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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.