How Lumber is Sized

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.

Further Information


  1. Just recently bought 1×4 and the were 3 3/8″ x 5/8, they were from New Zealand it is clear lumber I was expecting it to meet U.S. dimensions. Do they have a different standard and Lowes marked it to the closest american industry standard?

    • Hi, David,

      Danny says, “I’m not sure. Good question. But I know there have been a lot of complaints about dimensional lumber getting smaller than previous sizes. It generally happens with clear, smooth finish lumber.

      Good luck!”

  2. I just purchased 5 /4 deck boards 8 foot lentghs its about 1 inch over the nominal length of 8 feet the actual length was about 97 inches is this so you can true the ends before you install them?

    • Hi, Joseph!
      5/4 is the rough-sawn size, the measurement before sanding and surfacing.
      Thanks for your question!


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