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Episode 20 – Nominal Sizes

Nominal sizes are sizes that mean something other than the obvious.

For example a 1 1/2 inch pipe has no dimension that is 1 1/2 inches, but a 1 1/2  inch pipe is always the same size.   Similarly, there is no dimension on a 2X6 inch piece of lumber that is either 2 inches or 6 inches, but all 2 X 6 boards are the same size.

Many things are sized in these nominal units.  It means something, but not what most people would initially think it means.   It is just something you have to get used to.

A pdf of framing lumber and pipe sizes is down-loadable here.

An end view of a 2X4.


This is the picture discussed in the audio file showing how the corners of a 2X4 are rounded.  This makes it a little difficult to measure exactly to the edge of the 2X4.  The best way is to place another board or some other straight edge up against the side of this board and measure to that straight edge.



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2 comments to Episode 20 – Nominal Sizes

  • marbetu

    Hi and thanks,
    Imagine I buy enough lumber (2x4s) for 1/2 my house and build half of the frame.
    Then I get called away for a year.
    A year later, I have to buy the rest of the lumber for the house from a new supplier (different forest, different everything) as the local hardware has gone bust.
    Is it likely that the two lots of lumber will be different dimensions – width x depth (albeit slightly different?).

    • Gary

      Yes… They will be very very similar. The only way I ever got “burned” on something like that was I once had a house that was prefabricated, (factory built). The studs in that house were undersized. So to prevent a wall from having a “hump” in it if you were doing an addition, it would be necessary to rip every new board.
      My guess that was not a case of differences in nominal sizes but probably the manufacture of the house was ordering wood to just meet some standard and shaving pennies off each board.

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