Estimating the cordwood volume of a standing tree or a freshly cut stack is a common task for homeowners and firewood buyers. The process is complicated because trees are not perfect cylinders, and stacked firewood contains significant air space. Accurately determining the amount of usable wood requires understanding standardized measurements and separating the true volume of solid wood from the total space it occupies.
Defining the Standard Cord and Measurement Inputs
The official unit for measuring firewood volume in the United States and Canada is the standard cord. This unit is defined as 128 cubic feet of wood and air space combined. Traditionally, this volume is represented by a tightly stacked pile measuring four feet high, eight feet long, and four feet deep.
A face cord is often confused with a full cord, but it is not a standardized volume. It refers to a stack that is four feet high and eight feet long, but only as deep as the length of the firewood pieces, commonly 16 inches. This means a face cord of 16-inch wood is only one-third the volume of a standard cord, which is why measuring actual depth is necessary.
To estimate the yield of a standing tree, two fundamental measurements are required: Diameter at Breast Height (DBH) and Merchantable Height. DBH is the diameter of the tree trunk measured 4.5 feet above the ground, a standard forestry practice. This measurement is typically taken with a specialized diameter tape, or by measuring the circumference and dividing by pi (approximately 3.14). Merchantable height refers to the usable length of the trunk, usually up to where it tapers to a small diameter or where significant branching begins.
Calculating Wood Yield from a Standing Tree
Calculating the solid wood volume from a standing tree is challenging because the trunk tapers and is not a perfect geometric shape. Foresters often rely on pre-calculated volume tables specific to the tree species, which estimate the cord yield based on the measured DBH and merchantable height. For a simplified field estimate, the form factor method offers a way to approximate the volume before the tree is felled.
This method begins by calculating the basal area of the tree, which is the cross-sectional area of the trunk at DBH. The basal area is calculated using the formula for the area of a circle: Area = pi (Diameter/2)^2. This area is then multiplied by the merchantable height and a form factor, a number less than one that accounts for the tree’s natural taper. The simplified formula for volume is Volume = Basal Area Height Form Factor.
A typical form factor used for estimating firewood volume in hardwoods ranges from 0.33 to 0.44. This calculation yields the volume in cubic feet, which must then be divided by the solid wood content of a cord (approximately 70 to 90 cubic feet) to estimate the number of cords. The estimated volume must also be adjusted for defects like internal rot, cracks, and severe crookedness, which reduce the amount of usable wood. A visual inspection for signs of decay helps in applying a final percentage deduction to the initial volume estimate.
Measuring Stacked Firewood for a True Cord
Verifying a true standard cord involves measuring the cut and stacked wood to confirm it occupies the full 128 cubic feet of space. The pieces must be “racked and well stowed,” meaning they are aligned, parallel, and touching to minimize air space. The most reliable way to ensure a true cord is to construct a temporary frame or storage space with the required dimensions of eight feet long by four feet high and four feet deep.
The depth of the stack is where the greatest confusion occurs, especially with various log lengths. If the firewood pieces are cut to a standard 16-inch length, three parallel rows stacked side-by-side will create the necessary 48-inch (four-foot) depth for a full cord. Calculating the total volume involves multiplying the stack’s length, width, and height in feet, confirming the product equals 128 cubic feet.
Utilizing Online Tools and Species Conversion Factors
Modern tools simplify the estimation process by automating the volume calculations based on a tree’s measurements. Many university forestry extension services offer free online calculators or mobile applications where a user can input the DBH and merchantable height to receive an estimated cord yield. These tools use complex, scientifically derived volume equations and tables to provide a more accurate estimate than simple field formulas.
Once the physical volume in cords is determined, a crucial adjustment is made using species conversion factors, which relate to wood density and heat output. A cord of wood from a dense hardwood species, such as oak or hickory, contains more solid wood mass than a cord of a less dense softwood, such as pine, even though both occupy 128 cubic feet. This difference in density directly affects the British Thermal Unit (BTU) yield, which is the measure of heat energy produced upon burning.
For example, a cord of air-dried dense hardwood can yield up to 32 million BTUs, while a cord of less dense softwood may only yield around 12 to 18 million BTUs. Therefore, the estimate of wood volume must be converted into a usable heat value to understand the true benefit of the firewood. This conversion is why a smaller volume of a high-density wood can provide the same heating value as a larger volume of a low-density wood.