Formula
Cubic feet = length (ft) × width (ft) × depth (in) ÷ 12. Cubic yards = cubic feet ÷ 27. Example: an 8 ft × 4 ft bed at 12 in deep needs 8 × 4 × 12 ÷ 12 = 32 cubic feet, or 1.19 cubic yards.
Common mistakes
- Entering depth in feet instead of inches — that makes the result 12× too small.
- Forgetting settling: fresh soil settles after watering. Buy about 10% extra, or top up after the first watering.
- Using the outside dimensions of a bed with thick walls — measure the inside.