Figuring out the right amount of soil for your urban planter box is crucial for plant health and avoiding wasteful overfilling. An average-sized urban planter box, commonly found on balconies and patios, typically measures around 24 inches long, 12 inches wide, and 12 inches deep. To calculate the soil volume, you multiply these dimensions: Length x Width x Depth.
For this standard box, the calculation is 24" x 12" x 12" = 3,456 cubic inches. Since soil is usually sold by the cubic foot, you'll need to convert this. There are 1,728 cubic inches in a cubic foot. So, 3,456 / 1,728 = 2 cubic feet. Therefore, an average planter box typically holds about 2 cubic feet of soil.
This means you would need to purchase two 1-cubic-foot bags of potting mix to fill it. Remember, it's best to leave about an inch or two of space at the top for watering. For different box sizes, simply use the same formula: multiply the interior length, width, and depth (in inches) together, then divide by 1,728 to find the cubic feet needed for a thriving container garden.