Measure the inside glass size (1) of the frame, width and height, and subtract the width and height of the mat window opening respectively, as determined in Step Two above. Finally, divide these results by 2 to get the evenly centered mat margins.
For example, if you have a ready made frame that is 14" wide by 11" high and you are determining the borders for the 9 3/4" x 7 3/4" window opening as in the example above, you calculate as follows:
14" 9 3/4" = 4 1/4" -- divided by 2 = 2 1/8" (The mat will have a border of two and one eighth inces on the right and left side. See example diagram.)
11" 7 3/4 " = 3 1/4" divided by 2 = 1 5/8" (The mat will have a top and bottom border of one and five eighths inches each side. See example diagram.)
Note 1 (glass size): In general, the inside of a frame has an "allowance" of 1/8 inch to allow for minor irregularaties in glass, backing and mat. In other words, the materails cut for a frame should be about 1/8 inch less than the actual inner dimensions of the frame. This is why we suggest using the "glass size" for your calculations.
Note 2 (mat function): The practical function of a mat is to create an air space between the artwork and the glazing material used in the frame. This is especially important for sensitive artwork because humidity can condense on the glazing surface on the inside of the frame and, thus, cause damage to the art.
Note 3 (bottom weighting): As a general rule, we add a bit more space to the bottom mat margin than the other three sides. Partly this is a matter of common convention, and to a certain extent it is to also overcome a slight optical illusion: When a mat is the same width on all four sides, the image tends to look slightly closer to the bottom. Bottom weighting compensates for this illuison. It also gives more emphasis to the art work by presenting it higher in the frame.
You know the mat window opening size as calculated in Step Two above. Now you need to consider how wide you want your mat margins to be. This is an aesthetic and design decision for which there are no absolute rules. In general, besides the practical function of a mat, (2) the "aesthetic" function of a mat is to create a visual space between the art and the frame. The smaller the mat, the more it looks like a "stripe" around the image and this can create a visual dynamic that competes with the image. We want the image to be the center of visual focus. So, as a general rule, we recommend between 3 and 4 inches for mat margins on each side. If you have more wall space and/or want to make a more dramatic presentaiton, or if you are using Cut Art or Specialty mat designs in your mat, you might want to increase those dimensions considerably.
Please note that borders cannot be cut to any measurement less than 3/4. For the best aesthetic results we recommend your borders should be at least 1 1/2.
In any case, once you've determined the mat margin you want, simply add that number to the height and width of your mat window. In the 10" x 8" photo above, we have a mat window opening of 9 3/4" wide by 7 3/4" tall. We have a mat that is 3" on each side, 3" on the top, and bottom weighted to 3 1/2" on the bottom (3). Consequently, the outside dimension of our mat will be 15 3/4" wide and 14 1/4" high.
Does this make sense? If not, either e-mail us or give us a call 510-849-4444 and ask to speak with a manager about your mat design and measurements.
Specialty mats usually need some design advice, and we are here to help you with your mat and framing project.