How much soil does a 4 × 8 ft bed need?

Use the inside dimensions. An eight-foot board does not guarantee eight feet of growing space once the corners are assembled. A tape measurement taken across the soil gives a better starting point.

Depth makes the difference

For a true 32 square foot interior, each inch of fill takes 2⅔ cubic feet. At 6 inches the bed holds 16 cubic feet. At 9 inches it holds 24. At 12 inches it holds 32. These figures describe the shape you measured. Choose depth according to the bed and what will grow there.

Buy the difference

Imagine an established bed with six inches of usable soil and a nine-inch target. The three-inch gap takes 8 cubic feet. Ordering for the full nine inches would add 16 cubic feet you already have. Mark the finished level on a corner before moving material.

Read the bag label

At 1.5 cubic feet per bag, the eight-cubic-foot gap takes six bags. That leaves one cubic foot. Add a second area using the same product before rounding so that spare material can serve both.

Quick reference for an inside area of 32 square feet

Use the gap from the current surface to your target line. For help choosing what goes into that space, see the University of Maryland guide to filling raised beds.