Short answer: Most acrylic is made to metric or decimal thicknesses and sold under the nearest inch fraction: 1/4 in. acrylic is nominally 6 mm (0.236 in.), or 0.220 in. in many home-center sheets. On top of that, each sheet varies: extruded is held to about ±5% (0.224 to 0.248 in. for 6 mm Acrylite), while cast is made to ISO 7823-1, which allows 0.197 to 0.276 in. for 6 mm. Design slots and joints from the measured sheet, not the label.
Nominal size vs actual size
Acrylic is sold by a nominal thickness, a name for the size rather than a measurement, much like a 2×4 board isn't 2 by 4 inches. Most acrylic sheet in North America is made to metric thicknesses (3 mm, 6 mm, and so on) or to decimal sizes such as 0.093 or 0.220 in., and labeled with the nearest inch fraction. On top of that, every sheet varies a little across its surface, and cast sheet varies more than extruded.
Key facts
- 1/8 in. acrylic is usually 3 mm (0.118 in.); home-center “1/8 in.” can also be 0.093 or 0.100 in.
- 1/4 in. acrylic is usually 6 mm (0.236 in.) or 0.220 in. (5.6 mm).
- Cast sheet is made to ISO 7823-1, which allows ±(0.4 mm + 10% of nominal): ±1.0 mm on 6 mm sheet.
- Acrylite extruded sheet is held to about ±10% up to 3 mm and about ±5% above that. Plaskolite lists ±10% for Optix.
- Those are the allowed limits. Most sheets land much closer to nominal, but you can't count on it without measuring.
What each size actually measures
| Sold as | Usually made as | Extruded allowed range (Acrylite) | Cast allowed range (ISO 7823-1) |
|---|---|---|---|
| 1/16 in. | 1.5 mm (0.060 in.) | 0.054 to 0.066 in. | Rarely cast this thin |
| 1/8 in. | 3 mm (0.118 in.) | 0.106 to 0.130 in. | 0.091 to 0.146 in. |
| 3/16 in. | 4.5 mm (0.177 in.) | 0.168 to 0.186 in. | 0.144 to 0.211 in. |
| 1/4 in. (decimal) | 5.6 mm (0.220 in.) | 0.209 to 0.231 in. | 0.183 to 0.258 in. |
| 1/4 in. (metric) | 6 mm (0.236 in.) | 0.224 to 0.248 in. | 0.197 to 0.276 in. |
| 3/8 in. | 9 mm (0.354 in.) | 0.336 to 0.372 in. | 0.303 to 0.406 in. |
| 1/2 in. | 12 mm (0.472 in.) | 0.448 to 0.496 in. | 0.409 to 0.535 in. |
| 3/4 in. | 18 mm (0.709 in.) | 0.673 to 0.743 in. | 0.622 to 0.795 in. |
| 1 in. | 24 mm (0.945 in.) | 0.897 to 0.991 in. | 0.835 to 1.055 in. |
Ranges are from Acrylite's published tolerance tables for its extruded (Premium FF, OP3, P99, Satinice) and cast sheet; cast follows the ISO 7823-1 formula. Other makers are similar. Large cast sheets are often thicker in the middle than at the edges. More on cast vs extruded acrylic.
Why it matters
For a shelf, a cover, or a sign, it doesn't. It matters whenever one piece of acrylic has to fit into another:
- Tab and slot boxes. A slot drawn at 0.250 in. for "1/4 in." acrylic that measures 0.225 in. will be 0.025 in. loose, and the box will rattle.
- Press fits. A slot drawn at 0.236 in. may be too tight if your cast sheet came out at 0.250 in.
- Edge-to-edge joints in boxes. When one panel sits on the edge of another, the outside size of the box changes with the real thickness.
- Frames and channels. Thin spots can sit loose in a U-channel or frame rabbet; thick spots may not fit at all.
How to design around it
- Measure the sheet you'll use in a few places with calipers, and design slots to the thickest reading plus a small clearance (about 0.004 to 0.008 in. for a slip fit).
- Make slot widths easy to change. In your CAD program, set the slot width as one variable so you can adjust it in seconds.
- Remember the laser's kerf. The beam removes about 0.006 to 0.010 in. of material, which makes slots slightly wider and tabs slightly smaller. How to allow for laser kerf.
- Choose extruded for slotted kits if you don't need frosty engraving: its tighter tolerance makes fits more repeatable from sheet to sheet.
- Or just tell us. When you order, mention that your parts slot together, and we'll measure the sheet we're cutting from and adjust the slots for you. See tab and slot design.
Questions
Can I order acrylic in an exact thickness?
Not from stock sheet: every sheet varies a little. If a part needs an exact thickness, thicker sheet can be machined down, or we can pick the sheet closest to your number. Tell us the thickness and tolerance in your order notes.
Is 6 mm acrylic the same as 1/4 in. acrylic?
Usually, yes. Most sheet sold as 1/4 in. in the US is nominally 6 mm (0.236 in.), and some is 0.220 in. (5.6 mm). True 0.250 in. sheet exists but is less common.
Why is my “1/4 inch” sheet from the hardware store 0.220 in.?
Many home-center sheets, such as Plaskolite Optix, are made to decimal gauges: 0.093, 0.118, 0.177, 0.220, and 0.236 in. are all standard. The label rounds to the nearest fraction; the actual gauge is usually printed on the film or barcode label.
How thick is 1/8 in. acrylic really?
Usually 3 mm (0.118 in.). Extruded 3 mm sheet is allowed 0.106 to 0.130 in.; cast 3 mm may run 0.091 to 0.146 in. Some store sheet sold as 1/8 in. is really 0.093 or 0.100 in.