Since water conveniently weighs about 1 g/cm3 , variable with temperature, specific gravity was derived as an index metric to state the weight of other substances relative to water. The nice thing about using water as the reference measure was that early scientists could easily classify materials by whether they float on water (specific gravity less than 1.0) or whether they sink (specific gravity greater than 1.0).
With the specific gravity around 1.5, solid wood "substance", or lignocellulose as it is commonly called today, weighs around 1500 kg/m3 (93.6 lb/ft3), at theoretical most…no air, water, or other fluids in cell pores, which would decrease the weight of the wood per unit of volume. Wood also contains measurable quantities of organic "extractives" such as such as terpenes, resins, and polyphenols such as tannins, sugars, and oils. In addition, inorganic compounds such as silicates, carbonates, and phosphates appear in the wood as "infiltrates" and result in ash as the wood substance is decomposed. These extractives and infiltrates impregnate the lignocellulosic matrix and fill parts of the cavities of the wood. Ash, which makes up 0.5% to 2.0% of most woods, has a specific gravity of 1.6 to 2.8; the specific gravity of extractives varies depending on the substance. Together, the amount of ash and extractives in wood can vary from trace amounts to 30% and therefore affect the weight of wood differently according to species. 1
Thus far, we have been talking only of solid wood substance, which is not really wood as we know it. Wood of course, is comprised of cells, of which only the cell walls have the specific gravity stated above. Actual woods, the amazing composite of cell matrix in infinitely different shapes, sizes, and arrangements, much like a sponge made of lignocellulose, have much lower specific gravity than the theoretical maximum due to the amount of space in the matrix filled with air and water. And like a sponge, most woods float, and therefore have specific gravities less than 1.0; a few of the denser tropical hardwoods are actually heavier than water and sink. The most commonly referenced "heaviest wood", lignum vitae (Guaiacum officinale), has a specific gravity of 1.05 when green, which makes its weight about 1365 kg/m3, or 85 pounds per green cubic foot.
Now, green wood can have moisture content anywhere from 30% (denser woods) to over 200% (lighter woods). Let's talk about green wood in more detail.
First, that crazy moisture content calculation that confounds so many beginning wood science students. How, they wonder, can wood have more than 100% moisture?
It can't, of course. The moisture content calculation is simply a comparison of the mass of a sample of wood at any given moisture to its mass when "oven-dry", or when all the water has been removed from the sample. This is accomplished by drying the wood to a constant weight in a laboratory oven held at 101 to 105 degrees centigrade. The equation used for calculations is quite simple:
MC = (mgreen - mdry) / mdry (1)
So, to use a simple example, if a sample block of wood weighs 50 grams at original weighing, and 40 grams after being dried to 0% moisture content, then the moisture content of the original sample was (50-40)/40, or 25%. Now, suppose that original sample had weighed 100 grams. Then its original moisture content would have been (100-40)/40, or 150%.
We see in this example that an increase in moisture content results in the increased weight of green wood. This leads us also to the understanding that sapwood, with its higher moisture content in the field, often weighs more than heartwood. For softwood, this is practically always true. Hardwoods vary by species, and on average the moisture content in sapwood is only slightly higher. Table 4-1 of the 2010 Wood Handbook2 gives the heartwood and sapwood moisture contents of 40 North American hardwoods, and 28 North American softwoods. For the species in the table, the hardwood heartwood averages 81% moisture content, while the hardwood sapwood averaged 83%; the softwood heartwood, however, averages only 60%, while the softwood sapwood averaged 152%!
A similar situation exists between earlywood and latewood. Earlywood cells, formed in the fast-growing early weeks of growth when moisture movement is at its maximum, are necessarily larger with thinner cell walls to allow higher flow volume. As growth slows, the later cells formed take on a denser form with thicker cell walls and smaller cell lumina. Therefore, those species or specimens that exhibit wider bands of earlywood (or diffuse-porous species that exhibit no apparent latewood) will show more weight differential from green to dry than will those with significant bands of latewood.
The most technically correct way to calculate weight of wood gets somewhat tricky, because the specific gravity of woods changes with moisture content once the moisture content goes below 30%. 30% moisture content (plus or minus a couple of percentage points, based on the wood species and sample) is what we call the fiber saturation point of wood; above the fiber saturation point, the physical and mechanical properties of wood do not change as a function of moisture content. In other words, the specific gravity of wood does not change in wood that is above 30% moisture content. That is because the cellular structure of wood is "full" of what is called "bound water", the water chemically bonded to the wood. The structure of the wood is fully expanded at this point, and any additional water that increases the moisture content is "free" water residing in the cell pores and lumina.
Once moisture content goes below 30%, however, all the free water has been released through evaporation, and the bound water begins to be chemically driven from the wood substance. As it does so, the wood cells begin to shrink, again, just as a sponge does as it dries out. And as it shrinks, the specific gravity of the wood gets higher, and the wood becomes stiffer as the chemical properties change. Most species increase in density anywhere from 10 to 20% as they dry from 30% down to oven dry, 0%. (This, by the way, is why wood checks and splits as it dries.) So another way of thinking about this is that the density of green wood (that above 30%) is different (less) than the density of the wood as stated in most references, which are usually given as specific gravity at 12% moisture content, for the purposes of aiding those who work with wood.
As you may surmise from the paragraph above, capturing all the dynamics of changing wood density according to different moisture contents becomes pretty complicated for anything but basic research. The researcher must be able to calculate the density of the wood at the moisture content of interest. For this reason, specific gravity (which is always based on a wood's oven-dry mass) is usually expressed in one of three ways: 1) as specific gravity (oven-dry), which is the specific gravity of the wood when both mass and volume are measured at oven-dry; 2) as specific gravity (basic, or green), which is the specific gravity of wood when mass is measured at oven-dry and volume is measured green; and as specific gravity (12%), which is the specific gravity of wood when mass is measured at oven-dry and volume is measured at 12% moisture content.
For the purpose of our interest in calculating green weights of wood, we can ignore the complexities of wood shrinkage below 30% (as represented by sg(oven-dry) and sg(12%), and use sg(basic) in our calculations. (Note: if you are interested in calculating weights of a kiln-dried woods, you must use the specific gravity of the wood at that moisture content. That is when sg(12%) is most commonly used.)
Basic specific gravity, sg(basic) or sg(green), is that estimated by comparison of the wood's mass at 0% to its volume when green (30% MC or above.) Basic specific gravity is not given in most online reference pages, but they can be found for many species around the world in Tables 5-3, 5-4, and 5-5 of the 2010 Wood Handbook. (The basic specific gravity is identified by that at "green" moisture content in the tables, as opposed to the specific gravities listed at 12% moisture content.) Other good sources for basic specific gravity are the IWCS "Useful Woods of the World" books, and online at The Wood Database. The most comprehensive tables of specific gravities, both basic and at 12%, for North American woods that I know of is compiled in "Specific Gravity and Other Properties of Wood and Bark for 156 Tree Species in North America", by Patrick Miles and Brad Smith. This book is available free online.
To get on with the calculation of green weight...by the magic of algebra, we transform Equation 1 above to
mgreen = mdry * (1 + MC) (2)
Recall that mdry is the mass of dry wood and MC is the moisture content at which we wish to estimate the weight. We will use the mass of one cubic meter for mdry, and for MC we have a choice: we could plug in an assumed moisture content, one taken from a green sample, or one taken from reference. There are not many references on the green weight of wood; for this exercise, I have used Table 4-1 of the 2010 Wood Handbook, which gives green weight of heartwood and sapwood for 40 North American hardwoods and 28 North American softwoods.
Let's take the oaks and hickories first. Table 4-1 list 12 species of oaks and hickories, and they average. 59 in basic specific gravity. Recall that specific gravity is density in grams per cubic centimeter (g/cm3); since the denominator of the specific density is one cubic centimeter, and assuming we want to calculate the weight of a cubic meter, we simply have to multiply 0.59 * 1000 to get 590 kilograms as the mdry for Equation 2.
Next, we need to know what moisture content to expect for the species in question when green. We could then refer again to our Table 4-1 of the 2010 Wood Handbook, and calculate that the average moisture content (heartwood and sapwood) for all 12 is 71%. Now we have all we need. The calculation becomes:
Oak-Hickorygreen = 590 * (1 + 0.71) = 1009 kilograms/m3 , or 63 pounds/ft3
However, we know that with all the estimating and assuming we've been doing in these calculations, that our point estimate of 63 pounds per cubic foot is too precise. A better way to state our expected green weight of oaks and hickories would be to use estimates based on variation around our mean estimates. To keep it simple, I calculate a Low Range estimate and a High Range estimate for moisture content as 1/3 and 2/3 of the way between the stated values of heartwood and sapwood moisture content. Having done that, we find that the lowest low range estimate is for sand hickory, at 56%, and the highest high range estimate is 85% for water hickory. Substituting these values for the grand average MC used above, and using the specific gravities for the species from which we get the MC range values, we get:
Oak-Hickorygreenlow = 620 * (1 + 0.56) = 967 kilograms/m3 , or 60 pounds/ft3
Oak-Hickorygreenhigh = 610 * (1 + 0.85) = 1129 kilograms/m3 , or 70 pounds/ft3
So a correct statement in the original article should have been that oaks and hickories average between 60 and 70 pounds per cubic foot green. If you only have one estimate of green moisture content to work with, simply use that number for the MC in the equation, as I do in the next example.
The next species we consider here is live oak, Quercus virginiana. Its basic density is 0.80. If we use 80% as the moisture content again (it isn't listed on the Wood Handbook table 4-1), our estimate of live oak's green weight is:
Live Oakgreen = 800 * (1 + 0.8) = 1440 kilograms/m3, or 90 pounds/ft3
But we immediately notice that this estimate is pretty close to our previous calculation of the maximum theoretical weight of wood. What's wrong?
What we find is that there is a functional relationship between the specific gravity of a wood and the moisture content it can attain; in other words, at a certain point, the density of the wood limits how much moisture it can take in. Again a table from the 2010 Wood Handbook is helpful. Tables 4-6a and 4-6b give us the relationship between density of wood and moisture content. Unfortunately, the tables only go up to a specific gravity of 0.7, where the highest possible moisture content is listed as 72%. However, by extrapolating the table by the trend rate, we conclude that the highest possible moisture content of wood with a specific gravity of 0.8 to be 52%.
So, our better estimate of the weight of green live oak becomes:
Live Oakgreen = 800 * (1 + 0.52) = 1216 kilograms/m3, or 76 pounds/ft3
This technique works well for all wood species.
For those of you who just can't get enough of this sort of thing, I've developed an Excel spreadsheet that calculates dry and green wood weight of different species from specific gravity and moisture content data from the various tables I've alluded to in this article. The spreadsheet can be downloaded using the link in the left-hand table of this page. Also, if you have green moisture content data on species outside of North America, and you feel like sharing that data, please send it on and I will add those species with the appropriate calculations to the spreadsheet.
- Wood: Its Structure and Properties. Volume 1. Edited by F.F. Wangaard. 1981. The Pennsylvania State University Press. Page 199.
- Wood Handbook: Wood as an Engineering Material. 2010 Edition. The Forest Products Society.