This is how you calculate the overall measurements of a frame. A simple guide for measuring a photo.
Lay your picture on a flat work surface and check it is square with a set square. Measure both the parallel and vertical sides and take the shortest measurement as your picture size.
To get the final window size you need to subtract 5 mils from both the top and sides.
The size of the frame is the sum of the size of the picture plus the width of its surrounding mount card, which by convention, is typically between 45 mils and 50 mils for an average-sized artwork.
Then add an extra 5 mil to the vertical measurement of the surrounding mount card to get the final size of the framed picture excluding the width of the moulding chosen.