A medium-sized urban planter box usually needs about 9 to 10 cubic feet (roughly 255 to 285 liters) of soil. That estimate assumes a common medium-sized size of about 4 feet long, 2 feet wide, and 18 inches deep, and it allows for a little headspace and drainage. If your planter has different dimensions, the amount can change quickly, so it is best to measure before you buy.
The basic formula is simple: length × width × depth = volume. Use feet for all three measurements to get cubic feet. For example, a box that is 4 feet long, 2 feet wide, and 1.5 feet deep has a total volume of 12 cubic feet. But you should not fill it all the way to the rim. Leave about 2 to 3 inches of space at the top so you can water without soil spilling over. That brings the practical soil volume down to roughly 10 cubic feet. If you also add a drainage layer or fill part of the bottom with lightweight material, you may need about 9 cubic feet of actual soil.
A slightly smaller medium planter, such as 3 feet long, 2 feet wide, and 18 inches deep, has a total volume of 9 cubic feet. After headspace, it may need only 7 to 8 cubic feet of soil. This shows why checking the internal dimensions matters more than guessing from the outside. Always measure inside the box, because thick walls and false bottoms reduce usable space.
Soil type also affects how much you need. A fluffy potting mix settles after watering, so plan to fill the box, water it slowly, and then top it up. A dense topsoil blend may not settle as much, but it is heavier and less suitable for many urban planters. For most vegetables, herbs, and flowers in containers, use a high-quality potting mix rather than garden soil. It drains better and keeps roots healthy.
Weight is another important factor, especially on balconies and rooftops. Soil can be heavy, particularly when wet. A 10-cubic-foot fill may weigh several hundred pounds depending on the mix and moisture level. If weight is a concern, choose a lightweight potting mix and confirm that your balcony or roof can support the load.
When buying soil, check the bag size. Many bags contain 1.5 or 2 cubic feet. For a 9- to 10-cubic-foot planter, you would need about five 2-cubic-foot bags or six to seven 1.5-cubic-foot bags. Buy a little extra, because settling and uneven filling often mean you need more than the first estimate.
In short, for a typical medium-sized urban planter box, plan on 9 to 10 cubic feet of soil. Measure the inside dimensions, subtract 2 to 3 inches of headspace, and adjust for drainage or lightweight fill. That approach will give you a healthy, well-filled planter without wasting soil or overloading your space.