Why Gallery Mats Have a Wider Bottom Border
Look closely at framed work in a gallery and you will notice something odd about the mats: the bottom border is often wider than the other three. It is not a mistake, and it is not decoration. It is a correction for how the eye reads a rectangle.
A mat with four exactly equal borders looks bottom-light — the image appears to sit slightly low in its opening, as though it is sliding down. Adding a little extra to the bottom makes it look centred. The convention is called a weighted bottom, and it has been standard framing practice for a very long time.
How much extra
There is no fixed rule, but the common range is 10% to 25% more on the bottom than on the sides. On a 2-inch mat, that means a bottom border somewhere between 2.2 and 2.5 inches, with the top and both sides staying at 2.
The larger the mat, the more the weighting can carry. On a narrow 1-inch border the effect is barely perceptible and probably not worth the complication; on a 4-inch gallery mat it is clearly visible and clearly better.
What it does to the outer dimensions
Weighting the bottom adds to the height only, and only once rather than twice — the extra goes on the bottom edge alone. An 8×10 print with 2-inch borders and a half-inch weighting produces a 12×14.5 opening rather than 12×14.
Since the mat border calculator applies one border width evenly on all sides, the way to use it here is to compute the even case first and then add the weighting to the height by hand. The width is unaffected.
Where the illusion comes from
The usual explanation is that the eye's centre of a rectangle sits slightly above its geometric centre, so a shape placed at the true centre reads as low. The same effect drives the convention of setting type slightly above centre on a page and mounting signage a little higher than the arithmetic suggests.
Whether or not that account is complete, the practical observation is robust: people asked to place an image so it "looks centred" in a rectangle consistently place it above the middle. Weighting the bottom is simply building that preference into the mat.
When not to weight
Three cases where an even mat is the better choice. Square artwork in a square mat, where weighting reintroduces an asymmetry the composition has deliberately avoided. Work that will be hung well above eye level, where the viewing angle already tilts the perception. And grids of small frames, where consistency across the group matters more than the refinement within each one.
The last of these is the common one. A gallery wall of nine identical frames looks better with nine identical even mats than with nine weighted ones, because at that scale the eye reads the grid, not the individual mat.
Decide before cutting, not after
Weighting cannot be added later. It changes the frame opening's height, which changes the frame, which changes the moulding order — so it belongs in the first pass through the numbers, alongside the border width itself.
If you are unsure, cut a test mat from cheap board and look at it on the wall from across the room. The effect is one of those things that is difficult to judge from a drawing and immediately obvious in the object.