5.1 Density and specific gravity are cornerstone terms that help define many useful properties of wood and wood-based products. These terms designate concepts that have distinct definitions though they relate to the same characteristic (mass in a unit volume). Generally, in the US and Canada, density of wood is measured in terms of specific gravity, or relative density. In the wood-based composites industry and internationally the term density is often preferred.

5.2 The basic density and basic specific gravity of wood are used in the forestry industry for calculating the oven-dry weight of wood fiber contained in a known wood volume of various wood species. Thus, it serves as an indicator of the amount of wood pulp that could be produced, the workability of the material or its shipping weight. This information is referenced in various resources, including Wood Handbook.⁵ Note that specific gravity varies within a tree, between trees, and between species. Since the specific gravity of wood cell wall substance is practically constant for all species (approximately 1.53), it is apparent that individual specific gravity value is indicative of the amount of wood cell wall substance present. It affords a rapid and valuable method for selection of wood for specific uses. In US and Canadian building codes, the oven-dry specific gravity is correlated to various strength characteristics of wood products (e.g., compression perpendicular to grain, shear strength and fastener holding capacity).

5.3 It is often desirable to know the density or specific gravity of a living tree, a structural member already in place, a log cross section, a segment of a research element, or the earlywood or latewood layer. Therefore, it is possible that specimens will be large or small, regular or irregular in shape, and at a variety of moisture contents. These test methods give procedures that include all of these variables and provides for calculation of density and specific gravity values to degrees of precision generally needed.

5.4 In the wood-based composites industry, the product density or specific gravity also provides an important indicator of potential product attributes. For wood-based materials, the same test methods are used; however, the measurements typically combine the mass from the wood substance with any resin, wax, or other solid additives present in the material. These properties are not to be confused with equivalent specific gravity of structural composite lumber used to characterize its fastener-holding capacity determined in accordance with D5456.