To grasp basic measuring for picture framing, think of it like this. A frame is like a foot in a shoe. The size conforms to what fits inside of it. If you know the foot size, you know the shoe size. If you know the size of the contents you are loading into the frame, you know the frame size.
That’s because the frame size is always considered to be the size of the recess at the back of the frame, which is called “the rabbet”. The frame size is never considered to be the frame measured from edge to edge, in spite of the picture above.
A Simple Example
Let’s say your art, glass and backing measures 16″x20″ edge to edge. If you do not include a mat, your frame size will be 16″x20″. Simple as that.
However, when you add a mat, things get more complicated. That same 16″x20″ with a mat can be as large as 22″x26″, depending on how wide the mat’s borders are.
Adding a Mat
Let’s say the mats borders are 2″ wide. That’s 2″ on either side of the mat’s window. If the window is 16″ wide, then the width of the mat will be 20″ — because 2″+16″+2″= 20″. The mat’s height will be be 24″ — because 2″+20+2″ = 24″. Because the frame size will conform to the size of the mat, the frame size will be 20″x24″.
Typically, however, framers make the window of the mat smaller than the art. That way the art is supported along the edges by the overlap of the mat. The rule of thumb is to have the edges of the mat’s window overlap the edges of the art by a quarter-inch on each edge. Keep in mind that a mat is a two dimensional object. It is measured by height and a width. On the width dimension there are two borders, one on either side of the window. Each of these borders overlaps the image by a quarter of an inch, meaning that across the entire width of the mat the window is smaller by two-quarters of an inch, or a half inch.
Using this approach, the window for a 16″x20″ would be 15.5″ x 19.5″. Now borders of 2.25″ would result in the frame size of 20″x24″ — because 2.25″+15.5″+2.25″ = 20″. And 2.25″+19.5″+2.25″ = 24″.
This is basic measuring for picture framing. Depending on what you want to frame, the approach can be different. But by grasping basic measuring for picture framing you have a foundation on which to build.