What basal area is
Basal area is the cross-sectional area of tree trunks measured at breast height, 4.5 feet above the ground, added up for all trees in an area. Foresters express it in square feet per acre in the United States, or square meters per hectare elsewhere. It combines the number and size of trees in one figure, and is the most common measure of how crowded a stand is.
Formulas
For one tree with a diameter at breast height (DBH) in inches:
Basal area (ft²) = 0.005454 × DBH²
The constant is π ÷ (4 × 144), which converts a circle’s area from square inches to square feet. In metric units, basal area (m²) = π × (DBH in m)² ÷ 4.
For a stand, count the trees on a plot of known area, multiply by the average basal area per tree and divide by the plot area.
Example
On a 1/10-acre plot you count 12 trees with an average DBH of 12 in:
- basal area per tree: 0.005454 × 12² ≈ 0.785 ft²;
- trees per acre: 12 ÷ 0.1 = 120;
- basal area: 120 × 0.785 ≈ 94 ft² per acre.
Use the quadratic mean diameter, the diameter of the tree of average basal area, rather than the simple average when diameters vary a lot.
What the numbers mean
| Basal area (ft²/acre) | Stand condition |
|---|---|
| under 60 | open or understocked |
| 60 to 120 | typical managed stand |
| 120 to 150 | well stocked to crowded |
| over 150 | dense; growth slows and trees become stressed |
Target values depend on the species and goal. Pine plantations are often thinned when they reach about 120 to 150 ft² per acre, back to about 70 to 90. Stocking charts, such as the Gingrich chart for upland hardwoods, combine basal area and trees per acre to judge whether a stand needs thinning.
Measuring in the field
- Measure DBH with a diameter tape, which reads diameter directly from the circumference.
- A prism or angle gauge speeds things up: with a basal area factor 10 prism, each tree that appears “in” counts for 10 ft² per acre, without measuring any diameter.
- Measure several plots and average them; one plot rarely represents a whole woodlot.
A consulting forester or your state forestry agency can help set targets for timber, wildlife or fire risk.