Understanding Density and Specific Gravity in Wood Selection
When selecting wood slabs for an epoxy river table or a hybrid tabletop, weight, handling, and material behavior are critical. Wood reference manuals frequently describe species using both density and specific gravity. While these terms are closely related, understanding the precise technical difference helps builders evaluate slab properties accurately.
According to the Wood Handbook (General Technical Report FPL-GTR-282) published by the USDA Forest Products Laboratory, the density of a substance is defined as the ratio of its mass to its volume. It is expressed in international units as kilograms per cubic meter, in inch-pound units as pounds per cubic foot, or in centimeter-gram-second units as grams per cubic centimeter. Specific gravity, also known as relative density, is the ratio of the density of the wood to the density of pure water at a reference temperature of 4 °C (39 °F). At this temperature, water has a density of 1.000 g cm–3 (which equals 1,000 kg m–3 or 62.43 lb ft–3). Because it is a direct ratio comparing wood to water, specific gravity has no units.
Why Wood Density and Volume Shift with Moisture
For inert materials like steel or aluminum, density at room temperature remains constant because they do not absorb moisture. Even materials like stone and brick that take on water maintain a constant exterior volume; their mass increases while volume remains stationary, producing a simple linear relationship between density and water content.
Wood behaves differently because both its mass and its volume depend directly on moisture content. When wood absorbs water below its fiber saturation point, the cell walls swell, increasing the physical volume of the slab. When it dries, the slab shrinks. Because water adds mass while drying reduces volume, calculating the density of wood at a given moisture content requires accounting for both changing variables simultaneously.
The Three Volume Bases: Green, Ovendry, and 12% Moisture Content
To establish fair comparisons across different tree species, reference tables standardize their measurements. In all cases documented by the USDA Forest Products Laboratory, the specific gravity of wood is calculated using its ovendry mass. However, because volume changes constantly as wood dries, specific gravity must be tied to a defined volume condition. The three common volume bases are:
- Green volume (Basic Specific Gravity): Referred to as basic specific gravity, this pairs ovendry mass with green wood volume before any drying shrinkage has occurred.
- 12% moisture content volume: This pairs ovendry mass with the volume of the wood after it has dried to 12% moisture content, a standard reference point for seasoned lumber.
- Ovendry volume: This pairs ovendry mass with the volume after all moisture has been driven out in an oven.
Because wood shrinks as it dries from green down to ovendry conditions, the measured volume decreases while the ovendry mass remains identical. Consequently, a species will always show a higher numerical specific gravity on an ovendry volume basis than on a green volume basis.
How to Read Specific Gravity Numbers for Epoxy Builds
Published species tables typically provide specific gravity based on green volume, 12% moisture content volume, or both. Understanding these values helps you anticipate tabletop weight and resin application needs:
- Overall weight ranges: Most species fall between an ovendry density of about 320 and 720 kg m–3 (20 and 45 lb ft–3). However, the full spectrum ranges from extremely light woods like balsa at about 160 kg m–3 (10 lb ft–3) to dense imported woods exceeding 1,040 kg m–3 (65 lb ft–3).
- Natural tree variation: Published values represent species averages. A coefficient of variation of about 10% describes the natural variability of ovendry density within common domestic species, caused by natural growth factors such as earlywood-to-latewood ratios and heartwood-to-sapwood ratios.
- Estimating shrinkage: When total volumetric shrinkage from green to ovendry is not known, it can be approximated by multiplying the basic specific gravity by 26.5.
Shop Checklist: Evaluating Slabs for Wood and Epoxy Tabletops
- Check the reference column: When looking up slab properties, confirm whether the specific gravity is listed on a green or a 12% moisture content volume basis.
- Account for species weight: Use the density corresponding to your slab’s expected moisture content when calculating finished tabletop mass, supporting base design, and hardware needs.
- Plan for internal variation: Factor in an expected variation of roughly 10% from published density numbers when dealing with different sections of domestic lumber.
- Confirm dryness before pouring: Ensure the wood is thoroughly seasoned and stabilized before sealing and pouring, following the resin maker’s data sheet for application parameters.
Sources
Published October 29, 2026. Sources are linked in the text.
Header image: original graphic by BigEpoxy.com.
Staff note: When calculating total volumetric shrinkage from the green condition down to ovendry, multiplying the basic specific gravity by 26.5 provides a reliable baseline estimate whenever species-specific shrinkage values are unavailable in standard tables. (Source: USDA Forest Products Laboratory, Wood Handbook (FPL-GTR-282, 2021).)