The short answer depends on the interior size of your planter, but a typical urban planter box—about 4 feet long, 2 feet wide, and 12 inches deep—needs roughly 8 cubic feet of potting mix to fill it to the rim. In practice, you should leave 2 to 3 inches of headspace for watering and mulch, so plan on about 6.5 to 7 cubic feet. That is close to 5 bags of 1.5 cubic feet, 4 bags of 2 cubic feet, or about 185 to 200 liters.
To calculate exactly, measure the inside length, width, and desired soil depth in feet. Multiply length x width x depth. For example, 4 ft x 2 ft x 1 ft = 8 cubic feet. If your planter is 3 ft x 1.5 ft x 1.5 ft, it holds 6.75 cubic feet. A smaller balcony box, 2 ft x 1 ft x 1 ft, holds only 2 cubic feet. If you use inches, convert to feet first: 12 inches = 1 foot, 18 inches = 1.5 feet, 24 inches = 2 feet.
Then subtract 10 to 20 percent for headspace, settling, and any drainage reservoir or false bottom. For a 4 x 2 x 1 ft box, 8 cubic feet minus about 15 percent leaves about 6.8 cubic feet. If you are filling a self-watering planter, follow the manufacturer's fill line; do not fill the water reservoir with soil.
Conversions:
- 1 cubic foot = about 28.3 liters.
- 1 cubic yard = 27 cubic feet.
- 1.5 cubic feet is a common bag size, often sold as 40 quarts.
- 2 cubic feet is another common bag size, often sold as 50 to 60 quarts.
So 8 cubic feet is 0.30 cubic yards, or about 226 liters. After headspace, 7 cubic feet is about 0.26 cubic yards, or 198 liters.
For most urban planters, use potting mix rather than garden soil or topsoil. Potting mix is lighter, holds moisture, and drains well, which matters on balconies and rooftops. Garden soil can become heavy and compacted in containers. If you want to save money, mix potting mix with compost and a little perlite or vermiculite. Fill the box in layers, water gently as you go, and top off after the soil settles. Do not pack it down hard; roots need air.
Weight is also important. Wet potting mix can weigh 40 to 60 pounds per cubic foot, so a full 4 x 2 x 1 ft planter may weigh 300 to 450 pounds or more with the box. Check your balcony, roof, or railing limits before filling large planters.
In short: measure your box, multiply length x width x depth, subtract 10 to 20 percent, and buy bags based on cubic feet. For a typical 4 x 2 x 1 ft urban planter box, you will need about 6.5 to 7 cubic feet of potting mix—roughly 5 bags of 1.5 cubic feet or 4 bags of 2 cubic feet.