For an average-sized urban planter box, the short answer is usually about 8 to 12 cubic feet of soil, but the exact amount depends on the box’s length, width, and depth. A common urban planter box is around 4 feet long, 2 feet wide, and 12 inches deep. That gives you 4 × 2 × 1 = 8 cubic feet of soil. If the box is 18 inches deep, the same footprint needs 4 × 2 × 1.5 = 12 cubic feet.
To calculate your own planter, measure the inside dimensions in feet and multiply length × width × depth. For example, a 120 cm × 60 cm × 30 cm planter equals about 0.216 cubic meters, or roughly 216 liters. Buy about 10–15% extra soil to allow for settling and to fill the box to 1–2 inches below the rim. Bagged soil is often sold in 1.5 or 2 cubic-foot bags, so an 8-cubic-foot planter needs about 6 bags of 1.5 cubic feet or 4 bags of 2 cubic feet, plus a little extra.
Use a lightweight potting mix or raised-bed mix rather than heavy garden soil alone, especially if the planter is on a balcony or roof. Make sure the box has drainage, and if it has a built-in water reservoir or false bottom, subtract that space from your soil volume. For most average urban planter boxes, planning for 8–12 cubic feet of soil and buying slightly more than your calculation is the safest approach.