The Board foot (fbm) volume that is calculated assumes that the tree falls in the Mesavage and Girard's form class 78.

The tree fbm volume is calculated based on the number of 16 foot sawlogs the tree contains in merchantable lumber. The diameter of the tree is measured at breast height(DBH 4½ feet above the ground)

Examples:

Tree height = 99 feet to a top diameter of 8 inches
99 ÷ 16 = 6 full 16-foot sections. The trim allowance would make up the rest.
Enter a 6 in the number of 16 foot sawlogs field of the calculator.

tree height of 15 feet
include a trim allowance and you would end up with a 14 foot sawlog.
Enter 0.875 in the number of 16 foot sawlogs field of the calculator.

tree height of 45 feet
include a trim allowance and you would end up with 2 x 16 foot sawlogs and one 12 foot section.
Enter 2.75 in the number of 16 foot sawlogs field of the calculator.

Wiant's International 1/4 Formula:

Wiant's Formula:
V = (0.55743L² + 41.51275L - 29.37337)+ (2.78043 - 0.04516L² - 8.77272L)d+ (0.04177 - 0.01578L² + 0.59042L)d² where V = volume in mbf
d = diameter measured at breast height in inches
L = number of 16 foot saw logs