How it works
Volume follows the usual length × width × depth arithmetic. Bag count then divides that volume by a standard bag size — 2 cubic feet is the most common US retail bag; 30 litres is a common Australian bag — so check the size printed on the bag you're actually buying, since it does vary by brand.
Worked example
A 10 ft × 5 ft garden bed, mulched 3 inches deep.
- Convert depth to feet: 3 ÷ 12 = 0.25 ft.
- Volume: 10 × 5 × 0.25 = 12.5 ft³ (0.46 yd³).
- Bags: 12.5 ÷ 2 = 7 bags (rounded up).
You'd need about 12.5 ft³ of mulch, or 7 bags at 2 ft³ each.
Common questions
How deep should a mulch layer be?
2–3 inches (50–75 mm) is typical for most garden beds — enough to suppress weeds and retain moisture without smothering plant roots or crowns. Piling mulch much deeper, especially against stems and trunks, can trap moisture and encourage rot.
Why does the bag size matter so much?
Bag sizes vary by brand and region — some US bags are 2 ft³, others are sold by weight or a different volume entirely. Always check the volume printed on the actual bag rather than assuming 2 ft³, since using the wrong figure will throw off the count.
Should I buy mulch loose (bulk) or bagged?
For larger beds, bulk mulch delivered by the cubic yard is usually cheaper per unit volume than bags, though it needs somewhere to be dumped and moved from. Bags suit smaller beds or top-ups where a bulk delivery minimum doesn't make sense.