Frame Size Calculator

How the frame size calculator works

The frame size you order is the frame’s inside (rabbet) measurement — the opening the artwork, mat, and glass drop into — not the outer edge of the moulding. With no mat, that inside size is simply your print size: an 8 × 10″ print takes an 8 × 10″ frame. Add a mat and the frame has to grow to hold it.

With a mat, frame size = mat outside = window + borders. The window is your print minus the overlap (about ¼″ per side), and the borders are whatever width you choose. So the frame you buy is the print, minus a little for the overlap, plus twice your border in each direction. Switch to inches with fractions or centimeters and the values convert in place.

The calculator also gives the glass and backing cut, which drop in about ⅛″ smaller than the rabbet so they seat without binding — size those on their own with the glass size calculator. To set the borders themselves, including bottom-weighting, start from the print; to go the other way and check whether a print fits a frame you already own, use will it fit.

One practical note: custom frames can be ordered to the exact size the calculator returns, but stock frames come in standard steps (8×10, 11×14, 16×20…). When the math lands between sizes, round up to the next stock frame and let the borders run a touch wider — wider mats almost always look better than narrow ones. In the UK and Australia the mat is a mount, but “frame size” means the same inside measurement everywhere.

Bare frame or matted?

Whether to mat at all comes down to look and protection. A bare frame at the print’s exact size is clean and cheap, and it works well for posters, open-edition prints, and anything with its own built-in border. A mat lifts the glass off the artwork — which matters for photographs and anything that could stick to the glazing — and gives the eye somewhere to rest, which is why most framed photos use one. The cost is a bigger frame and a little more money.

Working from a frame you already own? Measure the rabbet — the inside opening — and feed that to will it fit to see which prints suit it, with or without a mat. A frame’s stamped size is its rabbet, so an “11×14” frame is 11×14 inside even though its outside is larger by the width of the moulding. Order new frames the same way: by the inside size you want to fill.

What about the rabbet?

The rabbet (sometimes spelled rebate) is the L-shaped step cut into the back of the moulding that the glass, mat, art, and backing all drop into. Frame size equals the mat outside, which equals that rabbet opening — so when the calculator hands you, say, 11½ × 13½″, that is the inside the whole sandwich must fill. The lip of the rabbet then folds over the front edge and covers about ⅛–¼″ of whatever sits beneath it.

That lip is why a snug, no-mat fit needs a little care: a print cut to the exact rabbet size loses a sliver of its image under the lip on every side, and a print cut a hair over won’t seat at all. A mat sidesteps the worry entirely — the lip lands on the wide mat border, never on the artwork. The glass size calculator sizes the pane to clear that rabbet, and the glass vs acrylic guide helps you choose what to put in it.

Reading the “Nearest standard frame”

Alongside the exact size, the calculator names the closest off-the-shelf frame so you can skip a custom order when one is near enough. It only suggests a stock size when your result lands within about a quarter inch of it — close enough that letting the borders run a touch wider or narrower will fill the difference invisibly. When nothing stock is that close, the “Nearest standard frame” line reads “No standard match (custom size)” — your signal that no off-the-shelf frame falls within the snap tolerance, so the size is a true custom order. That’s common with bottom-weighted mats and metric prints; either order the exact size from a custom framer, or step up to the next larger stock frame and widen the borders to suit.

Worked example: an 8×10 print with a 2″ mat

Take an 8 × 10″ print and a 2″ border. The ¼″ overlap makes the window 7½ × 9½″; adding 2″ on every side makes the frame 11½ × 13½″. The glass and backing come in at about 11⅜ × 13⅜″. Prefer a stock frame? Round up to an 11×14 and the side borders simply land a little wider than 2″. Bottom-weight the mat — 2″ top, 2½″ bottom — and the frame grows to 11½ × 14″.

Common print sizes and the frame to order

Common mistakes

• Ordering by the frame’s outside size. Frames are sold by the inside (rabbet) opening. Measure or order by that, not the outer edge of the moulding.
• Forgetting the mat enlarges the frame. A 2″ mat adds 4″ to each dimension, so an 8×10 print needs roughly an 11×14 frame once it’s matted — not an 8×10.
• Sizing the frame to the print when using a mat. With a mat, the frame equals the mat’s outside (window + borders), not the print size.
• Buying glass to the frame’s exact size. Glass and backing are cut about ⅛″ smaller so they drop in — see the glass size calculator.