What the calculator does
For fixed-area plots, the total sampled acreage is the plot size multiplied by the number of plots. Tree density and measured volume can then be expanded from the sampled acreage to a per-acre basis.
Example
Ten 1/10-acre plots sample one acre in total. If those plots contain 86 tally trees, the simple cruise estimate is 86 trees per acre.
Representative plots matter more than the arithmetic. A mathematically correct expansion can still be misleading if plots are poorly distributed, too few in number, or placed subjectively.
Method reference: USDA Forest Service Timber Cruising Handbook and University of Florida IFAS Timber Inventory: A Primer for Landowners.