That's a great question! For our standard medium-sized urban planter box, which measures approximately 24 inches long, 12 inches wide, and 12 inches deep, you will typically need about 1.5 cubic feet of potting soil to fill it.
Here's a simple way to calculate it yourself: multiply the Length (in feet) x Width (in feet) x Depth (in feet). For this example: 2 ft (L) x 1 ft (W) x 1 ft (D) = 2 cubic feet. Since you don't usually fill the planter to the very brim to allow for watering space, we recommend calculating for a depth of about 10-11 inches instead. This adjustment gives you the more accurate 1.5 cubic feet estimate.
Most bags of potting mix are sold in quantities like 1 cubic foot or 1.5 cubic feet. For this planter, one 1.5-cubic-foot bag should be perfect. We always recommend using a high-quality potting mix, not garden soil, as it provides better drainage and aeration for your plants' roots in a container environment. Happy planting