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Comment by wfleming | Hacker News Reader

2x4s are 2x4 before they're seasoned, though. The lumber is dried after dimensioning and shrinks as it loses moisture. But we still label it by the original dimensions because lumber is priced by the board foot (which is itself a weird archaic measure) of the lumber when it was cut.

For comparison, sheet goods aren't priced this way. A 3/4" sheet of plywood really is 3/4", because the plywood was manufactured that way and as a manufactured product there is no seasoning.

You're not being ripped off because a 2x4 is actually smaller. A sawmill had a tree, and they cut off a hunk of that tree, and they charge by how much wood they cut off. The dimensions of that piece of wood changed after they cut it (which in and of itself is a service they provide - it would be a hassle if you had to buy green lumber and dry it yourself).

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I work in sawmill technology. Way back in history 2x4 were cut to 2" x 4". In a modern sawmill 2x4s are never cut to 2" x 4". They are cut to a thickness and width such that given saw deviation, and variable drying shrinkage nearly all boards will cleanly plane down to 1.5" x 3.5". The exact target dimensions will depend on how well the mill can control their saw deviations and the statistical range of shrinkage they expect for the wood they are cutting. Reducing dimension targets by controlling saw deviation and understanding drying shrinkage is a big part of sawmill efficiency and profitability.

Fair enough, thanks for those details. Out of curiosity, do you know roughly what the target dimensions usually are now a days? I'm curious how much the efficiency gains are.

It's interesting that lumber kind of looks like shrinkflation (what the comment I was originally responding to suggested), but it's more that technical improvements have allowed producing the same finished product with less input material. But we still label the stuff by the amount of input material it used to take for historical reasons, which at a glance looks like shrinkflation.

https://en.m.wikipedia.org/wiki/Lumber

And what's is worse is how hard it is to know the true size of everything you find in stores in the USA. The same applies to pipes of all types. But even things like a drawer slide is often not the advertised length. Saw blades thinkness is converted from 2.5mm to whatever the closest inch the salesperson felt that day.

From having spent quite a lot of time in big box stores in France and the USA. I can tell you that in the USA 3/4" (19mm) can be anything from 16mm to 24mm.

In short almost everything is built in metric and the USA likes to pretend it's not and gets confused by rounding up randomly.

2x4 in old houses will measure 2x4 and is "rough cut". Modern milling will turn rough cut true 2x4 into the clean cut "2x4" you use today by running them through additional planing process. I don't think any shrinkage is significant though.

3/4 plywood will measure less than 3/4.

3/4 inch plywood is usually actually about .7 inches

Does this vary more depending on the plywood grade, maybe? I have some shelving I made from AB birch, and it looks pretty close to .75. Maybe 1/64 scant. (Although I don't have calipers handy and just used a tape measure, so I'm not being super precise.)

Lots of plywood, especially birch, is metric-sized. You might have 18mm plywood on your hands, which would be about 1/32 under 3/4 inches.

The plywood I often buy at a big box home store is labeled as 23/32" (0.71875" or 1/32" less than 3/4") and it is actually that thickness. I don't recall seeing any 3/4" labeled plywood though.

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