The diameter of a round object is tricky to measure directly so the usual way is to start with the circumference and then calculate the diameter. The circumference of a tree is measured 4 1/2 feet above ground level unless it forks at that point, in which case it is measured at the narrowest point below the 4 1/2 ft mark. Once the circumference is known, calculating the diameter is straightforward.
Things You'll Need
 Tape measure
 Calculator

Measure the circumference of the tree trunk by holding the tape measure 4 1/2 feet above the ground  at about chest height  and wrapping it around the tree. Keep the tape tight against the trunk and try to keep it parallel to the ground. Record the value.

Convert the measurement into inches. There are 12 inches in 1 foot, so multiply the number of feet by 12 and then add any remaining inches.
For example, a tree circumference of 8 feet and 8 inches is 103 inches  (8 x 12) + 8 = 104.

Divide the circumference by pi, a mathematical constant usually given as 3.1415. The result is the diameter of the tree.
For example, given a circumference of 52 inches, the diameter is 16.55 inches because 52 / 3.1415 = 16.55.
Tips & Warnings
 Keep the tape measure tight around the tree to get an accurate result. A loose tape, or a tape entangled in small branches, adds additional inches to the result.
 Check the tree of dangers before standing under it. Dead branches, wasp and hornet nests and entangled overhead power lines can lead to injury or death.
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References
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