If your urban planter box is about three feet long, the amount of soil you need depends on its width and depth. There is no single answer because a three-foot length is only one dimension. Measure the inside of the box in feet: length × width × depth. That gives you the volume in cubic feet.
For example, a common box that is 3 ft long, 1 ft wide, and 1 ft deep holds 3 cubic feet. If it is 2 ft wide and 1 ft deep, it holds 6 cubic feet. If it is 1 ft wide and 18 inches deep, or 1.5 ft, it holds 4.5 cubic feet. Most urban planter boxes fall in the 3 to 6 cubic foot range when they are about three feet long.
Then add about 10% extra. Soil settles after watering, and you will also want to leave 1 to 2 inches of space below the rim so water does not overflow. For a 3-cubic-foot box, buy about 3.3 cubic feet. That means one 2-cubic-foot bag plus one 1-cubic-foot bag, or two 2-cubic-foot bags if you want a little extra. For a 6-cubic-foot box, plan on about 6.6 cubic feet, which is four 2-cubic-foot bags or seven 1-cubic-foot bags.
If you are comparing bag sizes, remember that 1 cubic foot is about 7.5 US gallons or 28 liters. So 3 cubic feet is about 22 gallons or 85 liters, and 6 cubic feet is about 45 gallons or 170 liters.
Use a quality potting mix rather than garden soil. Potting mix is lighter, drains well, and gives roots air in a contained planter. For vegetables like tomatoes, peppers, and cucumbers, aim for at least 12 to 18 inches of soil depth. For herbs, lettuce, and flowers, 8 to 12 inches is often sufficient.
To be exact, measure your box’s interior width and depth, multiply by 3 feet, add 10%, and round up to the nearest bag size. That will give you enough soil to fill your urban planter box without too much waste.