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Nominal Lumber Size Dimensions and Conversion Chart

by Jeff Sawyer 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.

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{ 2 comments… read them below or add one }

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

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.

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