by Eric Meier

In my experience, specific gravity is without a doubt the single most abused and vaguely used term in woodworking terminology.

Technically, specific gravity is a measure of the ratio of a wood’s density as compared to water. (So if a wood is of the same density as water, the specific gravity would be 1.00.) However, as with any density measurement for wood, it is greatly dependent upon the wood’s moisture content: the more moisture the wood contains, the denser it will be. The chief problem arises is that there is no standardized way between woodworkers and botanists to express specific gravity: and there is no implicit or assumed values. (At least with average dried weight, the moisture content is generally assumed to be at 12% unless otherwise noted.)

There are several ways to express specific gravity for wood—the standard within botany uses a wood’s ovendry weight (meaning a moisture content of 0%, which is the lightest the wood can ever get), and its green volume, that is, when it’s freshly cut: having the largest possible volume. This may seem like a double-standard—to calculate this density from the wood’s dry weight, and its green volume—but this standardization, commonly called the “basic specific gravity,” prevents any irregularities or inconsistencies from occurring, mainly because it uses predictable extremes (i.e., lightest weight and largest volume) to calculate the SG value. Such a combination is a real-world impossibility: it’s useful within the scientific community, but is very confusing for the woodworking community.

Other specific gravity values used in botany include using both ovendry weight and volume, called ovendry specific gravity. Another is to use the ovendry weight, and the volume of the wood at 12% MC. The problem with these scientific measurements is that they use a non-existent ideal which never truly represents a given piece of wood at any one time. Since the weight is always based on the ovendry value, it tends to produce an artificially low impression of specific gravity.

In addition to the variety of measurements used in botany, woodworkers also use various standards to gauge specific gravity—usually based on real-world wood samples. Accordingly, specific gravity measurements referenced in woodworking will usually be a pairing of green weight and green volume, 12% MC weight and 12% MC volume, and so forth.

Clear as Mud

Between scientific and woodworking standards, there are at least five different ways to express specific gravity, and oftentimes sources (particularly woodworking publications) will make no attempt at identifying which standard is being referenced. For instance, American Beech (Fagus grandifolia) could be as low as .54 for its basic specific gravity, or up to .73 for its specific gravity based on 12% MC weight and volume. (And if the wood were still green and above its fiber saturation point, its specific gravity could be over 1.00, indicating that it would sink in water.) With such a wide disparity between specific gravity values, it’s not hard to see how confusing this measurement can become when no qualifying information accompanies the value.

On this website, every effort has been made to use clear and standardized numbers for specific gravity measurements. The first number is the basic specific gravity, based on the botanical standard of ovendry weight and green volume. The second number is meant for woodworkers, and is simply a snapshot of the wood’s specific gravity at 12% MC, (that is, both 12% MC weight and volume). Water weighs 1,000 kilograms per cubic meter, so taking the wood’s density (in metric units) and dividing by 1,000 yields its specific gravity in woodworking standards.

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Johhny dough
Johhny dough

Hi, I appreciate this site and come to it often. My question is in regards to btu values and certain species of wood. If a species of wood has a higher basic specific gravity than another, wouldn’t that species also have a higher btu value than a species with a lower basic SG? BTU charts that I can find on the net give varying values, sometimes significantly. Also, when I search through Wood Database I find that the ADW of some species of wood per cubic foot is higher than for some others species even though their basic SG is… Read more »

Dave
Dave

Does elevation affect wood density? Sea level vs. mountains?

Steve
Steve

There is an importance factor. When working with wood, as an engineer, one needs to be cognizant of weight, when used for dunnage, for floor loading.
This dunnage need to posses a reliable strength to support the loads.
Wood possesses great impact resistance.
Also, dimensionally, there are specific dimension at which wood is considered self extinguishing, when there is no exogenous heat source.
I am disappointed that some packagers of fat wood products list and price based on cf while others on lbs. It is rather annoying that one cannot compare, even with the vast internet.
Thanks

Biren Shah
Biren Shah

What is the density of Sal wood?

Alex Smith
Alex Smith

Density at 12% moisture content; and about 1.3m3 of seasoned sawn timber per tonne: aulacocarpa 800kg/m

Mrs Wood
Mrs Wood

What can be said about wood that is always used underwater, which species is the best, what is their SG, moisture level calculations, and specific gravity, etc?
Is wood that grows in water the best, for that situation eg as wharf pilings?

Babar Rana
Babar Rana

What is the range of specific gravity if wood generally?????

K.C.

Very interesting! Let me ask, would a piece of wood that’s not dried in an oven, go to it’s basic (0 mc) specific gravity over time?? Or would it settle at some other moisture content, due to natural humidity, perhaps?

pamodya pamunuwa
pamodya pamunuwa

I find this quite interesting and informative!