How to cut a multiple-opening (collage) mat

The short answer

To cut a multi-opening mat, lay the windows out as a uniform grid — equal opening sizes, equal gaps (gutters) and equal outside margins — then compute each window's left, top, right and bottom distance from the mat edge and cut against a straightedge with a bevel mat cutter.

Uniform grids are far easier to cut than mixed sizes, so start there. The whole job is two pieces of arithmetic and a steady hand: solve the grid once, then turn it into a per-window cut list. If you would rather skip the longhand, the multi-opening mat calculator prints the full cut list for you.

Cutter context: straight cuts and bevel cuts

A straight-line cutter trims the outside of the board. A bevel cutter cuts the window, usually with the board face-down so the angled edge opens cleanly toward the front. Hobby framers most often meet Logan and Dexter-style cutters: simple rail or handheld tools that ride a straightedge and let you start and stop each window cut on a mark. No brand changes the math. The cut list still tells you where each bevel line begins and ends.

The layout in symbols

Everything sits on a grid of R rows × C columns. Each window (the aperture, or opening) is wo wide × ho high. Around the outside you leave margins — top t, bottom b, left l, right r — and between every pair of openings you leave a gutter g (the bar of mount board, or mat, that separates two windows).

The trick that makes a collage mat look professional is hiding in those names: hold the gutter constant and hold the margins constant, and the spacing reads as deliberate. The moment one gutter is wider than the next, the whole grid looks like it slipped. So the symbols are not bookkeeping — they are the guarantee that every gap is identical before a blade touches the board.

There are two honest ways to solve a grid, depending on what you want to fix.

Compose mode — fix the opening, find the outside

You know the picture size you want to show, so you fix wo × ho, the margins and the gutter, then let the outside dimensions fall out:

• outside_W = l + r + C × wo + (C − 1) × g
• outside_H = t + b + R × ho + (R − 1) × g

Fit mode — fix the outside, find the opening

You already have a frame, so the outside is fixed and you back into the opening size that fills it evenly:

• wo = (outside_W − l − r − (C − 1) × g) / C
• ho = (outside_H − t − b − (R − 1) × g) / R

Cut coordinates

Solving the grid tells you the openings and the outside, but a cutter needs something more concrete: where does each window start and stop? Number the grid from the top-left corner, 0-indexed, so the first opening is (row 0, col 0). For the window at row i, col j:

• x_left = l + j × (wo + g)
• y_top = t + i × (ho + g)
• x_right = x_left + wo
• y_bottom = y_top + ho

Those four offsets — all measured from the same top-left corner — are exactly what a hand cutter or a custom-cut order form asks for. Get them right and the bevel cuts are mechanical.

Notice the pattern in the formula: each step across a column adds one opening width plus one gutter, and each step down a row adds one opening height plus one gutter. That is why a uniform grid is so forgiving — once you have the first window placed, every other window is just the same stride repeated. There is no fresh measurement to fumble, only the same wo + g or ho + g jump again and again.

One more habit pays off here: many framers like a slightly larger bottom margin than the top — a touch of bottom-weighting — so the finished piece does not look like it is sinking. That only changes t and b; the gutter and opening math stay identical. Keep t and b unequal but every gutter equal and the grid still reads as tidy.

Worked example: a 2×2 grid of 4 × 6″ openings

Say you want four photos shown at 4″ wide × 6″ high in a 2×2 grid, with 2″ margins all round and a ½″ gutter. This is compose mode, so the outside falls out of the formula.

1. outside_W = 2 + 2 + 2 × 4 + 1 × 0.5 = 12½″
2. outside_H = 2 + 2 + 2 × 6 + 1 × 0.5 = 16½″

Now the cut coordinates. Take the bottom-right window — that is (row 1, col 1), 0-indexed:

• x_left = 2 + 1 × (4 + 0.5) = 6½″
• y_top = 2 + 1 × (6 + 0.5) = 8½″
• x_right = 6½ + 4 = 10½″
• y_bottom = 8½ + 6 = 14½″

Repeat for the other three windows and you have the whole cut list. You can sanity-check this against the multi-opening mat calculator, and if you only need a single window's border instead, the mat opening calculator handles that case.

The full cut list

Here are all four windows' offsets from the mat's top-left corner. This is the table you tape to the bench.

When to use the calculator's cut table

Use the cut table when you are marking a real board, sending a layout to a custom cutter, or checking that a hand-drawn collage plan has not drifted. The preview tells you what the mat should look like; the table tells your blade where to go.

In the 2×2 example above, the calculator row for window (1, 1) reads: left 6½″, top 8½″, right 10½″, bottom 14½″. On the back of the board, mark all four of those offsets from the same top-left corner. Then cut the left and right vertical lines between 8½″ and 14½″, and the top and bottom horizontal lines between 6½″ and 10½″.

Do not re-measure the second column from the right edge or the second row from the bottom edge. That feels intuitive in the moment, but it mixes reference corners and turns tiny ruler errors into uneven gutters. One origin, one table, one pass through the windows.

Cutting it, step by step

With the cut list in hand, the physical job is short. Budget about three-quarters of an hour for a first four-window mat — most of that is careful marking, not cutting. The cutting itself takes only a few minutes once the lines are true.

1. Choose the grid. Rows × columns, opening size, gutter and margins. Keep everything uniform for your first one.
2. Compute the outside (compose mode) or the openings (fit mode) from the formula above.
3. Compute each window's coordinates — the four offsets per opening.
4. Mark them on the back of the board, every window measured from the same top-left corner, using a square.
5. Cut with a bevel cutter and straightedge, watching the start and stop of each pass so the blade does not overrun.
6. Check. Hold the mat to the light; confirm even gutters, clean corners and the right opening sizes.

Common mistakes

• Uneven gutters. The eye catches a 1/16″ difference across a grid instantly. Equal gutters are what make a collage mat look made, not improvised.
• Mixing reference corners. Measuring some windows from the left edge and others from the right guarantees drift. Always measure every window from the same corner — the top-left in our formulas.
• Ignoring bevel overcut. A hand cutter cuts slightly past each corner by design, because the blade enters the board at an angle. Plan for it: start and stop on your marks deliberately, and undercut a hair rather than over-running, so the corner releases cleanly instead of tearing. A few practice cuts on scrap board teach this faster than any instruction.
• Going non-uniform on attempt one. Different opening sizes are far harder to cut and align. Uniform grids are much easier, and there is no shame in starting there.
• Marking the front. Mark and cut on the back so pencil lines stay hidden and the bevel opens the right way toward the artwork.

A quick glossary

The terms vary by country — mat in the US is mount in the UK and AU — so here is the shorthand used above. For the full set, see the matting & framing glossary.

Gutter (g)

The bar of board left standing between two adjacent openings. Keep every gutter equal.

Opening / aperture (wo × ho)

The cut window the artwork shows through. In a uniform grid, every opening is the same size.

Margin (t, b, l, r)

The border of board between the outer openings and the edge of the mat, on each of the four sides.

Before you commit a sheet of board, it is worth checking your single-window measurements too — the how to measure for a mat guide walks through overlap and reveal so your openings land exactly where you expect.