The Four Numbers Behind Every Framing Job
A framing job is four numbers, computed in order. Get them in the right sequence and everything fits; work backwards from a frame you already own and you will usually find one of them is impossible.
The four are: the artwork size, the mat window, the frame opening, and the outer frame dimensions. Each one is derived from the one before it.
One: the artwork
Measure the area you intend to show, not the sheet. Prints often carry a white margin, and whether that margin is part of the presentation is a decision, not a given. For a signed edition it usually is; for a photograph it usually is not.
This single number propagates through everything that follows, so it is worth measuring twice. An 8×10 print is the running example below.
Two: the mat window
The window is cut slightly smaller than the artwork so the mat overlaps and holds the edge. At the usual 0.125-inch overlap, an 8×10 print gets a 7.75×9.75 window — a quarter inch off each dimension, because the overlap applies at both edges.
Cut the window to the artwork's exact size and the print falls through. Cut it much smaller and you start covering the image. An eighth of an inch per side is enough to hold and small enough not to show.
Three: the frame opening
The mat's outer size is the frame opening — the size the frame's rabbet has to accept. With a 2-inch border, the 8×10 becomes 12×14: eight plus two twos, ten plus two twos.
This is the number to give a supplier when ordering a frame, and it is the number people most often get wrong by applying the border once instead of twice. The mat border calculator and the frame size calculator both compute it and will agree: 12×14 either way.
Four: the outer dimensions
The moulding's face width sets how much larger the finished frame is than its opening, and again it applies twice. A 1.5-inch moulding around a 12×14 opening gives a 15×17 frame.
That is the number that decides whether the piece fits the wall, fits the car, and fits within whatever the framer's maximum size is. It is also the number nobody computes until it is too late to change the mat.
Running the sequence backwards
Starting from a frame you already have inverts the order but not the arithmetic. A ready-made 16×20 frame holding an 8×10 print needs a border of 4 inches on each side: sixteen minus eight is eight, halved is four; twenty minus ten is ten, halved is five.
Which does not come out even — 4 across and 5 down. The mat border calculator's reverse mode reports exactly this, flagging that the two are unequal, and the reason is that 8×10 and 16×20 are the same shape only in ratio, not after adding equal borders.
When the borders will not come out equal
This happens whenever the artwork's proportions differ from the frame opening's, which is most of the time. You then have three choices: accept unequal borders, crop the artwork to a proportion that works, or choose a different frame.
Unequal borders are not a failure. A common and deliberate variant is the weighted bottom, where the lower border is made larger than the other three on purpose. What you should avoid is unequal borders you did not choose, because a mat that is 4 inches at the sides and 5 at top and bottom looks like a mistake rather than a decision.