Soil quantity starts with the volume of the space you plan to fill. Measure the inside length and width, choose the finished depth, convert the result into cubic yards, and then use the product’s bag volume or delivery quantity. Soil settles and beds are rarely perfectly even, so a modest allowance is often practical.
Calculate bed volume
Volume = length × width × depth. A 12 ft × 4 ft bed filled to 12 inches has 48 cubic feet because 12 inches equals 1 foot. Divide by 27 to get 1.78 cubic yards before an allowance.
Allow for settling
A 10% allowance changes 48 cubic feet to 52.8 cubic feet, or about 1.96 cubic yards. The right allowance depends on soil blend, moisture, compaction, and the final level you want.
Convert to bags
Divide the required cubic feet by the volume printed on the bag. Bag weight does not reliably tell you the volume, so use the manufacturer’s stated yield.
Separate other layers
Drainage gravel, edging, liners, and existing soil should not be counted as planting soil unless they occupy the same material volume. Measure each layer separately.
Sources
The figures and rules in this guide are checked against:
- Raised Beds — Clemson Cooperative Extension, Home & Garden Information Center
- Raised Beds: Benefits and Maintenance — University of Florida IFAS Gardening Solutions
- Raised beds — Royal Horticultural Society
- Extension Gardener Handbook, Chapter 16: Vegetable Gardening — NC State Extension