Frame depth and rabbet calculator

The number nobody checks

Frame listings give the size, the finish and often the moulding width. The rabbet depth is buried in the specification if it appears at all — and it is the one measurement that decides whether the frame can hold what you intend to put in it.

The failure is not subtle. You assemble everything, the backing stands proud of the frame, the retaining points will not reach, and the only way to close it is to press hard — which bows the backing, pushes the artwork against the glazing, and in a humid room eventually bonds the two together.

What each component contributes

A single-matted print therefore needs about 0.26″; a double-matted one about 0.32″. A quarter-inch rabbet — common on slim decorative mouldings — will not close over a double mat.

Fixing a rabbet that is too shallow

Deepen it. Before assembly, run the moulding over a table saw or a router with a rabbeting bit. Straightforward on stock you are building from, impossible on an assembled frame without dismantling it.

Add an extender. A strip of timber glued to the back of the moulding effectively raises the rabbet wall. It is invisible from the front and is the standard trade solution for a frame that is otherwise right.

Drop a layer. A single mat instead of a double, or a thinner backing board, often recovers the eighth of an inch you need.

Buy differently. Which is usually the answer — see the moulding guide for how to read a rabbet specification before ordering.

Canvases are a different problem

No conventional frame has a rabbet deep enough for a stretched canvas. The solution is a floater frame: a deep, open-backed profile the canvas is screwed into from behind, sitting inside it with a deliberate gap all round. It reads as intentional, needs no glazing, and is what galleries use. See float and floater frames.