Board feet and log rules
In the United States, saw logs and lumber are measured in board feet. One board foot is a piece of wood 12 inches long, 12 inches wide and 1 inch thick. Because a round log loses wood to bark, slabs, saw kerf and shrinkage when it is sawn into square boards, foresters estimate the yield with log rules, formulas or tables that predict the board feet a mill can cut from a log of a given diameter and length.
Measuring a log
- Diameter: at the small end, inside the bark, in inches. Measure the average width if the end is oval.
- Length: in feet, usually in 2-foot steps, with 3 to 6 inches of extra length for trimming.
The Doyle rule
Board feet = (D − 4)² × L ÷ 16
Published in 1825, Doyle is easy to compute and still widely used in the South and Midwest. It deducts 4 inches from the diameter for slabs, which heavily underestimates small logs: a 10-inch, 16-foot log comes out at 36 board feet, little more than half the International figure. Around 30 inches the two rules agree, and above that Doyle gives more.
The International ¼-inch rule
This rule, developed by Judson Clark in 1906, computes each 4-foot section of the log separately, assumes the diameter grows by half an inch per 4 feet of length, and allows for a ¼-inch saw kerf. For a 4-foot section, board feet = 0.905 × (0.22 D² − 0.71 D). It is the closest of the common rules to the actual sawn yield, and many state and federal agencies use it.
Example
A 16-inch log, 16 feet long:
- International ¼-inch: about 181 board feet (published tables round to 180)
- Doyle: (16 − 4)² × 16 ÷ 16 = 144 board feet
- Cubic volume, assuming a mid-length diameter of 17 inches: about 25 cubic feet
That 37 board foot difference is why it matters which rule a timber sale uses.
Pricing timber
Logs and standing timber are usually priced per thousand board feet (MBF). A sale of 50 logs like the one above would be about 7.2 MBF by Doyle or 9 MBF by International ¼-inch, so the same price per MBF gives a very different total. Ask which rule is used, and compare offers on the same basis.
Other rules and units
- Scribner, based on diagrams of boards drawn inside circles, is common in the West.
- Cubic feet and cords are used for pulpwood and firewood. A standard cord is a stack 4 × 4 × 8 feet, or 128 cubic feet, that holds roughly 80 cubic feet of solid wood.
Limits
Log rules assume straight, sound logs. Defects such as rot, sweep, crook or large knots reduce the yield, and professional scalers deduct them. The cubic volume here uses Huber’s formula with an assumed taper of about 1 inch per 8 feet.