How to size a pattern to fit your panel opening

Every pattern on this site is built once, as geometry, and rebuilt to your exact panel width at download time, so sizing is mostly a matter of measuring your opening correctly and checking it against the pattern's declared range.

Measure the opening, not the glass

Measure the finished opening the panel needs to sit in, at its narrowest point if the frame is not perfectly square, and subtract a small clearance, typically 1/8 in to 3/16 in per side, so the finished panel is not fighting the frame.

Check the pattern's size range

Every pattern page on this site lists a minimum and maximum width. Choosing a size outside that range is refused before checkout, not silently allowed, because the cuttability checks are only verified within that declared range.

Why the checks are rerun per size

Interior angles do not change with panel size, since scaling a shape up or down does not change its angles. Piece width and glass area do change with size, in direct proportion. Rerunning the width and area checks at your exact chosen size, rather than only at one fixed size, is what confirms a small version of a pattern has not shrunk a piece below the 3/8 in cutoff.

Try this live on the free pattern with the Pattern Sizer tool, or see how much glass do I need for the area side of sizing.

Measure the opening in three places

Frames are rarely square. Measure the opening at the top, the middle and the bottom, then the same for the height, and work from the smallest figure. A panel built to the largest measurement will not go in, and a panel built to the average will bind on one corner. The difference is usually a few millimetres, which is exactly the amount that makes installation impossible.

Allow for the perimeter

The finished panel is not the same size as the glass you cut: a came border adds its own width around the edge, and a foiled panel usually gets a zinc or lead perimeter that does the same. Subtract that perimeter from the opening before choosing a panel width, and leave a small clearance so the panel seats without being forced.

Check the width against the declared range

Each pattern states the range across which its three checks pass. If your opening falls outside that range, the honest answer is a different pattern rather than a scaled one: scaling a pattern below its range produces slivers that cannot be cut, whatever the drawing looks like on paper.

Step by step sizing worksheet

Use a steel tape, a rigid ruler, a sharp pencil, masking tape, a notepad, and a square if the frame is open enough to check the corners. A flexible sewing tape is not accurate enough for this job, because a 1/16 in bend in the tape can become a tight panel later.

Label the opening before measuring. Mark top, bottom, left, and right with small pieces of masking tape. If the frame is already installed in a wall or cabinet door, measure from the actual seating surface, not from the visible front edge. Take each measurement twice, and write the smaller reading if the two readings differ by less than 1/16 in. If they differ by more, measure a third time.

Record the inside width at the top, center, and bottom of the opening.
Record the inside height at the left, center, and right of the opening.
Check both diagonals from corner to corner. A difference of 1/8 in over a small cabinet panel is noticeable, and a difference of 1/4 in usually needs frame correction or extra clearance.
Choose the smallest width as the controlling width, then subtract the border material and the fitting clearance.
Keep the final written size in inches to the nearest 1/16 in, or in millimetres to the nearest 1 mm. Do not mix units on the same worksheet.

When the pattern asks for a width, enter the final panel width after deductions, not the raw frame opening. If you need a reminder on what makes a resized drawing practical to cut, read the cuttable pattern rules before choosing a very small size.

Working figures for clearances and deductions

The exact deduction depends on the frame, the border method, and whether the panel will be installed with putty, clips, stops, or a removable retaining strip. The figures below are practical starting points for checking your calculation before you commit to cutting glass.

Situation	Typical figure to allow	What it prevents
Small cabinet door panel under 18 in wide	1/8 in total fitting clearance if the frame is square	Binding during installation
Medium panel from 18 in to 36 in wide	3/16 in to 1/4 in total fitting clearance	Tight fit caused by slight frame twist
Old wood frame with uneven rabbets	At least 1/4 in total fitting clearance after checking all contact points	Pressure on one corner or one long edge
Diagonal difference in the opening	Up to 1/8 in may be handled with clearance, more should be investigated	A rectangular panel forced into a racked opening
Measuring tolerance for hand work	Write sizes to 1/16 in or 1 mm	False precision that cannot be built reliably

For a simple check, draw a rectangle on paper and write every deduction on the drawing. For example, start with the smallest inside width, subtract the left seating allowance if it affects the finished panel, subtract the right seating allowance if it affects the finished panel, then subtract the fitting clearance. This written chain is slower than mental arithmetic, but it catches most wrong size orders.

Common sizing mistakes and recovery

The most common mistake is measuring the old broken panel instead of the frame. That old panel may have been trimmed small, ground unevenly, or installed with a border that is not being reused. If the old panel is the only reference, measure it, then still measure the empty opening and compare the two numbers before using either one.

A second mistake is forgetting that resizing changes the narrow pieces faster than the eye expects. A pattern that looks comfortable at 24 in wide may become difficult at 14 in wide because every narrow strip shrinks in the same ratio. If you are near the lower end of a pattern range, avoid textured glass with heavy relief in the smallest pieces, because scoring and breaking need more control.

If you ordered or printed a pattern too large, do not trim only the outer border unless the design has a plain border that can safely absorb the difference. Trimming decorative edge pieces can create thin points, short grain lines, or mismatched border widths. Recalculate from the opening and use a correctly sized pattern instead.

If the calculated size is only 1/16 in too large, do not assume grinding will solve it. Grinding a finished assembled panel is not the same as adjusting one loose piece. For foil work, compare the build method in foil versus came, because the perimeter choice affects the final outside size.

Special cases before you choose the final width

For arched, curved, or pointed openings, a width alone is not enough to prove the fit. Make a full size cardboard template of the opening and test it in the frame. Push it into the same position the glass will occupy. Mark any tight areas, then measure the widest and narrowest useful positions on the template.

For doors, measure with the door closed and also with it open if the frame flexes. A light cabinet door can change shape by 1/16 in when it is removed from its hinges. For exterior or damp locations, allow for seasonal wood movement and do not design the panel as a pressure fit.

For paired panels, size each opening separately. Do not assume the left and right doors are identical. A 1/8 in difference between two cabinet openings is common in older work, and copying one size to both sides can make one panel loose and the other too tight. If you are also estimating sheet glass before cutting, use the resized panel dimensions with the method in how much glass do I need.