That's an excellent question! Knowing the exact amount of soil needed saves you time, money, and trips to the garden center. For our standard-sized rectangular planter box, which measures 48 inches in length, 24 inches in width, and 12 inches in height (or 4ft x 2ft x 1ft), the calculation is straightforward.
You can find the soil volume in cubic feet using this simple formula: Length (ft) x Width (ft) x Height (ft) = Volume (cubic feet).
For our standard box:
4 ft (Length) x 2 ft (Width) x 1 ft (Height) = 8 cubic feet of soil.
This means you will need 8 cubic feet of potting mix to fill it to the brim. Most bagged soil is sold in units of cubic feet. Therefore, you would purchase eight 1-cubic-foot bags. Alternatively, if you buy in bulk, you would request 8 cubic feet from your supplier.
A Helpful Tip: We recommend leaving about an inch of space at the top of the planter box to prevent soil from washing out during watering. To account for this, you might calculate using a slightly reduced height (e.g., 11 inches instead of 12). For this box, that would require approximately 7.3 cubic feet of soil.
We always suggest purchasing a little extra—about 5-10% more—to account for settling after the first few waterings. So, for the full 8 cubic feet, consider getting 8.5 to 9 cubic feet total.
We hope this clear calculation helps you start your gardening project perfectly!