How the spacing math works
One acre contains 43,560 square feet. For rectangular planting, trees per acre are estimated by dividing 43,560 by the product of the row spacing and within-row spacing. Penn State Extension’s landowner tree-planting guide uses this same relationship and shows, for example, about 436 trees per acre at 10 × 10-foot spacing.
Planning for survival
The target-survival mode divides the desired established trees per acre by your expected survival rate to estimate how many seedlings would need to be planted before adding your handling reserve. USDA Forest Service reforestation guidance likewise treats expected survival as an important part of working backward from a desired future stocking level.
Not a planting prescription: Appropriate species, stock type, density, spacing, site preparation, browse protection, competition control, and regulatory requirements depend on your site and goals. Use this calculator for quantities, then confirm the prescription with a forester or planting professional.