Glass Size Calculator

How the glass size calculator works

Glass and acrylic are cut a touch smaller than the frame so they drop into the rabbet — the L-shaped lip on the back of the moulding that holds everything in. Enter your frame’s inside size and this tool gives the exact glass, acrylic, and backing cut, in inches with fractions or in centimeters.

The rule is glass = frame rabbet − clearance. The standard clearance is about ⅛″ overall — roughly 1/16″ on each side — so the pane seats without binding and has room to move with temperature. A 16 × 20″ frame therefore takes a 15⅞ × 19⅞″ pane. If you’re cutting glass to sit over a mat rather than into the rabbet, a tighter 1/16″ total clearance keeps the edge hidden under the lip.

The backing board — foam board or mat board behind the art — is cut to the same size as the glass, which surprises people who expect the back to be larger or smaller. It isn’t: the glass caps the front of the rabbet and the backing fills the back of the same rabbet, so they share one footprint and the whole stack — glass, mat, art, backing — slides in as a single sandwich held by the lip. Acrylic (plexiglass) is cut to those same dimensions too; it’s lighter and won’t shatter, which makes it the safer pick for large frames and busy rooms, but the numbers don’t change.

Measure the rabbet, not the outside of the frame — the inside opening the glass actually sits in. If you only know your print and mat sizes, the frame size calculator gives the rabbet, and will it fit confirms a print-and-frame pairing. UK framers call the pane the glazing; the cut is the same on either side of the Atlantic.

Glass, acrylic, and glazing types

The clearance is the same whatever glazing you choose. Standard glass, UV-filtering glass, and non-glare (etched) glass are all cut to the rabbet minus ⅛″ — only the price and the way they handle light differ. UV glazing is worth it for anything you’d be sad to see fade: photographs, watercolors, and prints hung in a bright room. Non-glare cuts reflections but can soften the image slightly, so it suits busy walls more than fine detail.

Ordering rather than cutting? Give the supplier the cut size, not the frame size, so there’s no confusion about whether the ⅛″ has already been taken off. If you’re cutting your own, score and snap glass to the calculated size; acrylic can be scored and snapped for thin sheets or cut with a fine-tooth blade for thicker ones — either way the target dimensions are identical to glass. Keep the protective film on acrylic until the piece is assembled to avoid scratches.

Which glazing should you choose?

The cut size is settled — the harder question is what to cut. Reaching for acrylic over glass mostly comes down to size, location, and what you’re protecting: acrylic is roughly half the weight and won’t break, so it’s the safer call for anything big, anything shipped, and anything hung over a bed or in a child’s room, while glass stays clearer and shrugs off scratches. Then there’s the upgrade question — whether plain glazing is fine or your piece wants UV filtering or an anti-reflective finish. Rather than re-litigate all of that here, the glass vs acrylic guide compares weight, clarity, UV protection, cost, and cleaning side by side so you can pick before you cut.

Worked example: glass for a 16×20 frame

You’ve got a 16 × 20″ frame and need a pane. Take ⅛″ off each dimension overall: 16 − ⅛ = 15⅞″ wide and 20 − ⅛ = 19⅞″ tall, so order 15⅞ × 19⅞″ glass. Cut the foam-board backing to the same 15⅞ × 19⅞″, and if you’d rather use acrylic, that’s 15⅞ × 19⅞″ too.

Common frame sizes and their glass cut

Common mistakes

• Measuring the outside of the frame. The glass sits in the rabbet, so measure the inside opening, not the moulding’s outer edge — they can differ by an inch or more.
• Cutting glass to the exact frame size. A pane the same size as the rabbet won’t drop in and can crack as it’s forced. Always leave the ⅛″ total clearance.
• Forgetting the backing. The backing is cut to the same size as the glass — order both at once so the sandwich seats flush.
• Assuming acrylic needs a different size. It doesn’t. Glass and acrylic share the cut; only the weight and safety differ.