The volume of wood required for heating or construction is commonly measured by a unit called a cord. This standard measurement allows for consistent trade of firewood and raw timber. Determining how many trees must be harvested to fill one of these large stacks is complex, as the answer depends entirely on the size, species, and quality of the trees being cut. The number can range from a single massive tree to dozens of very small ones.
Defining the Standard Cord
A standard cord is a precisely defined volume used in the wood industry. It is officially measured as a tightly stacked pile of wood that occupies a space of 4 feet wide, 4 feet high, and 8 feet long. This arrangement results in a total volume of 128 cubic feet.
This 128 cubic feet measurement includes the solid wood, air space between the pieces, the bark, and any irregularities in the wood shape. This makes the cord a measure of stacked volume, not solid wood volume.
The actual amount of solid wood within a standard cord typically ranges from 70 to 90 cubic feet. For mixed firewood, 85 cubic feet is often used as a reliable average. The variation in solid wood content depends on how neatly the wood is split and stacked, as poorly stacked wood contains more air space.
Factors Influencing the Tree Count
The largest variable determining the number of trees needed for a cord is the tree’s diameter at breast height (DBH), which is measured 4.5 feet above the ground. A small increase in diameter results in a disproportionately large increase in wood volume because the volume is related to the square of the radius. This means a 16-inch diameter tree can yield four times the wood of an 8-inch diameter tree.
The usable height of the trunk is another major factor, referring to how far up the tree the main stem is straight and thick enough to be worth cutting. Trees grown in a dense forest tend to be taller and straighter before branching out, increasing their usable volume compared to open-grown trees that spread out more.
Additionally, the wood species plays a role, with dense hardwoods like oak generally yielding more solid wood per cord compared to less dense softwoods like pine. Other factors, like the tree’s taper from base to top and the presence of internal rot, also affect the final usable volume. A tree with a severe taper or significant decay will yield less usable wood, which increases the total number of trees required to reach the 128 cubic feet standard.
Practical Estimation for the Average Tree
For typical mixed hardwood firewood, a practical rule of thumb suggests that a standard cord requires three to five large, mature trees. These are generally trees with a DBH of 18 to 20 inches, which produce a high volume of wood from their main trunks. A single, exceptionally large and tall tree with a DBH of 22 inches or more can sometimes yield a full cord by itself.
Harvesting smaller, younger trees significantly increases the number of individuals needed to achieve the same volume. For trees with a smaller diameter, such as those with a 10 to 12-inch DBH, the count typically ranges from six to ten trees per cord.
A tree with a 12-inch diameter and a usable height of 50 feet, for instance, is estimated to contain about 0.25 cords, meaning four such trees would be needed. Very young or small-diameter trees require a much higher number to make a cord. A tree with a DBH of just 8 inches requires approximately eight individuals to create a full cord of wood. This demonstrates that the decision to harvest large, mature timber or smaller growth is the greatest determinant of the final tree count.