Mulch and soil volume
Volume = length × width × depth
With length and width in feet and depth in inches, divide the depth by 12 first. A 20 × 5 ft bed covered 3 inches deep needs 20 × 5 × 0.25 = 25 cubic feet, which is 0.93 cubic yards or 13 bags of 2 cubic feet.
Recommended depths
- Bark or wood mulch: 2–3 inches (5–8 cm)
- Compost top dressing: 1–2 inches (2.5–5 cm)
- Topsoil for a new lawn: 4–6 inches (10–15 cm)
- Decorative gravel: 2 inches (5 cm)
One cubic yard covers about 108 sq ft at 3 inches deep. Bulk delivery is usually cheaper than bags once you need more than a cubic yard or two.