The weight of wood may not be very important when buying wood or firewood considering that the dimensions of the wood are what matters. But knowing the weight of wood may be important when it comes to other things. A builder considers lightweight, easy to carry but strong and resilient wood. A furniture maker also wants strong and light wood to make different furniture pieces. This is a guide to show you how much wood weighs and what is considered when it comes to weighing wood.
It is difficult to have a specific answer to this question because there are numerous pieces of wood and each one has specific qualities and properties that affect its weight. The weight of wood also varies whether it is seasoned or unseasoned and whether you are weighing early wood or late wood and more.
The composition of wood is like the human body. We are about 75% water with the remaining 25% made of flesh, bones and body fluids. Wood is also made up of a large percentage of water and the remaining would be the wood heart, grains, trunk and more.
Consider the amount of water inside a piece of wood. Wood is porous with every pore loaded with water. Since water weighs about 1 g/cm3 and variable with temperature, specific gravity was derived as an index metric. Knowing this value, you will be able to identify the weight of other substances relative to water. Using water as a reference measure also identifies which materials will be able to float on water and which ones will sink.
With the specific gravity around 1.5, solid wood weighs around 1500 kg/m3 (93.6 lb. /ft3). Consider air, water, and other fluids in cell pores, which would reduce the weight of the wood per unit of volume. Wood also contains large amounts of organic “extractives” like terpenes, resins, and polyphenols. Polyphenols are tannins, sugars, and oils. And aside from these materials, wood also has inorganic compounds such as silicates, carbonates, and phosphates. All these materials fill the parts of the cavities of the wood.
Wood is comprised of cells and only the cell walls have the specific gravity stated above. Actual composite of cell matrices varies in shape, size, and arrangement. Think of a sponge made of lignocellulose, have much lower specific gravity than the theoretical maximum because of the amount of space in the matrix that is composed of air and water.
And again similar to a sponge, most types of woods float and therefore have specific gravities with values less than 1.0. However, there are a few tropical hardwoods are actually heavier than water and thus will sink. The heaviest wood is lignum vitae (Guaiacum officinale), has a specific gravity of 1.05 when green. On the other hand, green wood has moisture content anywhere from 30% to over 200%.
When it comes to earlywood and latewood there are stark differences in weight. Earlywood cells are created during the fast-growing early weeks of growth. This is when moisture is at its maximum and is higher with thinner cell walls to allow improved flow volume. As wood starts to slow its growth, the cells that are formed take on a denser form and have thicker cell walls and smaller cell lumina. Therefore the younger a wood, the heavier it is.
Calculating the weight of wood
The most technically correct way to calculate the weight of wood is a bit complicated. This is because the specific gravity of woods changes with moisture content. Once the moisture of wood goes below 30% then wood begins to grow lighter.
30% moisture content is the fiber saturation point of wood. Above this level and the physical and mechanical properties of wood do not change with the moisture content. In other words, the specific gravity of wood does not change in wood that is above 30% moisture content. This is because the cellular structure of wood is “full” and the water has bonded to the wood. At this point, the structure of the wood has completely expanded and any additional water is “free” water in the cell pores and lumina.
When moisture content goes below 30%, all the free water has evaporated and the bound water begins to be driven from the wood material. Wood cells begin to shrink just like when a sponge dries out. As wood shrinks, the specific gravity of the wood increases and the wood becomes stiffer as the chemical properties change.
To answer how much does wood weigh is quite difficult. There are many factors to consider in determining the actual weight of wood. Also considering that wood is not just made of “wood” it contains a high percentage of water as well as other organic materials and extractives. The weight of wood also varies whether it is earlywood or latewood.