The commercial value of standing timber is determined by estimating the amount of marketable lumber that can be produced from a tree. This process, known as timber scaling or cruising, predicts the final product volume for inventory and payment purposes.
Calculating the board feet in a standing tree is always an estimation, never a precise measurement, because the accurate volume is only known after the tree is harvested and milled. These initial estimates rely on standardized measurements and mathematical models called log rules, which provide a consistent basis for transactions between buyers and sellers.
Defining the Board Foot
The board foot serves as the standard unit of commercial lumber measurement in North America, representing a specific volume of wood. It is defined as a piece of lumber that is 1 foot long, 1 foot wide, and 1 inch thick, totaling 144 cubic inches. This unit provides a universal metric for pricing and inventory tracking across different species and cuts.
The calculation is derived by multiplying the thickness and width in inches by the length in feet, and then dividing the total by twelve. For example, a standard 2×4 that is 8 feet long contains approximately 5.33 board feet. This standardization allows for comparing the cost and quantity of wood required for a project.
Collecting Measurements from Standing Trees
Accurately estimating a tree’s board footage begins with collecting two primary field measurements: Diameter at Breast Height (DBH) and merchantable height. DBH is a standardized measurement of the tree trunk’s width, taken at 4.5 feet (1.37 meters) above the ground on the uphill side of the tree. This height is used because it avoids the irregular swelling that often occurs at the tree’s base.
The most common tool for obtaining DBH is the diameter tape (D-tape), which is a flexible tape measure calibrated to convert the tree’s circumference directly into diameter. A quicker, though less precise, tool is the Biltmore stick, which is held a fixed distance from the eye and uses geometric principles to project the diameter onto a calibrated scale. For trees that are not perfectly circular, foresters typically take two diameter measurements at right angles and average them to account for the trunk’s elliptical shape.
The second measurement is merchantable height, which is the usable length of the trunk from the stump to the point where the diameter narrows past a specified minimum or where significant defects begin. A clinometer is used to determine this height by measuring the angle to the top and bottom of the merchantable section from a fixed horizontal distance away from the tree. The readings are then used in a trigonometric calculation to determine the vertical height of the usable log section.
Estimating Volume Using Log Rules
Once the Diameter at Breast Height and merchantable height are recorded, this data is converted into an estimated board foot volume using mathematical formulas known as log rules. Log rules are not based on the actual cubic volume of the wood but rather on the theoretical amount of lumber that can be sawn from a log of a specific diameter and length. Three of the most widely recognized are the Doyle, Scribner, and International 1/4-Inch Rules.
Doyle Rule
The Doyle Rule, developed around 1825, is a formula rule that is simple to calculate but significantly underestimates the board footage of small-diameter logs. This rule includes a large allowance for waste material, such as slabs and sawdust, which disproportionately penalizes smaller logs. It is still commonly used in many regions, particularly for hardwood sales, often with a corresponding adjustment in the price per board foot to compensate for the low volume estimate.
Scribner Rule
The Scribner Rule is a diagram rule, meaning its values were originally determined by drawing circles representing log ends and fitting diagrams of 1-inch thick boards inside them. The most common variation, the Scribner Decimal C Rule, rounds the volume to the nearest ten board feet for convenience. The Scribner Rule provides a volume estimate that is closer to the actual yield for intermediate-sized logs but tends to underestimate larger logs and does not account for log taper.
International 1/4-Inch Rule
The International 1/4-Inch Rule, developed in 1906, is considered the most consistent and mathematically accurate of the three rules. This formula-based method incorporates an allowance for a 1/4-inch saw kerf and a fixed amount of log taper over the length of the log. Because it reflects the geometric reality of sawing a log more accurately, the International Rule is often used as the standard reference.
Why Actual Yield May Differ from Estimates
The board foot volume calculated using log rules rarely matches the quantity of lumber actually produced at the sawmill due to wood loss during processing. Saw kerf is one factor, which is the width of the saw blade that turns a portion of the wood into sawdust during each cut. While modern band saws and thin-kerf technology have reduced this waste, the material loss still prevents the full theoretical volume from being recovered.
Internal defects within the log also contribute to a lower yield because they must be cut out to produce marketable lumber. Defects like internal rot, excessive knots, or “sweep” (a curve or bend in the log) reduce the usable length and width of the final boards. The milling efficiency and the sawing pattern chosen by the mill operator also influence the final product volume.