Figuring out lumber dimensions can be confusing at first.
In fact, the entire first trip to the lumber yard can be intimidating to a new woodworker. However, it is worthwhile getting comfortable going to a place where you can pick and choose the wood you want to work with.
Usually when buying wood you will be using board feet (bf) or linear feet
(lf) as the measure of the lumber's dimension. One board foot is one
square foot (1” x 12” x 12” or its equivalent). For example a board that
is 1” x 6” x 24” is also 1 bf, as is a board that is 2” x 6” x 12”.
To calculate a board foot multiply the length of board in inches, by the width of board in inches, by the thickness of board (yes, in inches), then divide by 144 (square inches in a foot).
A linear foot means that an individual board is sold by length rather
than board foot. Wood that has been milled to less than ¾, for instance,
is almost always sold by linear foot.
To find the price simple multiply the price/lf by the length of the board. For example a piece of 1/2" cherry that is 6" wide might be sold by the linear foot. So a 6 foot board would cost 6 times the price/linear foot.
To further confuse things when talking about lumber dimensions, hardwood
isn’t referred to by inches, but by quarter inch. A board that is one
inch thick is called 4/4 (pronounced “four quarter”) one that is 1 ½” is
6/4 (six quarter) , 2” is called 8/4 (“eight quarter”), etc.
Softwood used for construction purposes, on the other hand, is referred to by inches. For example a 2" x 4" pine board is a "two by four," a 1" x 12 is a "one by twelve," etc.
Now that you know the terminology for lumber dimensions you can get down to the actual lumber sizes.
When dealing with dimensional lumber (wood that is cut to standard sizes) there are two ways to refer to its size: nominal size and actual size.
Nominal sizes are the rounded up version of the actual size. It is the size of the rough wood before it is milled to it's standardized size.
For hardwoods the nominal sizes are rounded to the nearest 1/4" for softwood (construction lumber) it is the nearest inch. This represents the size of the plank before it is milled to the final size.
Actual size is, as it sounds, the size of the board in front of you. It refers to the standardized size of the lumber.
For example, if you go the the lumber yard and see a sign for 4/4 cherry, that is it's nominal size. If it has been milled it will actually be 3/4 (well, to be precise it will probably be 13/16 since that is usually the milled thickness). If it is rough lumber it will be close to one inch in thickness.
Pricing will be figured using the nominal sizes (see above for figuring board feet). So, when figuring out the total board feet for pricing purposes, a board milled to 13/16 will still be considered 4/4, since that is the size of the original (rough) board. In other words, if you have a 3/4" x 12 x 12 piece of wood you still have to pay for a full bf (board foot), not 3/4 of a bf.
A general rule of thumb is that nominal sizes are the actual thickness of rough lumber and actual size is the standardized size. The idea is that you are paying for the same amount of wood. The nominal size tells you the size of the rough board, while the actual size is what it has been finished to.
1 x 2
1 x 3
1 x 4
2 x 4
2 x 6
2 x 8
3/4 x 1 1/2
3.4 x 2 1/2
3/4 x 3 1/2
1 1/2 x 3 1/2
1 1/2 x 5 1/2
1 1/2 x 7 1/2
thickness in inches
thickness in inches
Lumber dimensions for hardwoods usually has an actual thickness of ¼”
less than the nominal. With construction lumber, it is ½” less up to a
certain size (2" x 8" wide, I think) where it becomes 3/4” smaller. So the
familiar 2x4 is actually 1 1/2" x 3 1/2."
It does become clear over time, really it does. Take a trip and bring your tape measure. It's fun digging through all that wood!