Figuring out how much soil to buy for your new planter boxes is a simple calculation that prevents waste and ensures your plants have the perfect foundation. First, you need to determine the volume of your planter box. Measure the interior length, width, and depth in feet. Then, multiply these three numbers together (Length x Width x Depth). This gives you the volume in cubic feet.
For example, a box 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. Most bagged potting soil is sold in cubic feet, so you would need 12 bags if each is 1 cubic foot. However, if your box is very deep, you can use a technique called "hügelkultur" or fill the bottom third with lightweight materials like broken pots or perlite to save soil and improve drainage.
Remember, soil settles. It's wise to purchase about 5-10% more than your calculated volume to account for this and for future top-ups. For irregularly shaped boxes, break the shape down into regular rectangles, calculate each volume, and add them together. With this quick math, you can shop confidently, knowing you're buying just the right amount for your urban garden to flourish.