Add the layers before you cut the moulding
The default stack here is an ordinary job: 0.08 inches of glazing, two mat boards at a sixteenth each, a sheet on a mount at another sixteenth, an eighth of backing and a sixteenth for the dust cover and whatever the fitting adds. That comes to 0.455 inches. It is nothing like the number most people carry in their heads, because each layer on its own is trivial.
Against a 5/8 rabbet with a sixteenth held back for the hardware, that leaves 0.107 of an inch spare and the job goes together. Against a 7/16 rabbet, which plenty of narrow profiles have, it is short and the frame will not close. Nothing about the front of the frame tells you which of those you have.
The layer that moves the answer most is the backing. Foam board and hardboard differ by more than every other layer put together, and that decision often gets made at the fitting table by whatever is on the shelf rather than at the design stage where it belongs.
What to do when it does not fit
Three routes and the page ranks none of them. Extend the rabbet by applying a strip to the back of the moulding, which is the standard fix and is invisible on the wall; it makes the frame stand off the wall further, which matters if you are working to a tight reveal. Take a layer out, which usually means one mat instead of two and is a design change rather than a technical one. Or move to a deeper profile, which almost always means a wider face as well, so the frame you end up with looks different from the one you drew.
What is not a route is forcing it. A package that has to be pressed to get the backing flat is loading the glazing against the lip at the corners, and thin picture glass under a point load is the definition of a sheet about to break in your hands.
The cut sizes are not the frame size
Follow the chain. The package is nominally 16 by 20. The rabbet gets cut with slack in it, so it is 16-1/8 by 20-1/8. The glazing and the boards then get an undercut so they do not bind in the corners, which brings them to 16-1/16 by 20-1/16 at the default sixteenth.
So the piece of glass in a 16 by 20 frame is a sixteenth of an inch wider than sixteen inches. This is exactly the size of error that makes a nesting plan collapse. A 32 by 40 sheet appears to hold four of them at 2 across and 2 down; at 16-1/16 across it holds one across, because 32 divided by 16-1/16 is 1.99. The page reports the real count off your real cut size, and the recommendation that falls out of it is to pick the sheet against the cut size rather than against the number on the frame.
Glazing, backing and mat outside all get the same cut size on purpose. Whichever one is largest is the one that stops the package seating, and it will do so invisibly with the backing already taped on.
Undercut: enough and not more
A sixteenth across the whole dimension — a thirty-second each side — is what the field defaults to. It exists because rabbets are not perfectly square in the corners, glass is not perfectly rectangular, and a package that binds on one corner sits proud on the other three.
Push it too far and the failure mode changes rather than disappearing. A mat cut an eighth under the rabbet can slide far enough sideways that its edge appears from under the lip at one side, showing a line of white core along one edge and nothing along the other. That is the fault that reads as amateur from across a room, and it is caused by too much clearance rather than too little.
What is not in here
No material property, no archival claim, no light or UV figure, and no statement about what any glazing type does or does not do to anything behind it. Every thickness in the form is yours to supply from the sheet you are actually buying, and nominal labels on glazing are exactly that.
Nor does the page have a view on what a package should be made of. If the piece matters, the materials question goes to a conservator before it goes to a calculator, and the conservator will have opinions about layers that this arithmetic cannot represent.
Questions people ask
How deep does a picture frame rabbet need to be?
Deep enough for whatever you are putting in it plus room for the hardware, and that is a number you build rather than look up. The default stack here — glazing, two mats, a mounted sheet, backing and a fitting allowance — comes to 0.455 inches, and with a sixteenth held back it needs 0.5175 of rabbet. A 5/8 rabbet takes it with 0.107 to spare; a 7/16 does not take it at all. Measure the rabbet of the actual moulding rather than trusting a catalogue depth, because the same profile number varies between runs.
What is a rabbet extension and when do I need one?
A strip applied to the back of the moulding that deepens the rabbet, used when the package is thicker than the frame will take. The page reports the shortfall rounded up to a sixteenth so you know what depth of strip to add. The trade-off is that the frame then stands further off the wall, which is only a problem where you are working to a tight reveal or where the piece sits alongside others that do not have one.
Why is my glass bigger than the frame size?
Because the frame size is the package size, the rabbet is cut larger than the package to give it slack, and the glass is then cut just under the rabbet. On the defaults that chain runs 16 to 16-1/8 to 16-1/16, so the glass in a 16 by 20 frame measures a sixteenth over sixteen inches. It matters most when you are nesting: a 32 by 40 sheet looks like four lites and delivers one across, because 16-1/16 will not go into 32 twice.
How much undercut should the glazing and boards get?
A sixteenth across the whole dimension, so a thirty-second each side, is what the field defaults to, and it is a bench figure rather than a standard. It absorbs the fact that rabbet corners are not square and sheets are not perfectly rectangular. Too much clearance has its own failure: a mat that can slide an eighth will show its white core along one edge and nothing along the other, which reads worse than a package that is slightly tight.
Does this tell me which glazing to use?
No, and it deliberately says nothing about the properties of any glazing. No UV figure, no light level, no archival claim, no comparison between materials beyond the thickness and price you type in yourself. Those questions belong to the supplier data sheet for the specific product and, for anything of real value, to a conservator. What the page will do is tell you whether the sheet you have chosen fits the frame you have chosen, which is the part that otherwise gets discovered at the fitting table.