The Frame Order Size corresponds to the size of the artwork and the rabbet recess on the frame's rear. For example, if you have a painting that measures 18" x 24", you would order an 18" x 24" frame. The actual opening of the frame, when viewed from the front, will be slightly smaller. (see Figure 1)
To calculate the approximate outer dimensions of a frame, take the artwork's width (or height) and add the width of the frame molding less rabbet width to each side of your artwork. For example, An 18" x 24" artwork that has a 5" frame molding width and 3/8" rabbet width would measure an outer frame dimension of approximately 27-1/4" x 33-1/4". (The math: 18" + (5" - 3/8") + (5" - 3/8") = 27-1/4" for the overall frame height and 24" + (5" - 3/8") + (5" - 3/8") = 33-1/4" for the overall frame width.) (see Figure 2)