Raised Bed Soil Calculator

How this calculator works

The volume of a bed is its footprint times its depth. For a rectangle, the footprint is length times width in feet. For a round bed, it is pi times the radius squared. Depth is entered in inches and converted to feet by dividing by 12. Multiply by the number of beds, then add your settling allowance, and you have the cubic feet of soil to buy. Dividing by 27 gives cubic yards, because a cubic yard is 3 feet by 3 feet by 3 feet.

A worked example

A 4-foot by 8-foot bed filled to 12 inches deep holds 4 × 8 × 1 = 32 cubic feet, which is about 1.19 cubic yards. With a 10 percent settling allowance, you need 35.2 cubic feet, or about 1.3 cubic yards. In 1.5-cubic-foot bags, that is 24 bags. If you need more than a couple of cubic yards, bulk delivery is almost always cheaper than bags.

Choosing a mix

Most extension guidance for raised vegetable beds favours a loose, well-drained mix that is rich in organic matter. A common homemade blend is about 60 percent topsoil and 40 percent compost. A lighter option, popularised by square foot gardening, uses equal parts compost, peat moss or coir, and vermiculite. Bagged raised-bed mixes are convenient but cost more per cubic foot. Avoid filling a bed entirely with compost or with heavy clay, because both can cause drainage and nutrient problems.

Where these numbers come from

Volume uses standard geometry. The settling allowance and mix proportions reflect common state extension and gardening guidance, which varies by source, so treat them as starting points. Bag sizes shown are those commonly sold in the United States. Always check the cubic-foot rating printed on the bag, because some products are sold by weight or by quart.