The formula
Cubic feet = length (ft) × width (ft) × depth (in) ÷ 12. Divide by 27 to get cubic yards. For a 4 × 8 bed at 12 inches: 4 × 8 × 12 ÷ 12 = 32 cubic feet, or 1.19 cubic yards.
Quick reference
| Bed | 6 in deep | 12 in deep | 18 in deep | 24 in deep |
|---|---|---|---|---|
| 2 × 4 ft | 4 cu ft | 8 cu ft | 12 cu ft | 16 cu ft |
| 3 × 6 ft | 9 cu ft | 18 cu ft | 27 cu ft | 36 cu ft |
| 4 × 4 ft | 8 cu ft | 16 cu ft | 24 cu ft | 32 cu ft |
| 4 × 8 ft | 16 cu ft | 32 cu ft | 48 cu ft | 64 cu ft |
Bags or bulk?
Bags are convenient for small beds. Once you need a cubic yard or more, compare a bulk delivery from a local supplier. One cubic yard is 27 cubic feet, or 18 bags of 1.5 cubic feet.
Plan for settling
New soil settles, often noticeably in the first season. Many gardeners add around 10 to 20% extra or top up with compost later. That's a rule of thumb, and the real amount depends on your mix.
About the wet weight
The range uses 55 to 90 pounds per cubic foot as a rough estimate for a moist mix. Published figures vary a lot by soil type. For anything above ground level, read our balcony planter weight guide first.