for 8 x 10 picture opening will be 7 3/4" x 9 3/4"
and if you want 3" matting around the picture you add 6" to the opening size which your frame size will be 13 3/4" x 15 3/4" if you have frame, you should put the opening of the frame size in (out size of mat) and put your picture size in (opening) section and we calculate the rest
MAT OPENING MEANS YOUR PICTURE SIZE.
if you choose 8 x 10 as opening we will cut your opening 8 x 10 and make sure your opening is little smaller than your picture which will cover edge of your picture.
MAT SIZE MEANS MAT FINISH SIZE(FRAME SIZE)