How Much Moulding to Buy for a Frame

Buying moulding is where a framing project stops being theoretical. You need a length of stock, and the length is not the frame's perimeter, because every mitred corner consumes material that ends up as sawdust.

The frame size calculator handles both parts. It builds outward from the artwork to the finished outer dimensions, computes the perimeter, and then adds an allowance for mitre waste.

Building outward, in two steps

Two numbers separate the artwork from the finished frame. The mat border sets the rabbet opening — the size of the recess the frame's inner lip has to accept. The moulding width, meaning the visible face of the profile from rabbet edge to outer edge, sets how much larger the outside is than that opening.

An 8×10 print with a 2-inch mat gives a 12×14 rabbet opening. Add a 1.5-inch moulding and the outer frame measures 15×17. Both steps double the added dimension, because the mat and the moulding each appear on two opposite sides.

Where the 10% goes

A 15×17 frame has a perimeter of 64 inches. The calculator returns 70.4 inches of moulding to buy — a 10% allowance on top.

That allowance is not padding for mistakes. A mitre cut removes a wedge of material at each of the four corners, and the length lost depends on the moulding's width: a wide profile wastes more than a narrow one because the 45-degree cut runs further across the stock. Ten per cent is a reasonable general figure that covers the four corners plus the width of the saw kerf.

Why the allowance matters more on small frames

The waste is roughly proportional to the number of corners and the width of the profile, not to the frame's size. Four corners of a 3-inch-wide moulding waste about the same absolute length whether the frame is small or large — which means the waste is a much larger proportion of a small frame's stock.

On a large frame, 10% is generous. On a small frame with a wide profile, it can be tight. If you are cutting a 6×8 frame from 3-inch moulding, buy more than the calculator says.

Order lengths, not totals

The calculator returns a single total, and stock comes in fixed lengths. Seventy inches does not mean one 70-inch piece is enough: you need four pieces of specific lengths, and how they nest inside the stock determines what you actually buy.

For the 15×17 frame, the four pieces are two at 17 inches and two at 15 inches, each plus the mitre allowance. Two eight-foot lengths would be wasteful; one might not nest. Work out the cut list before ordering, using the total only as a sanity check.

Measure the profile, not the frame

The moulding width the calculator wants is the visible face width — how much of the frame you see from the front. It is easy to confuse with the profile's overall depth or with the rabbet depth, neither of which affects the outer dimensions.

If you are working from a supplier's specification, the figure is often listed as "face width" or simply "width", with "depth" or "rabbet" given separately. Measure a physical sample if you have one: lay it face down, measure across from the outer edge to the inner lip, and that is the number.

Check the fit before you cut

One final sanity check worth doing on paper. The rabbet opening from the frame size calculator should match the outer mat size from the mat border calculator for the same artwork and border — both are the artwork plus twice the border, so both should return 12×14 for the example above.

If those two numbers disagree, one of the inputs is wrong, and you will find out at assembly when the mat does not drop into the frame. Better to find out while the moulding is still uncut.