Is 3/4 inch plywood really 3/4 inch?

The single most common reason a plywood cabinet comes out loose is that the drawing assumed 19 mm and the sheet was 17.6 mm. Nothing about the plan was wrong; the stock was different from the label. Here is the whole picture.

The number on the label is nominal

Sanded plywood is sold under its nominal thickness, and it has been undersized for decades. The actual numbers, which is what your dado cares about:

Baltic birch and most European sheet goods are actually metric and often measure closer to their nominal value; North American sanded pine is the one that shrinks. Cheap sheathing can run 16.5 mm and still be called 3/4.

Why it matters at the joining stage, not the cutting stage

Panel sizes are forgiving: a side that is 0.8 mm thin is invisible. A groove is not. A dado cut for 19 mm stock in an 18.2 mm panel is 0.8 mm loose over its full depth — enough for the joint to feel fine on the bench and fail under load, because glue needs contact and pocket screws need the parts to sit still.

The reverse is worse: a groove cut too tight splits the plywood along a face veneer, usually on the last panel of the sheet.

How to measure in three minutes

1. Use calipers, not a tape. A tape reads to 1 mm at best; you need 0.1 mm.
2. Measure at three points along the sheet edge — ends and middle. Plywood varies within a sheet and between sheets of the same lift.
3. Avoid the last 10 mm at each corner, where the edge is often crushed or rounded.
4. Write the three numbers on the sheet in pencil. Use the largest if you are cutting a groove by hand; the average if you trust your fence.

The five-minute test groove

Set the router or dado stack to the measured thickness plus 0.005 in (0.12 mm). Cut a 150 mm groove in an offcut, push a piece of the same stock into it, and lift. It should hold by friction with no rock. If it drops out, close the cutter by half the play and repeat. This single test replaces every argument about nominal sizes.

If your stock is 17.5 mm and the plan says 3/4

Two honest options:

• Adjust the groove. This is what we do. The drawing keeps the nominal 3/4 in dimension, and the notes tell you to size the dado off the measured sheet. Nothing else on the plan changes.
• Change the sheet. If you need a true 18 mm and your supplier only has 17.5, buy 18 mm birch or a different brand. Do not shim a dado with veneer tape — it fails at the worst moment.

One more trap: face veneer thickness. On cheap stock the face can be 0.3 mm. A groove that breaks through it exposes the core and the joint looks wrong even when it is structurally fine.

What the plans assume, stated plainly

• Drawings show nominal 3/4 in (18 mm) for carcases and shelves, 1/4 in (6 mm) for backs.
• Every dado note says: size to measured stock thickness plus 0.005 in.
• Joinery tolerance is ±1/16 in on the drawing, and the test-groove step is printed on the assembly sheet.
• If a plan uses a 1/2 in or 1/4 in panel anywhere, that sheet says so explicitly.

The 24 × 16 × 36 cabinet is the clearest example: three fixed shelves in 3/8 in dados, all sized off your measured sheet. If you want to see the note in context, the file list shows which sheet carries it.