How the estimate works
A rectangular bed uses length × width × fill depth. A straight-sided round pot uses π × radius² × fill depth. A tapered round pot uses the truncated-cone formula: π × depth × (top radius² + top radius × bottom radius + bottom radius²) ÷ 3.
For example, a 48 × 24 inch bed filled 12 inches deep needs 8 cu ft, about 226.53 liters, before any allowance. That is four 2 cu ft bags. Bags round up to whole units; bag size must be a volume, not a weight.
Before filling your planter
Check drainage, plant needs and the planter’s loaded-weight guidance. Measure inside the container and subtract space already occupied. For irregular shapes, use smaller measured sections rather than assuming the outer dimensions equal the usable volume.
Plan your watering routine · Prepare an in-ground bed · Calculate a large material order