What is a Board Foot?
A board foot is a specialized volumetric unit of measurement for lumber in North America. It is defined as a piece of wood 12 inches wide, 12 inches long, and 1 inch thick (144 cubic inches).
Estimating how many board feet exist inside a raw log is called 'scaling.' The scaler must account for the wood that will be lost to the kerf (the width of the sawblade), the slabs (the rounded outer edges of the log that cannot make square boards), and natural taper.
The Doyle Log Rule
Developed in 1825, the Doyle Rule relies on a mathematical formula rather than mill tests. It heavily penalizes small logs, assuming massive amounts of wood will be lost to slabs. Because of this, buyers prefer Doyle because it drastically underestimates the yield of small logs, allowing them to buy wood cheaply.
The Formula: Board Feet = [(D - 4) / 4]² x L. Where 'D' is the diameter at the small end of the log inside the bark (in inches), and 'L' is the log length (in feet).
Worked Example: A log is 16 feet long with a 10-inch diameter. [(10 - 4)/4]² x 16 = [1.5]² x 16 = 2.25 x 16 = 36 Board Feet.
Scribner and International 1/4-Inch Rules
The Scribner Rule (1846) is not a formula, but an empirical table created by drawing circles and physically diagramming how many 1-inch boards could fit inside, assuming a 1/4-inch saw kerf. It is highly accurate for medium logs but underestimates large logs.
The International 1/4-Inch Rule (1906) is the most scientifically accurate. It uses a complex mathematical formula that accurately accounts for a 1/4-inch saw kerf, slab allowance, and a standard log taper of 1/2 inch per 4 feet of length. It is widely used by state and federal forest agencies.
Scaling Deductions for Defects
Log scale formulas assume the tree is perfectly straight and solid. In reality, logs have rot, sweep (curvature), ring shake, and ant holes.
A licensed scaler will calculate the gross board footage using the chosen rule (e.g., Doyle), then physically measure the defects and subtract that volume mathematically to arrive at the Net Board Feet, which is what the mill actually pays for.
Frequently asked questions
Why is diameter only measured at the small end of the log?
Because boards must be cut straight. The diameter of the small end represents the maximum width of a continuous, straight board that can be extracted from the entire length of the log. The flared wood at the large end becomes slab waste or short boards.
Should I sell my timber using Doyle or International?
Landowners generally prefer the International 1/4-Inch rule because it pays for the actual volume of the tree. Buyers prefer Doyle because it underestimates small timber, effectively giving them 'free' wood. The choice depends on local custom and contract negotiation.
What is 'overrun' in a sawmill?
Overrun occurs when a mill physically cuts more board feet of lumber from a log than the log scale predicted. Modern mills with highly efficient, ultra-thin bandsaws routinely achieve 10% to 20% overrun compared to the historical Doyle or Scribner estimates.
Does bark count towards the log diameter?
No. Log scaling is strictly performed 'inside the bark' (DIB). Bark cannot be made into lumber, so it is ignored in all scale calculations.