Timber Cruise Plot Expansion and Stand Volume Calculator

Table of contents

• How to use
• Common mistakes
• Definitions
• Methodology
• Limitations & assumptions
• Sources

Fixed-area circular plot: A sample plot with a set radius, so every accepted plot has the same area.

Plot expansion factor (plots per acre): The number of equal-sized sample plots that fit into one acre. Multiply average volume per plot by this factor to estimate volume per acre.

Sampled plot area (acres per plot): The land area inside one circular sample plot. It is not the area of the stand.

Board foot: A lumber volume unit used in timber cruising. The entered board-foot total must use one consistent log rule or volume basis.

MBF: Thousand board feet. One MBF equals 1,000 board feet.

Estimated volume per acre (board feet per acre): The expanded sample estimate for one acre, based on the average sampled plot volume.

Mistake: Entering a prism basal area factor as Plot radius (feet).
Fix: Use this calculator only for circular fixed-area plots. Enter the actual radius in feet.

Mistake: Entering board feet per acre in Total sampled plot volume (board feet).
Fix: Add the board-foot volumes from all included sample plots first. The calculator creates the per-acre estimate.

Mistake: Counting plots that were abandoned, duplicated, or a different size in Number of sample plots.
Fix: Count only accepted circular plots represented in Total sampled plot volume (board feet).

Mistake: Using plot acres instead of tract acres for Stand area (acres).
Fix: Enter the net forested acres of the whole stand. One plot's area appears separately as Sampled plot area (acres per plot).

Mistake: Mixing Doyle, Scribner, International, or another volume basis in Total sampled plot volume (board feet).
Fix: Use one log rule or volume basis for every plot total before calculating Estimated stand volume (board feet).

• Use only for circular fixed-area plots with one shared radius. Prism and other variable-radius point samples use a different expansion method.
• The calculator expands the board-foot total you enter. It does not estimate tree volume from DBH, height, species, merchantability, or log lengths.
• All sampled plot volumes must use the same log rule or volume basis. It does not convert Doyle, Scribner, International, or other board-foot rules.
• The stand estimate assumes the sampled plots represent the entered stand area. Uneven timber, too few plots, poor plot placement, or an incorrect stand boundary can change the real stand volume.
• Zero sampled volume is allowed and produces zero estimated volume per acre and stand volume. A positive plot radius, a whole number of at least one sample plot, and a positive stand area are required.

Calculation method

The calculator finds the area of one circular fixed-area plot, converts that area to acres, and takes its inverse for the expansion factor. One acre contains 43,560 square feet. [1]

Sampled plot area (square feet) = π x Plot radius (feet)^2

Sampled plot area (acres per plot) = Sampled plot area (square feet) / 43,560

Plot expansion factor (plots per acre) = 1 / Sampled plot area (acres per plot)

The entered total plot volume is divided by the number of accepted equal-area plots to find the average sampled volume per plot.

Average sampled volume per plot (board feet) = Total sampled plot volume (board feet) / Number of sample plots

Estimated volume per acre (board feet per acre) = Average sampled volume per plot x Plot expansion factor

Estimated stand volume (board feet) = Estimated volume per acre x Stand area (acres)

Estimated stand volume (MBF) = Estimated stand volume (board feet) / 1,000

Worked example

With a 37.2-foot radius, one plot covers about 0.099804 acres, so the expansion factor is about 10.0196 plots per acre. If five plots contain 5,000 board feet total, the average is 1,000 board feet per plot. The estimate is about 10,019.65 board feet per acre and about 500,982.29 board feet, or 500.98 MBF, for 50 acres.