How much soil does a raised bed need?
Multiply length × width (in feet) × height (in inches) ÷ 12 to get cubic feet. A common 8 × 4 ft bed that is 12 inches tall holds 32 cubic feet – about 1.2 cubic yards or 22 bags of 1.5 cu ft.
Saving soil in tall beds
Most vegetables need 12 inches of good soil; deep-rooted crops like tomatoes and carrots like 18 inches or more. For beds taller than 18–24 inches you can fill the bottom with logs, branches and leaves (often called hügelkultur) and calculate soil only for the top layer.
Which soil mix is best?
The 60/40 topsoil-compost mix is affordable and works well for larger beds. The equal-thirds mix of compost, peat moss (or coco coir) and vermiculite is lighter, drains well and is popular for square-foot gardening, but costs more. Avoid filling beds with plain garden soil alone – it compacts and drains poorly.
Plan for settling
New beds settle 10–20% in the first season. Fill to the top and add compost each spring.