Framefit / the layout guide
A clear opening.
A deliberate border.
Framefit places one rectangular opening within one rectangular mat. It starts with the paper behind the opening, then makes the hidden overlap and visible borders explicit.
Paper size and opening size are different
Measure the actual sheet that sits behind the mat. Enter a uniform overlap at each edge: the amount of paper the mat will hide. The same overlap is taken from the left, right, top and bottom.
Opening width = paper width − 2 × overlap
Opening height = paper height − 2 × overlap
A 210 × 297 mm sheet with a 3 mm overlap produces a 204 × 291 mm visible opening. If your image area is smaller than the paper, first decide whether the mat should reveal paper around the image or crop into that area. The calculator cannot infer the picture's composition.
Zero overlap is available for measuring a flush opening, but it supplies no edge coverage. An overlap equal to half of either paper dimension is rejected because it leaves no opening.
Three ways to size the border
Existing mat, centred opening: enter the mat's outside width and height. The opening is centred independently on each axis. Left equals right and top equals bottom. All four borders will only match when the leftover width and height match.
Left = right = (outer width − opening width) ÷ 2
Top = bottom = (outer height − opening height) ÷ 2
Equal border: enter one visible border. Framefit derives the outer mat dimensions by adding that border on both sides of each opening dimension. This is useful when choosing a mat size rather than fitting an existing one.
Offset opening: retain a chosen outside size and move the opening. Positive horizontal offset moves it right, enlarging the left border. Positive vertical offset moves it up, enlarging the bottom border. A negative number reverses that direction.
Example 1: an A4 sheet in a 400 × 500 mm mat
Paper is 210 × 297 mm, with 3 mm overlap. The opening is 204 × 291 mm. The horizontal remainder is 400 − 204 = 196 mm, producing 98 mm left and right borders. The vertical remainder is 500 − 291 = 209 mm, producing 104.5 mm top and bottom borders.
These are distances from the outside mat edge to the finished opening edge on the face of the mat. They are not the paper's exposed edge position.
Example 2: an equal 50 mm border
Use the same sheet and overlap, then choose an equal 50 mm border. Outer width is 204 + 100 = 304 mm; outer height is 291 + 100 = 391 mm. Every visible border is exactly 50 mm. This derived size is not automatically a standard frame size.
Example 3: a wider bottom border
Return to the 400 × 500 mm mat and choose a vertical offset of +10 mm. The opening moves 10 mm upward. Its top border becomes 104.5 − 10 = 94.5 mm; its bottom border becomes 104.5 + 10 = 114.5 mm. Left and right remain 98 mm when horizontal offset is zero.
The word “bottom-weighted” describes that visual arrangement, not a fixed optical correction. Framefit applies the offset you enter; it does not guess which border looks best with your picture.
Why a layout can be rejected
The complete paper must remain inside the outside mat rectangle. A border therefore has to be at least as large as the paper overlap at that edge. If the opening fits but the paper would extend beyond the mat, Framefit reports the conflict instead of returning a plan with hidden overhang.
A negative offset is valid as long as all edges still fit. Inputs accept up to two decimal places in millimetres. Halving an odd hundredth can create a border with three decimal places, such as 50.005 mm; that value is retained rather than rounded down to a different layout.
Marking and cutting
The plan gives the finished opening rectangle, including its top-left position measured from the outside mat edge. Read it from the face: left and top measurements are not automatically mirrored for working on the back. If your cutting method uses reverse-side marks, convert those references deliberately.
The diagram is proportional on screen but is not a full-size cutting template. Printing scales it to the page. Use the written dimensions, measure your actual sheet and verify the cutter setup on suitable spare material. Blade angle, bevel, board thickness, overcuts, backing, mounting method, conservation choices and frame rebate allowances are outside this geometry model.
Save a plan with its assumptions
Copy the cut plan or print the result after a successful calculation. It includes the mode, paper, overlap, mat outside size, opening size and four borders. Your measurements remain in this tab, with no automatic account or server storage. Read how the page handles data.