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A lot of plywood products are slightly undersized by 1/32", e.g. 23/32 instead of 3/4, 15/32 in 1/2, etc. The undersizing often requires very careful math and measurements when constructing things, or the purchase of specialty hardware such as the many plywood specific router bits.

Why is the plywood undersized?

  • I don't know for sure, but I bet if it's not finish grade, it's so you can put veneer on it to finish the surface and then it will be the correct finish size. Just a guess on my part though. – Pᴀᴜʟsᴛᴇʀ2 Jun 29 '15 at 0:33
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    Quickly searching, I found this article. It states that the compression of the plywood's layers isn't predictable, so manufacturer are allowed to be off up to 3/64 but only undersized not oversized. Is that true, I don't know... Kinda makes sense... – Maxime Morin Jun 29 '15 at 2:01
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    undersized is certainly easier to account for than too thick. – bowlturner Jun 29 '15 at 2:16
  • just an observation but all of the Baltic birch sheets I've bought are dead on the correct dimensions – Steven Jun 29 '15 at 13:12
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This seems to be a matter of manufacturing and in some cases quality (in the sense of You get what you paid for) which is more speaking of the choice of wood, type and thickness of core and type and thickness of the veneer. The one thing you didn't explicitly imply either is that the thickness of a sheet of plywood could also be inconsistently think throughout the sheet, which is also a pain. To quote the article that Maxime Morin found which also concurs with bowlturners comment:

The reasoning is that an undersized thinner panel will still fit in a groove cut at the nominal size, but an oversized panel would not.

It is hard to predict what each sheet class of plywood will end up like post manufacturing which is why there are grading systems1. It is also important to keep in mind that moisture will play a small part in this ( again.. more deals with quality.)

Looking at forums from people discussing this issue is just accepted as fact that this is a possibility that needs to be accounted for. One of the many reasons why I own digital caliper. Tool manufacturers are also aware of this because you see products like undersized plywood router bits which are

Designed to cut slop-free dadoes for today's slightly thinner sheet goods

Quoted from product description for Freud 89-650 Four Piece Undersized Plywood Router Bit Set

This is one reason why MDF has such a cult following. It is easier to get a more consistent thickness and appearance (no knots!). MDF is recommended when you need to have consistent dimensions and don't mind the extra weight.

I'm trying to see if there is any worthy technical reference articles that discuss this variance but have not found any as of yet.

I can't find it again but there was a comment from another WW.SE user who mentioned something along the lines of "This is why measuring in the field is important". I believe the point being made was don't assume measurements. Measure twice, cut once.

1. Grading systems vary by region and can vary between manufacturers. The sentence is included to bolster the point of not every sheet is created equal.

  • Rant: It's always bothered me that the undersizing is so consistent you can buy an undersized router bit and stores actually sell some undersized sheets as 15/32" instead of 1/2", for example, but others as the nominal thickness even though they are actually undersized. Were the 15/32" originally spec'd as 1/2", but there were so many undersized sheets that all vendors were forced to carry undersized ply? Why does it seem like we see more undersized ply than actual-sized ply, and if it's so consistent, why wasn't the manufacturing process adjusted to start off all sheets 1/32" thicker? – rob Jun 29 '15 at 17:52
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    @rob I feel the obvious answer is it costs money. – Matt Jun 29 '15 at 17:57
  • You are also talking about the consistent boards you see. There could be a bunch that didn't pass quality inspection and were sent somewhere else away from your judging eyes. – Matt Jun 29 '15 at 20:58
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    Ha! I think the bar for quality must be pretty low for anything going to a big-box store; I've always figured they get the crap that nobody else wants. I'm pretty sure my local Lowe's is the "somewhere else." – rob Jun 29 '15 at 21:28
  • Measure twice, check that you are using the right measurement, cut once. I've burned myself on occasion by being exact but having misremembered,misread, or interchanged the measurement. Even a story stick can't save you if you're cutting the wrong part. – keshlam Jul 6 '15 at 6:21

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