Are you saying that you want to frame a piece of glass, mirror or other object, rather that cutting the glass to to fit the frame? If so, use this formula.
The first thing you must do some measuring on the frame. In the first attached photo, you will see the profile of a frame. Obviously, your dimensions will be different than the example.
Take two small pieces of the frame and lay them back to back.
In this example, the frames width is 2", the overall width of the two pieces is 4". But, in this case, the overall width is meaningless. The rabbet depth is 5/8". Now, measure the distance between the two rabbets.
You will see that the distance between rabbets is 2-3/4"
Lets say the object that you want to frame is 10 x 12. Measure one side of the object that you want to frame.
Add the measurement between the rabbets (2-3/4") to the measurement of one side of object (10"), plus 1/8" to allow for wood movement or out of square glass. The length of the side, when measured at the long points, will be 12-7/8". (2-3/4" + 10" + 1/8" = 12-7/8"
Now, do the same thing with the other dimension. (2-3/4" + 12" + 1/8" = 14-7/8"
Hope this helps.