# Calculating the Green Weight of Wood Species

How much does wood weigh? The question is simple enough for dry wood, but more difficult to when freshly cut.

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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 mostâ€¦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. 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.

Reference:

1. Wood: Its Structure and Properties. Volume 1. Edited by F.F. Wangaard. 1981. The Pennsylvania State University Press. Page 199.
2. Wood Handbook: Wood as an Engineering Material. 2010 Edition. The Forest Products Society.

## Green Wood Weight Calculator

Green heartwood and sapwood moisture content data taken from Table 4-1, 2010 Edition of the Wood Handbook.

### Hardwoods

Moisture Content, % Weight of one cubic meter, in kilograms Weight of one cubic foot, in pounds Genus Species Common Basic SG SG @ 12% Heartwood Sapwood Average Range Low Range High 12% MC Average Green Low Green High Green 12% MC Acer saccharinum Maple, Silver 0.44 0.47 58 97 78 71 84 526 781 752 810 33 49 47 51 Acer saccharum Maple, Sugar 0.56 0.63 65 72 69 67 70 706 944 937 950 44 59 58 59 Betula papyrifera Birch, Paper 0.48 0.55 89 72 81 78 83 616 866 853 880 38 54 53 55 Betula lenta Birch, Sweet 0.6 0.65 75 70 73 72 73 728 1035 1030 1040 45 65 64 65 Betula alleghaniensis Birch, Yellow 0.55 0.62 74 72 73 73 73 694 952 950 953 43 59 59 60 Carya cordiformis Hickory, Bitternut 0.6 0.66 80 54 67 63 71 739 1002 976 1028 46 63 61 64 Carya tomentosa Hickory, Mockernut 0.64 0.72 70 52 61 58 64 806 1030 1011 1050 50 64 63 66 Carya glabra Hickory, Pignut 0.66 0.75 71 49 60 56 64 840 1056 1032 1080 52 66 64 67 Carya ovalis Hickory, Red 0.62 0.68 69 52 61 58 63 762 995 978 1013 48 62 61 63 Carya pallida Hickory, Sand 0.62 0.68 68 50 59 56 62 762 986 967 1004 48 62 60 63 Carya aquatica Hickory, Water 0.61 0.62 97 62 80 74 85 694 1095 1059 1131 43 68 66 71 Celtis occidentalis Hackberry 0.49 0.53 61 65 63 62 64 594 799 795 802 37 50 50 50 Fagus grandifolia Beech, American 0.56 0.64 55 72 64 61 66 717 916 900 931 45 57 56 58 Fraxinus americana Ash, White 0.55 0.6 46 44 45 45 45 672 798 796 799 42 50 50 50 Juglans nigra Walnut, Black 0.51 0.55 90 73 82 79 84 616 926 911 940 38 58 57 59 Liquidambar styraciflua Sweetgum 0.46 0.52 79 137 108 98 118 582 957 912 1001 36 60 57 63 Liriodendron tulipifera Yellow-poplar 0.4 0.42 83 106 95 91 98 470 778 763 793 29 49 48 50 Magnolia grandiflora Magnolia, Southern 0.46 0.5 80 104 92 88 96 560 883 865 902 35 55 54 56 Malus sylvestris Apple 0.61 0.67 81 74 78 76 79 750 1083 1076 1090 47 68 67 68 Nyssa sylvatica Tupelo, Black 0.46 0.5 87 115 101 96 106 560 925 903 946 35 58 56 59 Nyssa biflora Tupelo, Swamp 0.46 0.5 101 108 105 103 106 560 941 935 946 35 59 58 59 Nyssa aquatica Tupelo, Water 0.46 0.5 150 116 133 127 139 560 1072 1046 1098 35 67 65 69 Platanus occidentalis Sycamore, American 0.46 0.49 114 130 122 119 125 549 1021 1009 1033 34 64 63 65 Populus tremuloides Aspen, Quaking 0.35 0.38 95 113 104 101 107 426 714 704 725 27 45 44 45 Populus deltoides Cottonwood, Eastern 0.37 0.4 162 146 154 151 157 448 940 930 950 28 59 58 59 Quercus kelloggii Oak, California Black 0.56 0.61 76 75 76 75 76 683 983 982 984 43 61 61 61 Quercus rubra Oak, Northern Red 0.56 0.63 80 69 75 73 76 706 977 967 987 44 61 60 62 Quercus falcata Oak, Southern Red 0.52 0.59 83 75 79 78 80 661 931 924 938 41 58 58 59 Quercus nigra Oak, Water 0.56 0.63 81 81 81 81 81 706 1014 1014 1014 44 63 63 63 Quercus alba Oak, White 0.6 0.68 64 78 71 69 73 762 1026 1012 1040 48 64 63 65 Quercus phellos Oak, Willow 0.56 0.69 82 74 78 77 79 773 997 989 1004 48 62 62 63 Tilia americana Basswood, American 0.32 0.37 81 133 107 98 116 414 662 635 690 26 41 40 43 Ulmus americana Elm, American 0.46 0.5 95 92 94 93 94 560 890 888 892 35 56 55 56 Ulmus crassifolia Elm, Cedar 0.59 0.64 66 61 64 63 64 717 965 960 970 45 60 60 61 Ulmus thomasii Elm, Rock 0.57 0.63 44 57 51 48 53 706 858 846 870 44 54 53 54 Average 0.52 0.58 81 83 82 646 937 923 951 40 58 58 59

### Softwoods

Moisture Content, % Weight of one cubic meter, in kilograms Weight of one cubic foot, in pounds Genus Species Common Basic SG SG @ 12% Heartwood Sapwood Average Range Low Range High 12% MC Average Green Low Green High Green 12% MC Abies balsamii Fir, Balsam 0.33 0.35 88 173 131 116 145 392 761 714 807 24 47 45 50 Abies grandis Fir, Grand 0.35 0.37 91 136 114 106 121 414 747 721 774 26 47 45 48 Abies procera Fir, Noble 0.37 0.39 34 115 75 61 88 437 646 596 696 27 40 37 43 Abies amabilis Fir, Pacific Silver 0.4 0.43 55 164 110 91 128 482 838 765 911 30 52 48 57 Abies concolor Fir, White 0.37 0.39 98 160 129 119 139 437 847 809 886 27 53 51 55 Calocedrus decurrens Cedar, Incense 0.35 0.37 40 213 127 98 155 414 793 692 894 26 49 43 56 Chamaecyparis lawsoniana Cedar, Port Orford 0.39 0.43 50 98 74 66 82 482 679 647 710 30 42 40 44 Cupressus nootkatensis Cedar, Yellow 0.42 0.44 32 166 99 77 121 493 836 742 930 31 52 46 58 Juniperus virginiana Cedar, Eastern red 0.44 0.47 37 115 76 63 89 526 774 717 832 33 48 45 52 Larix occidentalis Larch, Western 0.48 0.52 54 119 87 76 97 582 895 843 947 36 56 53 59 Picea mariana Spruce, Black 0.41 0.42 52 113 83 72 93 470 748 707 790 29 47 44 49 Picea engelmannii Spruce, Engelmann 0.33 0.35 51 173 112 92 132 392 700 633 767 24 44 39 48 Picea sitchensis Spruce, Sitka 0.37 0.4 41 142 92 75 108 448 709 646 771 28 44 40 48 Pinus strobus Pine, Eastern White 0.34 0.35 98 219 159 138 179 392 879 810 947 24 55 51 59 Pinus taeda Pine, Loblolly 0.47 0.51 33 110 72 59 84 571 806 746 866 36 50 47 54 Pinus contorta Pine, Lodgepole 0.38 0.41 41 120 81 67 94 459 686 636 736 29 43 40 46 Pinus palustris Pine, Longleaf 0.54 0.59 31 106 69 56 81 661 910 842 977 41 57 53 61 Pinus ponderosa Pine, Ponderosa 0.38 0.4 40 148 94 76 112 448 737 669 806 28 46 42 50 Pinus resinosa Pine, Red 0.41 0.46 32 134 83 66 100 515 750 681 820 32 47 42 51 Pinus echinata Pine, Shortleaf 0.47 0.51 32 122 77 62 92 571 832 761 902 36 52 48 56 Pinus lambertiana Pine, Sugar 0.34 0.36 98 219 159 138 179 403 879 810 947 25 55 51 59 Pinus monticola Pine, Western White 0.36 0.48 62 148 105 91 119 538 738 686 790 34 46 43 49 Pseudotsuga menziesii Douglas-fir, Coastal 0.45 0.48 37 115 76 63 89 538 792 734 851 34 49 46 53 Sequoia sempervirens Redwood, Old Growth 0.38 0.4 86 210 148 127 169 448 942 864 1021 28 59 54 64 Taxodium distichum Baldcypress 0.42 0.46 121 171 146 138 154 515 1033 998 1068 32 65 62 67 Thuja plicata Cedar, Western Red 0.31 0.32 58 249 154 122 185 358 786 687 885 22 49 43 55 Tsuga canadensis Hemlock, Eastern 0.38 0.4 97 119 108 104 112 448 790 776 804 28 49 48 50 Tsuga heterophylla Hemlock, Western 0.42 0.45 85 170 128 113 142 504 956 896 1015 31 60 56 63 Average 0.40 0.43 60 152 106 476 803 744 862 30 50 46 54