By Gracie Knight, Forisk Intern

 

In the Pacific Northwest (PNW), logs are typically scaled based on the Scribner Decimal C Log Rule that estimates the board feet of lumber recoverable from a log based on small end diameter and log length rather than weight. Two different Scribner Decimal C log rules are typically used on opposite sides of the Cascade Mountains. Long log Scribner is more commonly used in the coastal and western regions of Washington and Oregon, where the assumed maximum scaling length is 40 feet. Short log Scribner assumes a 20 foot maximum length and is standard for inland regions east of the Cascades. The two scales yield different volume estimates for the same tree. California Department of Tax suggests long log Scribner volumes should be increased 20% to convert to short log Scribner.

Since weight scaling is not widely used in the PNW, there is no well-established tons-to-MBF (thousand board feet) conversion rate. Different species vary in density. A board foot of a dense species like Western hemlock weighs more than a board foot of Douglas-fir. Wood density also varies geographically. For example, areas west of the Cascade Mountains receive more rainfall with milder temperatures than the east side. Westside Douglas-fir weighs more per MBF due to increased moisture and denser wood. With multiple commercial species and varying densities, finding reliable, consistent tons per MBF conversions for the PNW is challenging.

Forisk previously used 8 tons per MBF to estimate the weight of scaled logs in the PNW. This quarter, we reviewed the conversion against published conversions from Washington and Oregon Departments of Revenue (Figure). A tons per MBF conversion of 7.5 better represents the actual log sizes grown and procured by the forest industry west of the Cascades for Douglas-fir. We updated our conversion rate to 7.5 tons per MBF as a result. Notably, the conversion east of the Cascades appears closer to 5.5 or 6.

Figure. Department of Revenue conversions from Scribner MBF to tons

¹ Oregon Department of Revenue. (2020). Harvesting Timber and Timber Taxes. https://www.oregon.gov/DOR/forms/FormsPubs/harvesting-timber-taxes_441-616.pdf

² Washington Department of Revenue. (2025). Tax Reporting Instructions and Stumpage Value Determination Tables. https://dor.wa.gov/sites/default/files/2025-01/InstrSVtables2025_1stHalf.pdf