A medium urban planter box typically needs about 2 to 3 bags of potting soil if you are using standard 1-cubic-foot bags. If you are using larger 2-cubic-foot bags, plan on about 1 to 2 bags. The exact amount depends on the planter’s dimensions, how deeply you fill it, and whether the soil settles after watering.
For a common medium urban planter box, such as one measuring roughly 30 inches long, 12 inches wide, and 12 inches deep, the volume is about 2.5 cubic feet. That means you would usually need 2 to 3 bags of 1-cubic-foot potting soil, or about 1¼ to 1½ bags of 2-cubic-foot potting soil. A slightly smaller medium box, around 24 x 12 x 10 inches, may need closer to 1.7 cubic feet, or about 2 bags of 1-cubic-foot soil. A larger medium box, around 36 x 12 x 12 inches, may need about 3 cubic feet, or 3 bags of 1-cubic-foot soil.
To estimate accurately, measure the inside length, width, and depth of the planter box in inches, multiply those numbers together, and divide by 1,728 to get cubic feet. Then compare that number with the bag size printed on the potting soil package. Leave about 1 to 2 inches of space below the rim so you can water without overflow. Also, because potting soil settles after the first few waterings, it is wise to buy a little extra—especially if you are filling the box for the first time. Use a lightweight potting mix rather than heavy garden soil, and make sure the planter has drainage. In short, for most medium urban planter boxes, 2 to 3 bags of 1-cubic-foot potting soil is the practical answer.