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