Short answer: Measure the opening's width and height in three places each with a steel tape or ruler, take the smallest of each, then subtract clearance and order that finished size. For picture frames and indoor panels in a channel, about 1/16 in. less than the opening overall is typical; ACRYLITE's glazing guide also calls for cutting panels about 1/16 in. short of small openings. Large or outdoor panels need more room for thermal expansion, roughly 1/8 in. for panels up to 36 in. and more above that. Measure thickness too, from the rabbet or channel depth.
Key facts
- Order the part size, not the opening size. Subtract clearance before you order.
- Measure three times each way (top, middle, bottom; left, middle, right) and use the smallest.
- Frames and small indoor panels: about 1/16 in. smaller than the opening overall.
- Outdoor and large panels: Plaskolite's OPTIX-L guide lists expansion clearances of 1/8 in. up to 36 in., 1/4 in. for 36 to 60 in., 3/8 in. for 60 to 96 in., and 7/16 in. for 96 to 120 in.
- Acrylic moves: about 0.002 in. per °F on a 48 in. panel (ACRYLITE).
What do I need to measure?
- Width and height of the opening, frame rabbet, or channel, to the nearest 1/32 in. or 0.5 mm.
- Depth of the rabbet or channel, which limits how thick the acrylic can be.
- Corner shape: square, or rounded with a radius (measure the radius or trace the corner on paper).
- Holes and cutouts: position from one reference edge, plus diameter.
- How it's held: loose in a frame, screwed, glued, or clamped. That decides the clearance.
Use a steel tape or rule, not a cloth tape. For anything bigger than about 24 in., measure diagonals too: if they differ, the opening is out of square, and the part has to fit the smallest dimension.
How much clearance should I leave?
| Where it goes | Clearance (total, not per side) | Why |
|---|---|---|
| Picture frame | about 1/16 in. less than the rabbet | Frames are rarely perfectly square; acrylic must drop in without force |
| Indoor cabinet door insert, display case panel | about 1/16 in. | Easy fitting; small temperature range |
| Window insert, storm panel | 1/16 to 1/8 in., plus room for the gasket or seal | Cold glass side and sun exposure move the sheet |
| Outdoor glazing up to 36 in. | 1/8 in. | Plaskolite expansion table |
| Outdoor glazing 36 to 60 in. | 1/4 in. | Plaskolite expansion table |
| Outdoor glazing 60 to 96 in. | 3/8 in. | Plaskolite expansion table |
| Panel screwed to a surface | no edge clearance, but oversize the holes | Holes let the panel slide; see can you screw into acrylic |
The channel or rabbet also has to be deep enough that the panel stays engaged when it shrinks in winter. ACRYLITE recommends at least 7/16 in. of rabbet depth for exterior frames and installing panels without bolting through them so they can move.
Worked example: a picture frame
The frame's rabbet measures 11 31/32, 12, and 11 15/16 in. wide and 15 31/32, 16, and 15 15/16 in. tall. Take the smallest of each, 11 15/16 × 15 15/16 in., and subtract about 1/16 in.: order 11 7/8 × 15 7/8 in. A typical frame takes 1/8 in. or thinner; see acrylic vs glass for picture frames.
Worked example: an outdoor panel
A shed window opening is 30 × 42 in. in a wood frame with stops. The 30 in. side falls under 36 in. (1/8 in. clearance) and the 42 in. side falls in 36 to 60 in. (1/4 in.). Order 29 7/8 × 41 3/4 in. and bed it in a flexible glazing sealant or gasket. More in can acrylic be used outdoors and acrylic thermal expansion.
How do I measure for thickness?
Measure the rabbet or channel width with calipers if you have them. Remember that "1/4 in." acrylic is usually 6 mm (0.236 in.) and can vary by several thousandths either way; "1/8 in." is usually 3 mm (0.118 in.). If a channel is exactly 1/4 in. wide, 1/4 in. nominal sheet will usually fit; if it's tight, step down. Details in acrylic sheet actual thickness. For how thick a panel should be for strength, see what thickness acrylic do I need.
What if the shape isn't a rectangle?
Trace it. Tape paper over the opening or the old part, trace the edge with a pencil, and mark holes. Photograph the tracing straight on with a ruler beside it, or trace the old part directly. A cleaner option is a drawing: sketch it in any CAD or vector program at 1:1 scale and upload a DXF or SVG. For rectangles, circles, rounded rectangles, ovals, and rings, type the sizes into the instant quote and see the price as you go.
Questions
Should I measure the old acrylic piece or the opening?
Measure both if you can. If the old piece fit well, its size is your best guide. If it was loose, cracked, or warped, size from the opening and subtract clearance.
Can I order in millimeters?
Yes. Convert at 25.4 mm per inch, or upload a drawing in millimeters and tell us the units.
How close to my size will the part be?
Laser-cut parts are typically within about ±0.005 in. and routed parts about ±0.010 in. See acrylic cut-to-size tolerances.