How Much Soil or Mulch Do You Need for a Garden Bed?

Garden materials are sold by volume, in liters, cubic feet, cubic yards or cubic meters, so the job is to turn your bed's area and depth into a volume.

Step 1: Area

• Rectangle: length × width
• Circle: π × radius²
• Odd shapes: split them into rectangles and circles, or measure an average length and width.

Step 2: Volume

Use the same units throughout. Convert a depth in centimeters to meters by dividing by 100, or inches to feet by dividing by 12.

Example: a raised bed

A bed 2.4 m long and 1.2 m wide, filled 30 cm deep: 2.4 × 1.2 × 0.3 = 0.864 m³, or 864 liters. In 50-liter bags that's 17.3 bags, so buy 18.

Example: mulching a round bed

A circular bed 3 m across (radius 1.5 m), mulched 7.5 cm deep: π × 1.5² × 0.075 ≈ 0.53 m³.

Typical depths

• Mulch: usually 5–8 cm (2–3 in). Keep it a few centimeters away from plant stems and tree trunks.
• Topdressing beds or lawns with compost: about 1–2 cm.
• New raised beds: the depth of the bed, often 20–30 cm or more.

Unit conversions

• 1 m³ = 1,000 liters ≈ 35.3 cubic feet ≈ 1.31 cubic yards
• 1 cubic yard = 27 cubic feet ≈ 0.765 m³
• In feet: area (sq ft) × depth (in) ÷ 12 = cubic feet; divide by 27 for cubic yards.

Allow for settling

Loose soil and compost settle after watering. Buy a little extra for raised beds and top them up after a few weeks.