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How Lumber is Sized

By: Joe Truini

Did you know that a 2×4 does not actually measure two by four inches?

If you’re new to carpentry and woodworking you may be surprised to learn that lumber today is actually smaller in both width and thickness than its listed size. That’s because the listed dimensions for lumber are based on the rough sawn thickness and width of the board as it comes out of the sawmill before it is planed to make it smooth and uniform in size.

To determine the actual size of planed lumber from the listed dimensions:

  • Thickness: The actual thickness of planed lumber is 1/4″ less for boards 1” in thickness, and 1/2″ less for boards 2″ and thicker. So the actual thickness of a 1” planed board will be 3/4″ while a 2” thick planed board will measure 1½” thick.
  • Width: The actual width of planed lumber is 1/2″ less for boards 6” wide or narrower, and 3/4″ less for boards over 6” wide. So the actual width of a 6” planed board will be 5½” while the actual width of a 10” planed board will be 9¼” wide.

Watch this video to find out more.

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How Lumber is Sized