Figuring out how much soil you need for your new urban planter boxes is a simple math problem. You just need to calculate the volume of your boxes. Here's how to do it step by step.
First, measure the inside dimensions of your planter box in feet: Length, Width, and Depth (height). For a rectangular box, multiply these three numbers together:
Length (ft) x Width (ft) x Depth (ft) = Volume in cubic feet.
For example, a planter that is 4 feet long, 2 feet wide, and 1.5 feet deep has a volume of 4 x 2 x 1.5 = 12 cubic feet.
This number is the total soil capacity. However, you should not fill the box completely to the brim. Leave about 1-2 inches from the top for watering. You can subtract a small amount (about 0.5 cubic feet for a box this size) to account for this.
Most bagged potting soil is sold by the cubic foot (e.g., 1 cu ft, 1.5 cu ft bags) or by quarts/liters. Simply match your calculated cubic foot need to the bag sizes. For our 12-cubic-foot example, you would need about twelve 1-cubic-foot bags.
If your planter is circular, use the formula for the volume of a cylinder: 3.14 x (Radius in ft)² x Depth (ft). Measure the distance across the inside of the pot (diameter), divide by 2 to get the radius, and then calculate.
A key tip: always buy a little extra soil—about 5-10% more. Soil settles after the first watering, and having extra ensures your planters stay perfectly full. Now you can shop confidently, avoiding waste and ensuring your plants have the perfect foundation to grow.