Quick answer: A 20 m² bed at 7.5 cm deep needs 20 × 0.075 = 1.5 m³ of mulch, which is 1,500 L or 30 bags of 50 L.
How it's calculated
Volume = area × depth, with the depth in metres (7.5 cm = 0.075 m). One cubic metre is 1,000 litres, so bags = litres ÷ bag size, rounded up.
In US units, cubic yards = square feet × depth in inches ÷ 324, because a cubic yard is 27 ft³ and a foot is 12 inches (27 × 12 = 324).
University extension services recommend about 2–4 in (5–10 cm): 2–3 in for fine or composted material and 3–4 in for coarse bark. Keep mulch a few centimetres away from stems and tree trunks. In hot, dry Gulf gardens a layer of mulch cuts evaporation from the soil.
Worked examples
- 20 m² at 7.5 cm, 50 L bags: 1.5 m³ = 1,500 L → 30 bags
- 200 ft² at 3 in, 2 ft³ bags: 200 × 3 ÷ 324 = 1.85 yd³ = 50 ft³ → 25 bags
Coverage of 1 m³ of mulch
| Depth | Area covered |
|---|---|
| 2.5 cm | 40 m² |
| 5 cm | 20 m² |
| 7.5 cm | 13.33 m² |
| 10 cm | 10 m² |
Frequently asked questions
How deep should mulch be?
About 5–10 cm (2–4 in). Finer mulch at the low end, coarse bark at the high end; deeper layers can starve roots of air.
How much area does a cubic yard cover?
324 ft² at 1 in, 162 ft² at 2 in and 108 ft² at 3 in deep.
Should I add extra?
Mulch settles, so many gardeners order 5–10% more, or top up the next season.
How many 50-litre bags cover 10 m²?
Fifteen, at the usual 7.5 cm depth. That's 750 litres of mulch.
Does mulch help in hot Gulf summers?
A lot. It shades the soil, keeps the roots cooler and slows evaporation, so you water less. Just keep it a few centimetres back from the stems.
Can I use gravel or stones as mulch?
You can. Stone lasts for years and won't blow away, but it does nothing for the soil and gets hot in full sun. The gravel calculator will tell you how much to order.