The short answer is that a standard urban planter box usually needs about 6 to 9 cubic feet of potting mix. The exact amount depends on the box’s internal length, width, and depth. Many city balcony or patio planter boxes are around 4 feet long, 2 feet wide, and 10 to 12 inches deep. A 48" x 24" x 10" box has a volume of 4 x 2 x 0.83 = about 6.7 cubic feet. A 48" x 24" x 12" box holds 8 cubic feet. Because potting mix settles after watering, buy 10 to 15 percent more. That means roughly 7.5 to 9 cubic feet for a typical 4-foot planter box.
To calculate your own planter box soil volume, measure the inside dimensions. Convert inches to feet by dividing by 12. Then multiply length x width x depth. For example, a 36" x 18" x 12" box is 3 x 1.5 x 1 = 4.5 cubic feet. A 60" x 24" x 12" box is 5 x 2 x 1 = 10 cubic feet. If you plan to leave 1 to 2 inches of space below the rim, subtract a little from the depth. This space helps prevent overflow when you water.
Soil is usually sold by volume, not weight. Common bags contain 1 cubic foot, 1.5 cubic feet, or 2 cubic feet. One cubic foot equals about 25.7 dry quarts, so a 40-quart bag is about 1.5 cubic feet. For a 6.7-cubic-foot planter, you would need about five 1.5-cubic-foot bags or four 2-cubic-foot bags. For an 8-cubic-foot planter, buy about six 1.5-cubic-foot bags or four 2-cubic-foot bags. Always buy one extra bag if you can; it is useful for topping off after settling.
Use potting mix or raised-bed mix rather than heavy garden soil or cheap topsoil. Container mixes are lighter, drain better, and give roots air. A simple blend is 60 percent potting mix, 30 percent compost, and 10 percent perlite. For herbs and flowers, a quality potting mix alone can work well. For tomatoes, peppers, and other vegetables, add compost for nutrients. Do not add a gravel layer just for drainage unless your planter has no drainage holes. In most cases, gravel reduces root space and does not solve drainage problems. Instead, make sure the box has drainage holes and a saucer or feet to keep it slightly off the ground.
Finally, fill the box to 1 to 2 inches below the rim, water thoroughly, and let the soil settle. Then top it off if needed. Wet potting mix is heavy, so also check that your balcony or roof can support the weight. As a quick guide: a standard 4 ft x 2 ft x 10 in urban planter box needs about 7 to 8 cubic feet of soil; a 4 ft x 2 ft x 12 in box needs about 9 cubic feet. With those numbers, you can buy the right amount of soil without guessing.