Figuring out how much soil you need for your urban planter boxes is simpler than you might think. Whether you're filling rectangular raised beds, cylindrical pots, or irregular containers, the basic principle is the same: you're calculating volume. Here's how to do it step by step.
For standard rectangular or square planter boxes, use this formula: Length × Width × Depth = Volume. Measure all dimensions in the same units (feet work best). For example, a planter that's 4 feet long, 2 feet wide, and 1.5 feet deep would require 4 × 2 × 1.5 = 12 cubic feet of soil.
For cylindrical planters, use this formula: π × Radius² × Height. Measure the radius (half the diameter) and height in feet. For instance, a pot with a 1.5-foot radius and 2-foot height would need approximately 3.14 × (1.5)² × 2 = 14.13 cubic feet of soil.
Remember to account for settling and compression by adding 10-15% to your final calculation. Most bagged soil is sold in cubic feet, so you'll know exactly how many bags to purchase. For larger projects, soil is often sold by the cubic yard (1 cubic yard = 27 cubic feet).
Don't forget to consider drainage layers if you're using materials like gravel at the bottom—this will affect your soil volume calculation. With these simple calculations, you can confidently purchase the right amount of soil for your urban gardening project without waste or last-minute store runs.