The amount or volume of wood, whether it be logs or sawed lumber, is generally described in terms of board feet. Calculating board feet is the method used in the United States for pricing lumber. The size or dimensions of lumber are given as the “nominal” or “actual” dimensions in inches. The nominal dimensions refer to the original size of a piece of lumber. By the time the lumber is surfaced, processed and treated the actual dimensions for the piece are less by about a half inch. So if the nominal size of a piece of lumber is given as 2 x 4, the actual dimensions are closer to 1 ½ inch x 3 ½ inches. If the nominal thickness of a piece of lumber is 2 inches or less it is called a board. Dimension lumber refers to pieces ranging from 2 to 4 inches in nominal thickness. Lumber with thicknesses of 5 inches or greater are described as timbers. Dimension lumber and boards are generally made available in lengths of 2 foot increments ranging from 8 feet to 16 feet, but rafter boards can be obtained in lengths up to 24 feet.
A board foot is based on nominal dimensions and is defined as the volume of a piece of lumber 1 square foot in cross sectional area by 1 foot long. All dimensions must be expressed in the same units in order to calculate board feet. If the cross sectional area is equal to less than 1 square foot, and the area is measured in square inches, the length must also be converted to inches. If the area is given as a fraction of a square foot, the length is measured in feet.
For example, a piece of lumber 1 inch thick by 6 inches wide by 8 feet long and a piece 2 inches thick by 3 inches wide by 8 feet long both contain 4 board feet. A board 2 inches thick by 12 inches wide by 10 feet long and a piece of lumber 4 inches thick by 6 inches wide by 10 feet long both contain 20 board feet.
A Simple Method for Calculating Board Feet
Calculating board feet can be easily done by following the steps below.
- Find the cross sectional area of a piece of lumber in units of square feet by multiplying the width x the thickness.
- Divide the cross sectional area by 1 square foot. (This will still give a value with units of square feet)
- Multiply the results by the length (given in feet) of the piece to give a value with units of cubic feet (When base numbers are multiplied, the exponents are added. Therefore if feet are multiplied by square feet, the resulting units are cubic feet, which defines a volume).
The commercial timber industry typically deals in large volumes of lumber, so that prices for lumber are often quoted as dollars per thousand board feet.