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Nominal Lumber Size Dimensions and Conversion Chart

A part of me has always been bothered by the fact that the lumber business can’t come up with a straightforward method for measuring their products. Why can’t a 2×4 lumber size be exactly that: 2 inches by 4 inches? Instead, I’m forced to fill my mind with pointless mathematical puzzles. To clarify, when I have too 2x4s together, they are actually… 3′′ x 3′′…or is it 3.5′′ x 3.5′′? Let’s see.

Nominal Lumber Size Dimensions and Conversion Chart

The Difference Between Nominal and Actual Sizes

The “nominal” lumber size refers to the label that is placed on the timber that you purchase. The real size is always a bit smaller than the stated size. The nominal dimension refers to the size of the timber after it has been sawn. When a 2×4 is cut from a log, the dimensions are 2′′x4′′. After the board has been dry and planed, it will have reached its final dimension of 1 1/2′′x3 1/2′′.

This short conversion table will come in handy if your memory is anything like mine and you need a little assistance remembering the actual measurements of the timber you’ll use for your next project:

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″

Take a Look for Yourself

Whenever you go to Lowes or Home Depot, make sure to have a tape measure with you and go straight for the dimensional lumber sections. Compare the total length and width of a wood board (such as a 2×4), and then compare the measurements to the ones on the price tag on the shelf. Hmm. There’s something that doesn’t quite add up.

In the case that you can talk to a store employee who can assist you in explaining the difference, they’ll start saying terms like “nominal sizes” and “real sizes,” as well as “softwood lumber” and “hardwood lumber.” I am hoping that they will spend the time necessary to describe what these terms represent, it will be very helpful in making you feel more at ease about purchasing wood.

To start, you’ll need to understand that softwood (such as spruce, pine, and fir) is measured and sold in one manner, but hardwood (such as maple, oak, and cherry) is sold and measured in a completely another manner. If you get the two concepts mixed up, you’ll be in serious trouble. You’ll have to cut a piece or will be a few inches short.

Sometimes you’ll need to use a filler to ensure that the parts where boards come together are of the same size. For instance, the header of a door or window is typically constructed of two boards that are joined together and supported by an upright stud. When the two header boards (both 1 1/2 inches wide) are joined together, the vertical stud would be 3 1/2 inches wide, but the combined width of the two header boards will be just 3 inches. As a result, the answer is to cut a piece of 1/2′′ plywood to the same size as the two header boards and put it between them.