The formula to calculate the number of board feet that can be obtained from a log is:

\[ BF = \frac{(D - 4)^2 \cdot L}{16} \]

Where:

• \(BF\) is the number of board feet (bd-ft).
• \(D\) is the small diameter of the log (in.).
• \(L\) is the length of the log (ft).