Log Scale

Learn more about how to scale your logs on the Log Scale page.

Descriptions of the Scribner Log Scale, the Doyle Log Scale and the International Log Scale can be found here.

Scaling methods are estimates of volume; therefore, variability exists between the different methods based on log shape, density, size, taper, moisture content and human error.  Log scaling determines the quantity of wood and is done primarily to determine the value of individual logs. Scaling also is used to maintain accountability for paying loggers and landowners, to evaluate the financial performance of a sawmill, to ensure the quality of logs entering the mill yard, to assist in inventory and quality control, and to allow tracking of mill efficiency.

Board-foot log scaling methods

Board-foot log scaling methods have traditionally been the standard used with the industry and are still the most commonly used. This method estimates the volume of lumber that can be sawn from a log on a board-foot basis and requires that each log be scaled according to a specific log rule. A log rule is a mathematical formula or table that gives an estimate of the net yield of lumber in board feet for logs given a specific diameter and length. Log rules generally have allowances for losses in yield due to saw kerf, producing slabs, edging and shrinkage. More than 95 log rules are recognized in the US and Canada. Each log rule is different because of the way log taper, saw kerf, slabbing, edging and other manufacturing variables are calculated. Logs are not perfect cylinders, so each rule uses a different method to estimate the volume of a tapered, imperfect cylinder. Most log rules were developed prior to the 1930s, so they do not account for modern sawmill technology (thinner saw kerfs, better equipment, etc.), often underestimating today’s actual yield.

Common log rules

The three most common rules are the Doyle, Scribner Decimal C and International 1/4 –inch. 

Doyle Log Scale/Rule

The Doyle Scale/rule is a rule commonly used in hardwood log scaling in the southern United States. Edward Doyle developed this rule prior to 1850. This rule estimates the volume based on log length, diameter, slabs, edgings, shrinkage and production of sawdust. The allowance for slabs and edgings is too large for small logs and is too small for large logs. Also, the rule allows for only a 4.5 percent reduction of log volume for sawdust and shrinkage where most rules allow between 10-30 percent. There is no allowance for taper. The formula used in estimating lumber volumes using the Doyle rule is

Log Volume (bd. ft.) = (D – 4)² L/16

where D is the diameter inside the bark measured in inches at the small end of the log and L is the nominal log length measured in feet. It underestimates small logs and overestimates large logs.

Scribner Log Scale/Rule

J. M. Scribner developed the Scribner log rule in 1846. This rule is based on a series of diagrams outlining the sawing pattern for 1-inch lumber for each diameter and length class with a 1/4-inch allowance for saw kerf. Taper is not accounted for in the rule, so it under-estimates volumes for logs more than 16 feet in length. One disadvantage of this rule is that there is no uniform slab allowance for log diameters. A change was made to the basic Scribner rule in the early 1900s to make it easier to apply. This new rule is known as the Scribner Decimal C rule and is different from the standard Scribner rule in that all volumes are rounded to the nearest 10 board feet. For example, a log that scales 64 board feet would be rounded to 60 board feet. This rule is commonly used on the West Coast and in the South for pine saw logs. A formula which closely estimates the Scribner rule is

Log Volume (bd. ft.) = (0.79D² – 2D – 4 )(L/16)

where D is the diameter inside the bark measured in inches at the small end of the log and L is the nominal log length measured in feet.

International 1/4 - inch Log Scale/Rule

Judson C. Clark originally developed this formula-based rule around 1906 as the International1/8-inch rule. In 1917 he developed the International1/4 inch rule, which increased the saw kerf allowance to 1/4-inch in the formula. This 1/4-inch rule is the form used today. The rule is based on analysis of losses that occur during the conversion of sawlogs to lumber and is one of the few rules that incorporates taper. The rule assumes a taper of ½ inch in 4 feet. All calculated values are rounded to the nearest 5 board feet. This rule is generally used in timber sales in eastern national forests and state forests.  This is the most accurate rule of the three discussed. The formula used in estimating lumber volumes using the International 1/4 -inch rule is

For 4-foot lengths: BF = (0.199D² – 0.642D)

For 8-foot lengths: BF = (0.398D² – 1.086D – 0.27)

For 12-foot lengths: BF = (0.597D² – 1.330D – 0.72)

For 16-foot lengths: BF = (0.796D² – 1.375D – 1.23)

For 20-foot lengths: BF = (0.995D² – 1.221D – 1.72)

where D is the diameter inside the bark measured in inches at the small end of the log and BF is board feet.

Of the three rules discussed, the Doyle rule is the least accurate in lumber volume estimation. The rule’s inaccuracies are due to the method by which slabs and edgings are accounted for and the lack of allowance for log taper. This rule under scales small logs and over scales large logs. The main reason this rule is used so often in the southern US is that it has been used for scaling hardwood logs for so long that the volumes the rule gives have become the standards of value. The errors associated with the rule are usually well understood by both the buyer and seller of logs so the prices paid for logs are adjusted appropriately. The Scribner rule is an intermediate estimator of lumber volume for logs less than 28 inches in diameter and less than 16 feet in length. Its inaccuracies come from the lack of allowance for log taper and a uniform slab allowance for different log diameters.  The International 1/4 -inch rule is the most accurate of the rules discussed.  The log rule used will vary for different areas. In rules that scale low, the adjustment is usually made by increasing the cost per thousand board feet for logs. Often, such rules have been used for scaling hardwood logs for so long that the volumes the rule gives have become the standards of value. An example of this is the common use of the Doyle rule. The rule used is commonly chosen based on the tradition of an area rather than the rule’s accuracy in volume estimation.