Cut glass to the right size the first time. Enter your frame rabbet dimensions, get the exact glass cut with proper clearance built in.
Glass should never be cut to the exact inside dimension of your frame. As wood swells and shrinks with humidity, glass cut to the exact opening will bind, crack, or pop out. You need a small clearance — typically 1/8" total, meaning 1/16" per side.
Glass Cut Size
glass W = rabbet W − clearance
glass H = rabbet H − clearance
Standard clearance is 1/8" (0.125") total. Some shops use 3/32" (0.094") for tighter fit on smaller frames, or 1/4" (0.25") for large frames in humid climates. The rule: enough room for thermal movement, not so much that the glass visibly shifts inside the frame.
Enter frame rabbet dimensions. Get the glass cut size with proper clearance.
In a standard framing package, the order front-to-back is: glass, mat(s), artwork, mat backing, foam core or backing board, dust cover. The glass sits in the rabbet — the L-shaped channel cut into the inside edge of the moulding. The rabbet has a lip (the part you see from the front, overlapping the mat) and a depth (how far back it goes, determining how much material stack it can hold).
When you measure the rabbet, measure the opening — the inside dimension where the glass will sit, not the outer dimension of the frame. This is sometimes called "rabbet to rabbet" or "sight size."
Standard picture framing glass is 2.0mm (also called "single strength" or "SS"). For frames larger than ~20×30, use 2.5mm for rigidity — it'll flex less over time and is less likely to crack from its own weight. Conservation glass (UV-blocking) and museum glass (UV plus anti-reflective) are typically 2.0mm or 2.5mm with specialty coatings. Acrylic glazing is sized the same way but with looser clearance rules since acrylic has much more thermal expansion than glass.
The glass is only one layer in the rabbet. Behind it sits the mat (typically 1/16" thick), artwork, foam core or backing (1/8" to 3/16"), and dust cover. The total package thickness has to fit within the rabbet depth. If you're using a thick matboard, foam core backing, and a deep shadowbox spacer, you can exceed the rabbet depth of a shallow moulding — at which point the frame can't close properly. When in doubt, measure the full stack against your moulding's rabbet depth before cutting glass.
Glass cutting is linear — one rectangle per frame. Moulding cutting is the compounding problem: four cuts per frame, dozens of frames per day, multiple profiles, and a rack of remnants that may or may not work. Glass math takes 10 seconds. Moulding math, done right, takes 15–20 minutes.
RailChop handles the moulding side. Glass you still cut yourself.
Always measure to the smaller dimension of each pair. If the top of the rabbet is 16⅛" and the bottom is 16¼", measure 16⅛" as your rabbet width — the glass has to fit the tightest dimension. Use the clearance allowance to handle the variation.
No. Wood expands with humidity, and glass doesn't. A frame assembled in winter with tight-fit glass will bind or crack when humidity rises in summer. The clearance exists specifically to accommodate this movement. Even for short-term displays, tight-fit glass is a liability.
A quality glass cutter (score-and-break) will handle 2mm glass cleanly. For production shops, a bench cutter like Fletcher or Logan gives consistent cuts. Score firmly, one pass only, in a single continuous motion. The break should happen along the score line almost silently. Wavy breaks usually mean multiple passes on the score — which creates stress fractures.
None, for sizing purposes. UV-blocking glass is sized and cut the same way as standard glass — it's the coatings that differ, not the base material dimensions. The same is true for museum glass, which combines UV blocking with anti-reflective coating.
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