Why the bottom border is cut deeper
Take a 16 by 20 frame and an 11 by 14 print with a quarter inch of overlap all round. The window is 10-1/2 by 13-1/2, which leaves 5-1/2 inches of board to divide across the width and 6-1/2 inches to divide down the height. Split the height evenly and you get 3-1/4 top and bottom. Hang it and it looks wrong — the picture appears to have slid down the mat.
Nothing has moved. The eye takes the centre of the opening as the centre of the composition and reads the board around it, and above the opening it sees 3-1/4 inches of mat plus whatever moulding sits above that, while below it sees the same 3-1/4 inches from a lower angle. Anything hung above eye level foreshortens the bottom border further. Both errors run the same direction, so the correction runs one direction too: more board at the bottom.
At the default 20 percent the calculator gives a top of 2-15/16 and a bottom of 3-9/16, which puts the centre of the window 5/16 of an inch above the centre of the board. That is a small number and it is the whole effect. Ten percent is a correction you can only see by measuring; thirty percent starts to read as a deliberate design choice. There is no correct figure and this page publishes none.
The three borders have to add up to the board
This is where mats go wrong on the bench rather than on paper. Round the exact borders independently — top 2.9545 to 2-15/16, bottom 3.5455 to 3-9/16 — and the two rounded numbers plus the window height come to exactly 20 by luck rather than by construction. Change the artwork by a sixteenth and they will not.
So the page rounds the side and the top to a sixteenth, which are the two you set a fence to, and then derives the bottom as whatever is left of the board height. The bottom border is the one that absorbs the rounding, which is also the one where an extra sixteenth is invisible. The column printed under the cutter settings adds up every time, because it is constructed to.
Overlap is what holds the piece up
The window is smaller than the artwork, not the same size. A quarter inch of lap on each side is the usual starting point, and it is doing real work: it is the only thing stopping the sheet from dropping through the opening. Cut the window to the sheet size and the piece is held by nothing but whatever is behind it.
The exception is float mounting, where the piece sits on top of a board with its edges showing and the mat window is larger than the sheet rather than smaller. That is a different geometry and this page will report negative overlap for it, which is the signal that you are in the other case and want the window sized from the reveal you want rather than from the sheet.
Multi-opening mats and the bar between windows
Two openings side by side do not simply double the board. The block of openings is two windows plus the bar between them, and that bar reads visually as a border of its own — which is why a 2 inch bar reads as thin next to a 2-3/4 inch side border. The bar is doing the work of two borders at once, one for each opening, so it has to be at least as wide as the side border before it stops looking pinched.
The arithmetic in the page treats the openings as one rectangle and then puts borders round it, so the outside borders stay consistent and the bars are yours to set. What it does not do is space unequal openings, and that is the harder job: a row of different sized windows on one mat is usually laid out on centre lines rather than on gaps, and it wants drawing before it wants calculating.
Getting the boards, and the blade
A 32 by 40 sheet gives four 16 by 20 mats with nothing much left, and two if you turn the mat the other way, so the orientation is worth a look on any run. The page reports the better of the two. What it does not model is colour and grain: two mats cut from opposite ends of the same sheet can differ enough to see when they hang side by side, which matters for a set and not at all for a single piece.
Blade life is the other thing worth counting. The bevel figure is the total length of cut in the job, and a blade that starts to drag leaves a hooked bevel that shows on every corner. Framers change blades far more often than the arithmetic suggests they should, and a scrap test cut before each mat is cheaper than a mat.
Questions people ask
How much deeper should the bottom border be?
There is no standard and this page states none. Framers who work in percentages tend to land between 10 and 30 percent deeper than the top, and the higher the piece hangs the more correction it wants. On the default 16 by 20 frame with an 11 by 14 print, 20 percent gives 2-15/16 at the top and 3-9/16 at the bottom — a difference of 5/8 of an inch, which puts the window centre 5/16 above the board centre. Set the figure to zero if you want four equal margins and the page will give you those instead.
Why is the window smaller than my artwork?
Because the mat has to lap over the edge of the sheet or the sheet falls through the opening. The overlap field is that lap, applied to each side, so a quarter inch of overlap takes half an inch off each dimension of the window. An 11 by 14 print gives a 10-1/2 by 13-1/2 window at that setting. If you want the edges of the piece visible you are float mounting rather than matting, which is the opposite geometry and wants the window sized from the reveal you want round the sheet.
Do I measure the frame at the rabbet or the outside?
The rabbet — the inside step the mat and glass drop into. The outside of the moulding is bigger than the rabbet by twice the face width and has nothing to do with the mat size. A frame sold as 16 by 20 normally means a 16 by 20 rabbet, but mouldings vary and a tape in the rabbet takes ten seconds. The moulding page works the two dimensions against each other if you have the profile figures and not the frame.
Why does the bottom border come out at an odd fraction?
Because it is derived rather than chosen. The side and the top are the two settings you actually dial into a cutter, so those round to a sixteenth, and the bottom is then whatever is left of the board height once the top and the window are accounted for. That guarantees the three numbers add up to the board every time. An extra sixteenth on the bottom border is the least visible sixteenth on the whole mat, which is why it is the one that absorbs the rounding.
How many mats will I get out of a board?
It depends on the sheet your supplier sells and on which way you turn the mat, and the page reports the better of the two orientations. A 16 by 20 mat gives four out of a 32 by 40 sheet and two if you turn it. Order at least one board over what the arithmetic says on any job where the corners matter — the first cut with a new blade is a test cut, and a hooked corner is not fixable after the fact.