How Many Cubic Yards of Soil Do I Need for Raised Beds?

Raised bed soil is a straight volume calculation: length × width × fill depth, all in feet. Convert depth from inches by dividing by 12. A 4 × 8 bed filled 12 in deep is 4 × 8 × 1 = 32 ft³. The same bed at 18 in deep is 48 ft³, and at 6 in deep only 16 ft³.

Convert to purchase units. Bulk soil is sold by the cubic yard, and there are 27 ft³ in a yard, so 32 ÷ 27 = 1.19 cubic yards. Bagged soil is usually 1.5 ft³ or 2 ft³ per bag: 32 ÷ 2 = 16 bags. Bulk is dramatically cheaper past about 1 cubic yard — roughly the point where bags stop making sense — but requires somewhere to dump it and a wheelbarrow.

Use a blend rather than one product. The widely used mix is about 60% quality topsoil or garden soil, 30% finished compost, and 10% aeration material such as coarse perlite, pumice or pine bark fines. For our 32 ft³ bed that is 19 ft³ topsoil, 10 ft³ compost and 3 ft³ aeration. Straight bagged potting mix is expensive at this volume and straight topsoil compacts into a brick.

Fill depth depends on what you are growing. Six inches works for lettuce, herbs and other shallow-rooted greens if the bed sits on open ground the roots can reach into. Twelve inches is the practical default. Eighteen to twenty-four inches suits carrots, potatoes and other root crops, or any bed sitting on a hard surface like a patio or driveway.

Two things to plan for. Fill the bed slightly proud of the top edge, because a fresh mix settles 10–20% in the first season as the compost breaks down and irrigation packs it. And budget a couple of cubic feet of compost per bed each spring to top it back up — a raised bed loses volume every year, not because the soil leaves but because organic matter is consumed.