To determine the opening of the mat, measure the artwork. The mat must cover at least 1/8″ of the artwork on all sides, in order to prevent the edges from peeking through or actually falling through the mat opening. More of the edges of the artwork may be covered if desired, but the very minimum is 1/8″.
Consequently, if a photograph measures 8″ x 10″, the opening will be 7-3/4″ x 9-3/4″, or perhaps 7-1/2″ x 9- 1/2″. Always check the exact image size and where the opening will be positioned on the artwork. After the proper opening is established, add the amount of matting desired.
1. Add the amount of both side borders to the horizontal opening measurement.
2. Add the amount of both the top and bottom borders to the vertical opening measurement.
It is often useful to add extra matting to the border size so the measurements become round numbers which are easier to work with. For example:
An 8″ x 10″ photograph has an actual opening of 7-3/4″ x 9-3/4″. With a 2″ mat border on all four sides, the outside mat edge will be 11-3/4″ x 13-3/4″.
By adding 1/8″ to each border (now 2-1/8″), the result is a 12″ x 14″ mat, which will also be the size of the frame and glass.