Can I use 1/8 inch plywood with a 3 mm design?

Often yes for flat parts, but joints and slots may fit differently. Measure the sheet and adjust the design.

Is Baltic birch 3 mm really 3 mm?

It is nominally 3 mm and often close, but measure your actual sheet because tolerances vary.

Is 1/4 inch plywood 6 mm?

1/4 inch is 6.35 mm, and many sheets sold as 1/4 inch are thinner. Measure.

What tool do I need to measure plywood thickness?

A pair of digital or dial calipers, used at several points on the sheet.

Is 1/8 Inch Plywood the Same as 3 mm for Laser Cutting?

1/8 inch plywood is a board sold in the US by the fraction, 0.125 inches thick on paper, which equals 3.175 mm. "3 mm plywood" is sold by metric thickness, nominally 3.0 mm. The two are close enough that many makers treat them as the same, and close enough to cause trouble when you cut slots that must grip. A difference of 0.175 mm is as large as a typical laser kerf of 0.1 to 0.25 mm.

Nominal versus actual

Label	Exact thickness	Where you see it	What you actually get
1/8 in	3.175 mm	US hardware and craft stores	Often 2.5 to 3.2 mm, varies by brand
3 mm	3.0 mm	Baltic birch and import plywood	Commonly 2.7 to 3.1 mm, varies by batch
1/4 in	6.35 mm	US stores	Frequently thinner, sometimes around 5.5 to 6.0 mm
6 mm	6.0 mm	Baltic birch and import plywood	Commonly close to 6 mm, but measure it

The "actual" column is a typical spread, not a guarantee. Your own sheet is the only number that counts. If a figure above looks surprising for the board you hold, trust your calipers.

Why the difference matters

Finger joints, slots and tabs fit into material of a specific thickness. If your slot is cut for 3.175 mm and your plywood is 2.8 mm, the parts rattle. If your slot is cut for 3.0 mm and the plywood is 3.2 mm, you hammer parts in and the veneer cracks. Add kerf, and the correct slot size is: measured thickness minus kerf for a snug press fit, or measured thickness for a looser fit. See how to measure kerf and compensate for it.

Situation	Slot drawn at	Result with 2.8 mm plywood and about 0.15 mm kerf
Designed for 3.175 mm	3.175 mm	Cut slot about 3.3 mm, loose by about 0.5 mm
Designed for 3.0 mm	3.0 mm	Cut slot about 3.15 mm, loose by about 0.35 mm
Designed for measured 2.8 mm	2.8 mm	Cut slot about 2.95 mm, snug to slightly loose

How to measure and decide

Use calipers and measure the sheet in at least five spots, away from the edges.
Record the average, plus the lowest and highest.
Enter the average in your design, for example in our finger joint box generator, which accepts thickness of 3, 4, 6, 8 or 10 mm and kerf of 0 to 0.4 mm.
Cut a small test joint on scrap from the same sheet.
Adjust kerf or slot size until the fit is right and note the final values.

Pro tip: Write the measured thickness on the corner of each sheet with a pencil, so the next project does not start from a guess.

Worked example

You buy a sheet labelled 1/8 in and measure 2.9 mm. You design a box slot for 3.175 mm because that is what the label says. After cutting, your joints wobble by about 0.4 mm. You measure, update the design to 2.9 mm minus kerf, recut one joint, and it clicks together. The cost was one piece of scrap and a couple of minutes. The alternative was a whole box that does not hold glue well.

Factors that change the real thickness

Brand and manufacturer, even within the same label.
Humidity, since wood swells a little.
Veneer sanding, because sanded sheets lose a bit.
Batch, since the same supplier can ship different sheets.

Common mistake: Assuming a metric file from our library fits any plywood labelled 1/8 inch. A file made for 3 mm will fit 3.0 mm stock, not necessarily 3.175 mm stock. Check and adjust. Related reading: what plywood thickness is best for ornaments.

Next step

Measure the sheets you already own, write the results on them, and keep a small chart by the laser. Then read about laser cutting plywood to dial in your speed and power.

Is 1/8 Inch Plywood the Same as 3 mm Plywood for Laser Cutting?