How To Calculate Log Board Feet

A log is a round cylinder, but lumber is rectangular. When you calculate log board feet, you are trying to predict exactly how many rectangular boards you can cut out of that cylinder. This math is complicated by “saw kerf” (the wood turned into sawdust by the blade) and the rounded slabs that are cut off the sides and thrown away. Over the last two centuries, the timber industry developed three competing mathematical formulas to estimate this yield: the Doyle Rule, the Scribner Rule, and the International 1/4-Inch Rule. This guide compares how these three calculation methods work and why they produce drastically different board foot estimates for the exact same log.

Comparing The Three Main Log Scale Calculation Rules

No single log rule is universally “correct.” The rules were developed in different eras for different types of mills, and they treat the unavoidable waste of milling differently. It is critical to note that all three rules estimate gross scale volume only. They assume the log is perfectly straight and solid. They do not deduct board feet for defects like rot, sweeping curves, or insect damage.

1. The Doyle Log Rule Mathematical Calculation Method

The Doyle Rule is a simple mathematical formula developed in 1825. It is widely used for private timber sales in the Eastern and Southern United States. The Doyle formula assumes a massive 4-inch slab allowance (waste from squaring the log) and a thick 5/16-inch saw kerf. Because modern sawmills use much thinner blades and waste less wood, the Doyle rule is notorious for drastically underestimating the lumber yield of small logs.

The Doyle formula: Board Feet = ((Diameter - 4) squared) × (Length / 16). If you use the Doyle rule to sell small-diameter logs to a mill, the mill will likely recover far more board feet of lumber than they paid you for. However, for massive logs over 30 inches in diameter, the Doyle rule actually overestimates the yield.

2. The Scribner Log Rule Diagram Calculation Method

The Scribner Rule, published in 1846, is not a true mathematical formula. It is a “diagram rule.” The creator physically drew circles representing the ends of logs, then drew 1-inch thick boards inside those circles, leaving a 1/4-inch gap between each board for the saw blade (the kerf). You do not “calculate” Scribner; you measure the log and look up the volume on a pre-printed Scribner table. It is widely used in the Pacific Northwest and the Midwest.

Scribner is more accurate than Doyle for small logs, but it has a major flaw: it ignores log taper. It assumes a 16-foot log is a perfect cylinder based on its small-end diameter. Because actual logs are wider at the base than at the top, Scribner regularly underestimates the yield of long logs.

3. The International 1/4-Inch Rule Calculation Method

Developed in 1906, the International 1/4-Inch Rule is widely considered the most accurate of the three methods. It is the standard rule used by government agencies, the U.S. Forest Service, and professional forestry scientists.

The International rule uses a highly complex mathematical formula that accurately accounts for a modern 1/4-inch saw kerf, slab waste, and, most importantly, log taper. The formula assumes the log grows 1/2-inch in diameter for every 4 feet of length.

Because the formula is too difficult to run by hand in the field, foresters use lookup tables or digital calculators. The International rule produces the highest board foot estimates for small and medium logs, closely matching the actual lumber recovery of a modern sawmill.

The Standard Log Board Foot Calculation Procedure Guide

Regardless of which rule you use, you must measure the log exactly the same way to ensure the inputs are valid.

Step by step:

1. Measure the length of the log in feet. Most mills require logs to be cut slightly longer than the target length (e.g., 16 feet 4 inches) to allow for squaring the ends. You only input the target length (16 feet) into the formula.
2. Measure the diameter of the log at the small end. Do not measure the large end.
3. Measure the diameter inside the bark. Do not include the thickness of the bark.
4. If the log is not perfectly round, measure the shortest diameter and the longest diameter, then calculate the average.
5. Round the diameter down to the nearest whole inch.
6. Input the length and diameter into your chosen log scale table or calculator.

Worked example: Inputs: Log length 16 feet, average diameter inside bark 14 inches. Step 1 (Doyle calculation): ((14 - 4) squared) × (16 / 16) = 100 board feet. Step 2 (Scribner lookup): 114 board feet. Step 3 (International calculation): 135 board feet.

Result comparison: The exact same log yields 100, 114, or 135 board feet depending entirely on the rule chosen.

Because these differences represent hundreds or thousands of dollars in a commercial timber sale, the buyer and seller must explicitly agree on which rule to use before signing a contract. Use the Log Board Foot Calculator to instantly run your log dimensions through all three rules side-by-side without doing the manual math. Explore the Calcumatix construction calculators for related tools.

Sources and References

• U.S. Forest Service, National Forest Log Scaling Handbook
• University of Missouri Extension, Log Rules and Scaling