Measurements of Timber

One of the most important aspects of forestry is growing a crop of trees. However, a crop of trees is measured and sold differently than most familiar crops. Soybeans are sold by the bushel and cotton is sold by the bale or pound. Farmers receive a certain amount for each bushel or pound they sell. Two measures used to quantify the amount of wood in trees are weight (tons) and volume (board-feet). Measuring by weight or volume usually depends upon the market into which the trees will be sold. Most trees for pulp and small dimensional lumber are bought and sold by the ton whereas those intended for lumber or veneer production are usually measured and sold by board feet, however, they can be sold by the ton as well. Whichever unit is used, there are three basic measurements that are needed to determine the amount of wood in a tree.

Diameter at Breast Height (DBH)

The diameter of the tree measured 4-½ feet above the ground. This measurement is recorded in inches.

Merchantable Height

The usable height of the tree recorded in feet. This is usually measured from the point on the ground where it will be severed from the stump, approximately 1 foot above ground level, to the top part of the tree where either the diameter reaches a minimum of 4 inches or where the tree's crown will be removed.

Degree of Taper

The degree of taper in a tree is referred to as "Form Class". Form class is the ratio between the diameter (inside bark) at the top of the first 16-foot log and the outside bark diameter at breast height. For Example, assuming the tree will be cut leaving a 1-foot stump, a tree with a diameter of 16 inches at 17foot from ground level and a DBH of 20 inches has a form class of (16 ÷ 20 x 100) 80 percent. The higher the form class, the greater the volume per tree due to less stem taper. An average form class is usually determined for a stand of trees instead of measuring taper for every tree.

If these three values are known, the volume of wood can be determined. Once the volume is known, the weight can be obtained by multiplying the volume by the tree's density. All this can sound confusing but special charts and calculators have been made to simplify the process. As stated earlier, you really just need to know how to measure the tree's diameter, height and taper and what to do with those numbers once you get them.

Measuring Diameter (DBH):

Tree diameter is the first dimension required in measuring timber. Take this measurement at 4 1/2 feet above ground level on the uphill side of the tree. With a measuring tape, measure 4 ½ feet from the ground and see where it falls on your body. For many people it is the second button down from the shirt collar. Determine where it is on you and use this mark as a guide in measuring DBH. Many different methods can be used to measure a tree's diameter such as a Pi-tape, tree scale/Biltmore stick or a caliper. A Pi-tape is a special tape measure that is scaled to read the tree's diameter when wrapped around the tree at breast height. A Pi-tape can be bought at forestry supply stores or can be homemade. The Pi-tape is very accurate and simple to use. A homemade Pi-tape can be constructed from non-stretch material such as a¾" wide by 8' long leather strap. To construct a Pi-tape, you will need a suitable strip of material, a marking pen, and a regular tape measure. Measure 3-9/64" from the start of the material and scribe a mark labeled 1". From this mark, measure 3-9/64"again and scribe another mark labeled 2". Continue this process until you run out of material. Note: an 8-foot or 96 inch length of material will allow you to measure trees up to 30.55" diameter (96" ÷ 3.14 = 30.55"). When measuring trees, make sure that the tape is tight and level around the trunk at 4 ½' above the ground.
A Biltmore stick is a quicker method but takes some practice and experience to get accurate results. The stick is designed to determine a tree's diameter and height depending on how the stick is held. The stick can be purchased at forestry supply stores as well.
A sliding caliper is a tool with a fixed arm that forms a 90-degree corner with the measuring rail and an additional arm that slides up and down the measuring rail. A caliper is fast and accurate, but can be awkward to carry around. A caliper can be easily constructed from (2) yardsticks of the same thickness. One yardstick is used for the measuring rail, while the second is used to make the fixed arm and sliding arm.

Measuring Tree Height

The usable tree height is the second measurement used to find volume. Many different types of height-measuring devices are available. The Merritt hypsometer is a linear scale imprinted on one face of a tree scale/Biltmore stick. By learning a few tips, tree heights can also be measured with a regular retractable tape measure.
When using a tree scale stick to measure the merchantable length of a standing tree, you must first measure out 66 feet from the base of the tree. You can use a tape measure or mark a length of non-stretch string 66 feet to get this distance. With a little practice you will learn how to pace this distance. Always be sure you can see the entire length of the tree. Hold the stick vertical to the ground with the "number of 16-foot logs" side facing you and the zero end pointed toward the ground. Holding the stick in this position 25 inches from your eye, sight the zero end to appear to rest on the stump height of the tree. Then without moving your head, run your eye up the stick to the point where the top of the last merchantable log would be cut. Read the number of 16-foot logs that can be cut from the tree to the nearest one-half log (8 feet). The Merritt hypsometer scale is marked in 16-foot logs, so you will have to estimate the half logs. If the top of the last merchantable log falls halfway between two 16-foot log readings, then add one-half log to the reading.
A similar method can be used with a retractable tape measure and works on the same principle. In this case, walk 48 feet away from the tree. Hold the tape measure with the tape sticking straight up. Pull about 3' of the tape measure from the reel and lightly lock into place. NOTE: It is best to use a ½" steel retractable measure. Your hand should be level with your eye and 24" away from your eye. If you were to look straight ahead, you would see the top of your hand holding the body of the tape measure at the 3' mark. Without moving your head, look to the point where the tree will be topped off. Slowly let the tape measure retract until the end matches up with your line of sight near the top of the tree. Look at the position of the tape and note that every inch of height on the tape measure equates to 2 ft of tree length. For example, if the tape was stopped with 21" sticking out, then the height of the tree is (21x2) = 42 feet.

Measuring Taper (Form Class)

The amount of taper present in a tree affects the volume. Foresters classify different shapes of trees with varying amounts of taper into a "Form Class".

1. Measure the diameter-inside-bark of the tree 17' from the ground.
2. Measure the diameter-outside-bark of the tree 4 ½' from the ground.
3. Divide the value from step 1 by the value from step 2 and multiple by 100.

The value from Step 3 is refered to as the Form Class.
Volume tables have been constructed to quickly determine the volume of wood in a tree based on diameter, height and varying form class.

Determining Tree Volume

Volume tables determine the volume of wood in a standing tree based on DBH and merchantable height measurements. Volume tables are available based on different log rules (Doyle, Scribner, ¼"International) and various form classes. Once the average form class is determined for your timber stand, refer to the appropriate volume table of matching form class. A table similar to the one below can be used to tabulate your tree volume. After measuring the DBH and height, record those values along with the species type.
Selling timber is a business proposition, so it is important to know how many board feet of sawtimber exists. To determine timber volumes you must measure your trees. Otherwise, you would not know the value of your timber. Measuring timber is not difficult to learn and is one of the most important skills timberland owners should possess. The following table is an example form that can be used for a tree count:

Treenumber	Species	DBH(inches)	Number of 16' logs	Board foot volume
				Pine	Hardwood
Example:
1	Loblolly pine	18	3½	247	--
2	Southern red oak	18	3	--	231
3
4
5
6
7
8
9
10

There are conversion factors to convert board feet to weight; however, weight can also be determined directly. Just record the tree species, diameter, height and form class and use our timber calculator to determine weight.

Information provided from https://timberquote.com/education/howto/measure .