Matting
Multi-Opening Mat Calculator
Design an even collage mat — set the grid, margins, and gutter and get the exact opening size, mat outside, and a per-opening cut list, in inches or centimeters.
Measured from the mat’s outer edge to the nearest opening edge.
- Mat outside
- 16″ × 20″ 40.6 × 50.8 cm
- Opening size (each)
- 5 3/4″ × 7 3/4″ 14.6 × 19.7 cm
- Grid
- 2 × 2 (4 openings)
- Outer margin
- 2″ 5.1 cm
- Gutter
- 1/2″ 1.3 cm
How the multi-opening mat calculator works
A collage mat is one sheet of mat board with several windows cut into it, so a single frame can show a whole group of photos. This calculator lays those windows out on an even grid and gives you the exact opening size, the mat’s outside size, and a per-opening cut list — the numbers you actually mark before cutting.
Two measurements set the layout: the margin (the border around the whole block of openings) and the gutter (the strip of mat between neighboring openings). Choose how many rows and columns you want, and the tool solves the rest in one of two modes. In Fit mode you fix the mat’s outside size and it solves the opening that fills the grid: opening width = (outside width − left − right − (cols − 1) × gutter) ÷ cols, and the same for the height. In Compose mode you fix the opening size and it solves the outside size to order: outside width = left + right + cols × opening + (cols − 1) × gutter. Use Fit when the frame is fixed; use Compose when the photos are.
Whichever mode you use, the tool returns a cut list: for every window it gives the left, top, right, and bottom distance from one corner of the mat. Marking each opening from the same corner — rather than measuring the gaps between them — is what keeps a grid from drifting as you work across it.
Even spacing is the whole point of a grid, and it comes from two habits: use a single gutter value everywhere, and keep the outer margins equal on the left, right, and top. The tool then shares the leftover board equally between the openings, so four windows — or nine — line up exactly. Choosing the grid itself is mostly about photo count and shape: a run of portraits reads well as a 1 × 3, a set of square phone photos suits a 3 × 3, and a mixed handful usually lands on a 2 × 2 or 2 × 3. If you frame internationally, the same piece is sold as a multi-aperture mount in the UK and Australia and a passe-partout in France — the math is identical, only the words change.
Worked example: a 2×2 collage on a 16×20 mat
Say you have four roughly 5×7 photos and a 16 × 20″ frame. Set a 2 × 2 grid, 2″ margins, a ½″ gutter, and turn on bottom-weighting. Fit mode solves four identical 5¾ × 7½″ openings: (16 − 2 − 2 − ½) ÷ 2 = 5¾″ across, and (20 − 2 − 2½ − ½) ÷ 2 = 7½″ down — the extra ½″ goes to a 2½″ bottom margin so the block sits optically centered. With equal borders instead, the same grid gives slightly taller 5¾ × 7¾″ openings — that is the assumption behind the layouts table below. Bottom-weighting matters more on a collage than on a single photo, because the eye reads the whole block as one image, and a row of windows drifting toward the bottom edge looks unbalanced.
The cut list then reads the windows out from the top-left corner: the first opening starts 2″ in and 2″ down; its neighbor starts 8¼″ in (2″ margin + 5¾″ opening + ½″ gutter); the bottom row starts 10″ down. Need a single window instead? The mat opening calculator handles one photo, and will it fit checks the frame.
Second example: a 2×3 grid of 4×6 photos
Now compose a mat around photos you already have: six 4 × 6″ prints, ¼″ overlap, a 2″ margin, a ½″ gutter. Each window is the photo minus the overlap — 3½ × 5½″. In Compose mode the tool solves the outside to order: 2 + 2 + 3 × 3½ + 2 × ½ across and 2 + 2 + 2 × 5½ + 1 × ½ down both land on 15½ × 15½″.
The cut list reads each window from the top-left corner. The first opening (R1·C1) runs 2″ to 5½″ across and 2″ to 7½″ down. Its neighbor R1·C2 starts at 6″ across — the 2″ margin plus one 3½″ opening plus the ½″ gutter — and the bottom row drops to 8″ down. Marking all six from that one corner keeps the grid from drifting.
Share or print your layout
Every change is written into the page’s web address, so the Share link button copies a URL that reopens this exact layout — mode, grid, margins, gutter, bottom-weighting, and the unit you were working in. It’s the supported way to send a design to a framer: paste the link and they see the same openings and cut list you do, with nothing to install.
Print sends the page — preview and full cut-coordinate table — to your printer so you can mark the board with the numbers in front of you, and Copy drops a plain-text summary of the outside, opening, margins, and gutter onto your clipboard. None of it leaves your browser.
Popular collage layouts
| Layout | Photos | Mat outside | Even opening |
|---|---|---|---|
| 2 × 2 grid | 4 | 16 × 20″ | 5¾ × 7¾″ |
| 1 × 3 row | 3 | 20 × 16″ | 5 × 12″ |
| 2 × 3 grid | 6 | 20 × 16″ | 5 × 5¾″ |
| 3 × 3 grid | 9 | 20 × 20″ | 5 × 5″ |
Common mistakes
- Confusing the gutter with the margin. The margin is the border around the whole block; the gutter is the smaller strip between openings. Set them separately — a too-thin gutter makes a busy grid look cramped.
- Eyeballing the spacing. Mark every window from one corner using the cut list, not by measuring the gaps. Stacking small measurement errors across a grid is what leaves openings visibly out of line.
- Forgetting the overlap. Each window is cut ¼″ smaller than its photo on every side so the mat holds the print — a 5×7 photo needs a 4½ × 6½″ window, not a 5 × 7″ one.
- Trying to mix opening sizes. This tool lays out a uniform grid; one repeated size is far easier to cut cleanly. Mixed-size collages are a later feature.
Frequently asked questions
How do you measure for a multiple-opening mat?
Measure each photo, then subtract your overlap — usually ¼ inch on every side — to get each window. Decide how many rows and columns you want, an even margin around the whole block, and a single gutter between openings. Enter those and the calculator solves the rest, so you never have to do the division by hand.
How do you space the openings evenly?
Use one gutter value between every opening and keep the side and top margins equal. In Fit mode the tool takes the mat’s outside size, subtracts the margins and gutters, and divides what is left equally — so every window comes out identical and the spacing is dead even. Uneven spacing almost always comes from eyeballing the gaps instead of using a single gutter number.
What is the difference between Fit mode and Compose mode?
Fit mode starts from the mat’s outside size: you set the outside, margins, and gutter, and it solves the opening that fills the grid. Compose mode starts from the opening: you set the window size you want, plus margins and gutter, and it solves the outside size you will need to order. Use Fit when the frame is fixed and Compose when the photo size is fixed.
Can the openings be different sizes?
Not yet — the calculator launches with uniform openings: one window size in an even grid. Mixed sizes are planned for a later version. Uniform layouts are also far easier to cut accurately, so a single repeated size is the right starting point for a hand-cut collage mat anyway.
What is a collage or multi-aperture mat?
It is a single sheet of mat board cut with several windows so one frame can hold a group of photos. You will see it called a collage mat, a multi-opening mat, or — in the UK and Australia — a multi-aperture mount. Same thing: one mat, many openings.
How do I cut a multiple-opening mat accurately?
Work from the cut list. For each opening the calculator gives the left, top, right, and bottom offset from one corner of the mat, so you mark every window from the same reference edge instead of measuring the gaps between them. Our guide on how to cut a multi-opening mat walks through marking and bevel-cutting step by step.
Is there a picture frame calculator for a gallery wall?
It depends on the look. For a gallery-wall arrangement inside a single frame, this multi-opening mat calculator lays out a grid of evenly spaced windows in one mat, with a per-opening cut list, so a group of photos hangs as one framed piece. For separate frames spread across a wall, size each frame on its own with the frame size or mat border calculator and keep a consistent gap — often 2 to 3 inches — between frames for an even arrangement. Either way, you get the exact sizes before you cut or buy.