A typical urban planter box holds roughly 2 to 8 cubic feet of soil, with many common models in the 4 to 6 cubic feet range. The exact amount depends on the box’s inside dimensions. For example, a 24-inch by 12-inch by 12-inch planter is about 2 cubic feet, or roughly 15 gallons. A 36-inch by 18-inch by 18-inch box is about 6.75 cubic feet, or about 50 gallons. A larger 4-foot by 2-foot by 1-foot raised planter holds 8 cubic feet, or nearly 60 gallons.
To estimate soil capacity, measure the inside length, width, and depth in feet and multiply them: length × width × depth = cubic feet. If your measurements are in inches, multiply length × width × depth in inches and divide by 1,728. For larger projects, divide cubic feet by 27 to get cubic yards. One cubic foot is about 7.5 gallons or 26 dry quarts. Remember that soil settles, and drainage materials or false bottoms can reduce the actual amount needed. Fill most urban planter boxes with a lightweight, well-draining mix rather than heavy garden soil, especially on balconies or rooftops where weight matters.