A typical four-foot planter from our planter box factory generally holds about 4 to 6 cubic feet of soil. The exact amount depends on the interior width and depth, because “four-foot” describes the length only. A common 48 in × 12 in × 12 in planter has an interior volume of 6,912 cubic inches, which equals 4 cubic feet. A slightly deeper 48 in × 12 in × 16 in model holds about 5.3 cubic feet, while a wider 48 in × 18 in × 18 in planter holds about 9 cubic feet. For most standard four-foot models, plan on 4–6 cubic feet of loose potting mix, or roughly 30–45 gallons. Do not fill the soil to the very top; leave 1–2 inches below the rim for watering and mulch, and allow space for root balls and natural settling. Before ordering soil, confirm the interior dimensions from the factory specification sheet. This ensures your planter gets enough soil without overfilling or wasting mix.