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← Back to the full build guideStandard lumber dimensions differ from their labeled size. A "2x6" actually measures 1.5 inches thick by 5.5 inches tall. A "4x4" post is actually 3.5 inches square. This calculator uses the actual dimensions for all measurements.
The short side boards are cut shorter than the bed's exterior width because they fit between the long side boards. The formula is: short side length = (width in inches) minus 3 inches (1.5" per long board × 2). This gives you the correct finished exterior dimension.
Corner post length includes extra material below the bottom board that gets buried in the ground for anchoring. Taller beds use proportionally longer posts for stability.
Soil volume is calculated from the interior dimensions and rounded up. We use a 60/30/10 mix of topsoil, compost, and peat moss (or coconut coir) — the same recipe recommended in our build guide.