Most of us buy lumber that is either ‘dressed’ or ‘rough’. Dressed lumber (also called ‘surfaced lumber’) has been jointed and planed after it has been dried, while rough lumber has been dried but not planed. Although it’s convenient to buy dressed lumber by the piece from your local building supply store (especially if you don’t have the equipment to joint and plane the lumber yourself), lumberyards can provide a wider selection of lumber species, greater variety in board widths, lengths and thicknesses, and often at better value. Lumberyards generally stock rough lumber, but many will dress the lumber to your specifications for a reasonable surcharge.
The standard reference for buying lumber is the nominal measurement.
‘Nominal’ refers to the dimension of rough lumber before it is dried and planed into the dressed boards most of us buy. In nominal measurement the width and thickness of a board is measured in regular fractions such as 1″, 1-1/2″ and 2″. Sometimes you will see thickness referred to in ‘quarters’ – lumber is measured in quarters of an inch; thus 4/4 equals 1″, 8/4 equals 2″, 6/4 equals 1-1/2″, and so on.
When you buy a 1″ x 6″ piece of rough lumber you actually get a full 1″ thick by 6″ wide piece of wood. If the lumber has been dressed, that 1″ x 6″ piece of lumber will be somewhat smaller, typically 3/4″ thick by 5-1/2″ wide, which is a result of the loss of material through jointing and planing. That’s why a 2 x 4 actually measures 1-1/2″ by 3-1/2″ and a 2 x 6 measures 1-1/2″ by 5-1/2″. There are industry wide standards that regulate the minimum thickness and width for dressed lumber. The two most widely used standards are the National Hardwood Lumber Association (NHLA) and the American Softwood Lumber Standard (ASLS). These standards are voluntary, however, and you should always check the dressed dimensions with your supplier.
To calculate the cost of lumber, your supplier uses a standard unit of measurement called a ‘board foot’ (bf). It’s similar to buying nails by the pound or paint by the gallon. Calculating the board foot measure is simple if you think of it as a measurement of volume. A board foot is 144 cubic inches of wood, equivalent to a 12″ x 12″ x 1″ piece. Remember that if you are buying dressed lumber the finished size will be less than the nominal size.
To calculate board feet, multiply the nominal thickness in inches by the nominal width in inches by the length in inches and divide by 144. You can also measure the length in feet – you’ll then divide by 12 instead of 144. Typically any length less than a full foot is rounded down to the next lower whole foot.
In the following example we’ll calculate the bf of a piece of lumber that is 1″ thick, 6″ wide and 8′ long:
1 x 6 x 8 = 48
48 / 12 = 4 bf
Pricing is based on the nominal dimensions when sold by the board foot ($/bf) even it it’s been dressed to a finished size. When sold by the linear foot, the price varies depending on the width and thickness, however the basis for the price will usually be the board foot cost. Many retail outlets stock a selection of dressed boards in standard dimensions and price them by the piece. When comparison shopping, you can easily work backwards to establish the price based on the board foot measure. The accompanying illustration shows you four boards of various dimensions that are all 4 board feet. Different dimensions, same price.
The comparison chart provided gives the minimum actual sizes of lumber that has been dressed on four sides as compared to the nominal sizes. The chart also shows the board foot measure for a typical 6′ and 8′ length of each size.