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Nominal Lumber Size Dimensions and Conversion Chart

By on June 11, 2010
Nominal Lumber Size

In a way, it has always disturbed me that the lumber industry can’t come up with a simple format for measuring their product.  Why can’t a 2×4 be just that, 2″x4″?  Instead I have to clutter my head with needless math problems. Let’s see, when I hold two 2x4s together they are actually… 3″ x 3.5″ …or is it 3.5″ x 3.5″?

Nominal vs. Actual

The label put on the lumber you buy is called the “nominal” lumber size.  The actual size is always a little bit smaller.  Nominal refers to the dimension the lumber was sawn.  When a 2×4 is cut out of a log it is 2″x4″.  After the board is dried and planed it becomes its actual size of 1 1/2″x3 1/2″.

If your memory is like mine, and you need a little help remembering the actual dimensions of the lumber you’ll need for your next project, then here is a quick conversion chart:

Nominal       Actual
5/4″                 1 1/8″
1″                       3/4″
2″                    1 1/2″
3″                    2 1/2″
4″                    3 1/2″
6″                    5 1/2″
8″                    7 1/4″
10″                  9 1/4″
12 ”                11 1/4″

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*Tip – Sometimes you’ll need a filler to get sections where boards meet to be of equal size.  For example, the header to a door or window is usually made with two boards set together and resting on an upright stud.  The upright stud will be 3 1/2 inches wide, but the two header boards (each 1 1/2 inches wide) are only 3 inches when put together. So, the solution is to cut a piece of 1/2″ plywood to equal size, and sandwich it between the two header boards.

7 Comments

  1. Tom Newby

    January 6, 2013 at 8:47 pm

    In getting lumber at say Home Depot, the dem. are not the standerd, they are smaller. Why is this? It get very diffecalt to design things when the lumber dose not match up.

    Tom

  2. Jeff Sawyer

    January 7, 2013 at 5:19 pm

    Tom – Not quite sure what you’re asking. If by “standard,” you mean the board’s actual measurement, then it’s due to drying and planing the wood after it’s been sawn.

  3. kenneth young

    July 22, 2015 at 4:12 pm

    FIRST THE TREE IS CUT TO BASIC SIZES. FOR EXAMPLE 2X4, 2X8, ETC. THIS NEW WOOD HAS A NATURAL HIGH MOISTURE CONTENT AND WILL SHRINK WHEN KILN DRIED. THE PROCESS OF DRYING MIGHT KINK THE WOOD, SHRINK IT, ETC. THIS DRIED PIECE IS TRIMMED TO ACTUAL DIMENSION. HENCE A 2X4 BECOMES 1 1/2 X 3 1/2″, ETC.

  4. B. Rocco Bama

    September 14, 2015 at 9:00 am

    This helps to explain why a half gallon of most ice cream brands is only three pints. They have to dry & plane the ice cream which is why you get less.

  5. Ed Kramer

    October 27, 2016 at 12:26 pm

    You’ll notice that at 8″, the amount subtracted from the nominal size goes up to 3/4″ so the actual size drops to 1/4. The reason for this is that wider boards shrink a greater amount. That is a 6″ board may shrink 3/8″ while an 8″ and wider boards may shrink 1/2″ or 5/8″. So when the board is put through a thickness planer to make it smooth, 3/4″ is allowed so that the boards between 8″ and 12″ are 7 1/4″ and 11 1/4″ respectively.

    So why not just add on 3/4″ on the wider boards so they all come out at X 1/2″ instead of X 1/4″ when the board’s are wider? There are 3 reasons I can think of:
    1 – Because the sawyers, those who cut the trees into rough lumber, are set up to do so in 2″ increments.
    2 – It’s always been done that way. (H8 that answer. Don’t you?)
    3 – You don’t want to confuse the sawyers now… Do you?

  6. Bob Donegan

    December 15, 2016 at 4:33 pm

    if the tree cutters know, which they do the wood will shrink when it drys, why not wait till it drys then cot a 2×4 to a real 2×4 and a 1×2 or a 3/4×4 real true cuts. its not that they will lose money, i’m sure they will sell the sawdust also. that will make those of us beginners and retired to build things once come out right, just saying give us chance.

  7. Mel Ginsburg

    February 2, 2017 at 3:13 pm

    There was once a time when the dressed sizes were larger than the current standards. I suspect shrinkage due to drying does not fully explain the reduced dimensions of dressed lumber today oompared to the nominal size.