A medium-sized urban planter box usually holds far more soil than beginners expect. Start by measuring the inside of the box, not the outside. Use feet for easy math: length × width × depth = cubic feet. For example, a common medium box measuring 4 ft long, 2 ft wide, and 18 in deep has a volume of 4 × 2 × 1.5 = 12 cubic feet. If it is only 12 in deep, it holds 8 cubic feet.
Don’t fill it all the way to the rim. Leave 1–2 inches of space for watering and mulch. Also, if the planter has a drainage layer or a false bottom, subtract that space. For most medium urban planter boxes, plan on needing about 8–12 cubic feet of potting mix, depending on depth.
Bagged soil is often sold in cubic feet or quarts. A 40-quart bag is about 1.5 cubic feet. So 12 cubic feet needs around 8 bags of 40 quarts, while 8 cubic feet needs about 5 to 6 bags. A 2-cubic-foot bag makes the math easier: 4 bags for 8 cubic feet or 6 bags for 12 cubic feet. Buy one extra bag if you can; soil settles after watering and you may need to top off.
For vegetables and herbs, use a quality raised-bed or container mix, not heavy topsoil. It should drain well and hold moisture. Fill the box in layers, water gently as you go, and let the soil settle before planting. If you are growing shallow-rooted greens, 10–12 inches of soil may be enough. For tomatoes, peppers, or root crops, aim for at least 12–18 inches and fill close to the top. With those measurements and a little buffer, you can order or buy the right amount of soil without guesswork.