Quick answer

Multiply the bed's length (ft) by width (ft) by depth (ft) to get cubic feet. For a 4 ft x 8 ft x 10.5 in bed, that's 4 × 8 × 0.875 = 28 cubic feet. Always round up by 10% to account for settling.

Why Accurate Soil Volume Matters

Ordering too little soil leaves your bed half-empty and delays planting. Ordering too much wastes money and creates a disposal problem. Using the right volume also ensures your plants have enough root space and that the bed drains properly.

How to calculate your soil volume

  1. Measure the interior length and width: Use a tape measure inside the bed, not the outer edges. For a standard 4x8 bed, the interior is usually very close to 4 ft by 8 ft, but always check your specific model. Common mistake: measuring the outer dimensions, which overestimates volume.
  2. Convert depth to feet: Divide the depth in inches by 12. A 10.5-inch-deep bed becomes 10.5 ÷ 12 = 0.875 ft. For a 12-inch bed, it's exactly 1 ft. Checkpoint: use decimal feet, not inches, in the final calculation.
  3. Multiply length × width × depth: Multiply the three numbers: length (ft) × width (ft) × depth (ft) = cubic feet. For a 4x8 bed at 10.5 inches: 4 × 8 × 0.875 = 28 cubic feet. Add 10% for settling: 28 × 1.1 = 30.8 cubic feet. Order 31 cubic feet.

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