artwork size and your mat and reveal choices and derives the united inches, glass and backing size, mat openings, frame outer size, and chop length with corner allowance, then prints a shop cut-sheet alongside the customer quote. It works fully offline with no account or subscription, which removes the hand arithmetic that causes re-cuts at the saw.

How do double mats change the measurements?

A double mat adds a second board beneath the top mat, with its opening cut smaller so a reveal of the bottom colour shows — commonly around 1/4”, though it is a deliberate choice. The bottom-mat opening, the reveal width, and the resulting package thickness all need to be on the cut-sheet, and the deeper package means checking your rabbet depth. The full mechanics are covered in the single vs double mat guide.