Figuring out how many bags of potting soil you need for a medium-sized planter box is a common gardening puzzle. The answer isn't one-size-fits-all, but a quick calculation will solve it. First, measure your box's interior dimensions: Length, Width, and Depth (height). Multiply these three numbers together (L x W x D) to find the volume in cubic inches. Then, convert to cubic feet by dividing that number by 1,728 (since there are 1,728 cubic inches in a cubic foot).
Most standard bags of potting soil are sold in quantities of 1, 1.5, or 2 cubic feet. For example, a typical medium planter box measuring 36 inches long, 12 inches wide, and 12 inches deep has a volume of 5,184 cubic inches. Dividing by 1,728 gives you 3 cubic feet. Therefore, you would need two 1.5-cubic-foot bags or three 1-cubic-foot bags to fill it. Always buy one extra bag to account for settling and to have some on hand for top-ups. Remember, successful container gardening starts with the right foundation, and knowing your soil volume ensures you buy just the right amount.