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I know that a 2x4 is actually 1.5x3.5 because the 2 and 4 refer to the rough-sawn dimensions. But when planing a rough 1x2 and other 1 inch nominal thickness lumber into S4S, why do they only take 1/8" off each surface in the 1-inch dimension? Even on that size board mills take 1/4" off each side in the larger dimension.

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This has been answered elsewhere on the Internet, though there seem to be a few different stories running around.

The one I currently believe is that nominal 1" is actually 1" unsurfaced -- but what you buy as construction lumber is typically s4s (surfaced 4 sides). That means some wood has been planed away. To make that predictable, the convention was established that 1/8" could be removed from each side of a 1" thickness.

Yes, it's weird. But that"s how the industry has decided to label things. It's much like the fact that hard drives were often measured in metric base-10 gigabytes rather than the power of two that programmers normally would call a gigabyte -- annoying but there really isn't much we can do but be aware of it.

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  • I did find something that said that rough dimensions less than 2" get 1/4" taken off; and dimensions 2" (inclusive) and above get 1/2" taken off. However, it seems there is a continued if nonlinear correlation, insofar as a 2x8 is 1.5" by 7.25", meaning they planed off 3/4".
    – stannius
    Jul 15, 2016 at 6:39
  • a nominal 2x4 is actually 1.5x3.5", not the 1.5x3 you indicated. Was that a typo or are you getting gyped? Also, hard drives used to actually be labeled in 2^10 bytes until the marketing geniuses discovered they could sell smaller devices by selling in multiples of 1000 for the same price. :(
    – FreeMan
    Jul 15, 2016 at 20:35
  • Ok, I gotta go measure; I could swear two stacked 2-bys equalled a 4-by, but I am quite willing to believe I am being an idiot today.
    – keshlam
    Jul 15, 2016 at 21:35
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    Yes a 1x4 is 3/4 x 3.5, 1x6 is 5.5, but then as you get larger they start taking more off. 1x8 is 7.25, 1x10 = 9.25, 12= 11.25. Go figure.
    – Joshua
    Jul 16, 2016 at 22:04
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    Making a raw board flat and square does tend to get more difficult as the surface gets wider, as we've discussed before with regard to jointing boards. I suspect they're trying to allow for that... But it may not be that innocent. Note that this is one of the reasons serious woodworkers tend to prefer to surface and/or resaw their lumber from cured but otherwise raw stock.
    – keshlam
    Jul 16, 2016 at 23:04

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