When it comes to filling your standard urban planter box, the amount of soil you need largely depends on the specific dimensions of the box. However, most standard urban planter boxes (often designed for balconies, patios, or small gardens) measure around 3 feet long, 1.5 feet wide, and 1.5 feet deep. For such a box, you would typically need approximately 6.75 cubic feet of soil. This is calculated by multiplying length x width x depth (3 ft x 1.5 ft x 1.5 ft = 6.75 cu ft).
Keep in mind that if your planter is shallower (e.g., 10 inches deep for succulents) or deeper (e.g., 2 feet for root vegetables), the soil volume will change accordingly. A simple rule is: for every 3 feet of length and 1.5 feet of width, each 6 inches of depth adds about 2.25 cubic feet of soil. So a 12-inch deep box would need around 4.5 cubic feet, while a 24-inch deep box would require about 9 cubic feet.
Also, remember that soil settles after watering, so it's wise to buy slightly more soil (roughly 10% extra) to top off the planter after the initial fill. For most urban gardeners, buying three large 2-cubic-foot bags of high-quality potting mix or a specialty raised bed soil blend will cover the needs of a standard planter box. Always check your specific planter's dimensions before purchasing to ensure success for your vegetables, flowers, or herbs.