Mat Size is the same as the picture frame size (glass size from the back side of frame). A 16x20 Mat fits into a 16x20 picture frame.
Mat size is generally 2-6" larger than photo size. A 12x16 photo commonly fits into a 16x20 Mat (4" larger total, which is approx 2" mat borders on Each side)
Smaller photos, 3x5 or 4x6, commonly do not need as much matting. A 4x6 photo can fit nicely into a 5x7 Mat. Note, that only leaves a narrow .50" half inch mat border all around however.
Opening Size is generally .50" half inch smaller than the photo size. This reduction allows the photo to be mounted behind the Opening and not fall through.
A half inch may seem like a lot, but remember that is only a .25" mat overlap on each side. The width of a pencil. Very common and typical.
11x14 photo would need a 10.5x13.5 Opening size.
!! Documents, diplomas items with design elements close to edge of paper, or small 4x6 or smaller prints, can have an Opening .25" smaller. 8x10 diploma would use a 7.75x9.75 Opening request. 3.5x5 photo would have a 3.25x4.75 Opening request.
PRO TIP - if you choose a Custom Opening size on a product, we will cut it exactly as you type it in. We do not automatically reduce. That's why it is Custom.
If you choose a "standard" opening from the Opening Size dropdown box, example, 11x14 (10.5x13.5), the actual opening size cutout is the dimension in (10.5x13.5), to hold an 11x14 print.
Mat Boarder Size is the difference of the Mat Size and the Opening Size. THEN divided by 2.
16x20 Mat with a 12x16 (11.5x15.5) Opening will have 2.25" mat borders on each side.
16" Mat - 11.5" Opening = 4.5" total. However, that is 2.25" left side, 2.25" right side.