by Eric Meier
[arve url=”https://youtu.be/6XAJyj9Au6Y” /]
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.
This implies that the settings on a Wagner moisture meter should be adjusted down as the wood dries, but if you don’t know the MC, you wouldn’t know what to adjust to. Very confusing. I use Wagner’s suggested SG values (Wagner calibrates at 12%), and take the measured MC with a grain of salt, used purely as a general guide for how much the board has dried from air dry to final use. Clear as mud.
Eric, I’ve read your articles on specific gravity and moisture content and understand you provide both the BASIC and 12% specific gravity numbers for each species on your website. I use a Wagner MMC 220 moisture meter which requires that I adjust the meter for a given species by entering the specific gravity value for that species. While I’ve read your articles a couple of time, I’m not sure which specific gravity number that is listed in the “Moisture Content” area for a particular wood species I should enter in the Wagner meter (Basic or 12%) so that I’m getting… Read more »
You’d want to use the green SG numbers for a moisture meter.
Can i know what is factors that affect specific gravity?
Can i know how to get Hardness using Specific Gravity?
There is no relationship between Specific Gravity and hardness. Diamond specific gravity of 3.52 is about as hard as it gets mohs 10 and is all carbon. While graphite specific gravity 1.9-2.3 is very soft 1-3 mohs and also all carbon.
Hematite specific gravity 5.26 is only rated 5.5 – 6.5 mohs.
Technically what you say is true — for minerals. But there most certainly is a strong relationship between specific gravity and hardness in wood. There’s so much of a relationship between the two that the USDA published a paper on how to estimate hardness from specific gravity. https://www.fpl.fs.fed.us/documnts/fplrp/fpl_rp643.pdf
Eric, when you calculate the Janka hardness based on specific gravity, are you using the basic specific gravity or the 12% MC specific gravity (or something else)? For example, for red mallee burl, you list a Janka hardness of 2,490 lbf based on specific gravity, but I can’t tell whether you’re using .79 or 1.05. And I assume for red mallee burl you’re using the tropical, rather than temperate, hardwood equation given by Wiemann and Green, right?
Many thanks for such a wonderful site. I use it all the time.
Using green SG, and yes, using the tropical equation.
Hello Mr. Eric, Thanks for the Video. is it ok to use the formula as below?
Specific gravity was calculated with the recommended formula: Wg = Wd(1 + M/100)
Where: Wg is the wood gravity.
Wd is the wood density (Weight/ size)
M is moisture content.
Wood Density is 442kg/m3, Weight 1.028kg, Humidity 11.6% dimensions of wood piece 1660x70x20mm.
base on the above formula I am getting SG 493. is it correct? where i am doing a mistake? attached the calculation sheet.
Thank you for your help
I’m curious what application you are wanting to know specific gravity for? You already have the wood density, so that is what I would use. Easiest way is to just take density and divide by 1000, so SG = .442. If you are trying to find BASIC specific gravity, that is another story, and a much longer explanation. To get an accurate BASIC SG number, you would first need to know the volume of the wood when green and also the weight of the wood at 0% moisture. Not really practical or helpful in most instances.
Hello Mr. Eric,
I would like to thank you for the precious information in the article!
And I want to ask if the Wagner Meters MMC220 can give a trustful measurment of the Humidity Content of a wood.
Would you recommend that meter to be used for that?
I don’t have experience with that exact model, but it looks like a quality unit.
Thanks for the reply.
Which one would you recommend please?
Hello. Do you have any data on the specific gravity of the following two timber species from Papua New Guinea, when they are freshly cut (green)?
Canarium indicum (aka “Red Canarium”)
Canarium oleosum (aka “Grey Canarium”)
I am not asking about any of the other many Canarium species found all around the planet, just these two, please.
Many thanks for any help you can provide!
Canarium indicum 0.560
Canarium oleosum 0.482
These are BASIC specific gravity numbers, so please take this into account if trying to calculate average dried weight.
Thank you, Eric. This is great. If I may please ask one really simple, follow-up question: The numbers above are “specific gravity numbers.” Is this the same as what these species would weigh in log form, when green and freshly cut? Apologies for the very elementary follow-up question. I just want to be absolutely sure that I understand you.
No, it’s very confusing. See the third paragraph of this article for an explanation.
Thank you, Eric. Sorry to say that there did not appear to be an article attached to your comment above. Did I miss something? Sincere thanks.
You are commenting on the very page of the article — look up.
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 »
Does elevation affect wood density? Sea level vs. mountains?
Density is a function of an object’s volume and mass. Specifically, it is the mass in a unit of volume. Mass is constant everywhere in the universe. Therefore altitude has no influence on density.
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.
What is the density of Sal wood?
Density at 12% moisture content; and about 1.3m3 of seasoned sawn timber per tonne: aulacocarpa 800kg/m
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?
Submerged wood reaches what’s called its fiber saturation point (FSP) which varies a bit for each wood species. Generally, it is somewhere between 25-30% moisture content — so after this point (the FSP) the wood will no longer expand or change dimensionally. See this article for more info: https://www.wood-database.com/wood-articles/wood-and-moisture/
What is the range of specific gravity if wood generally?????
Off the top of my head, I’d say average for most woods is .4 – -9 specific gravity. Extreme ends would be more like .1 – 1.4.
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?
No, it would reach what is known as equilibrium moisture content, which is, as you guessed, dependent on relative humidity. (This is why almost all the data on the site here is standardized to a 12% moisture content, as this is believed to reflect a good “average” moisture level.
I find this quite interesting and informative!