Mulch Calculator (Australia): How Much Mulch Do I Need?
Short answer: multiply the area in m² by the depth in metres to get cubic metres. A 20 m² bed at 75 mm needs 20 × 0.075 = 1.5 m³, which is 1,500 litres or 30 bags of 50 L. The calculator below does this for rectangles, circles, tree rings and triangles. It also adds settling and gives bags, wheelbarrow loads, weight and cost.
Mulch Calculator
Work out how much mulch you need in cubic metres or cubic yards, plus bags, wheelbarrow loads, weight and cost.
Estimates only. Bag sizes vary by brand, so check the label. Coverage changes with the mulch type and how evenly you spread it. Bulk suppliers often have minimum loads, so check before you order.
| Depth | Area |
|---|---|
| 1 in | 324 sq ft |
| 2 in | 162 sq ft |
| 3 in | 108 sq ft |
| 4 in | 81 sq ft |
| Depth | Area |
|---|---|
| 25 mm | 40 m² |
| 50 mm | 20 m² |
| 75 mm | 13.3 m² |
| 100 mm | 10 m² |
Step 1: Pick a depth (it changes the answer more than anything else)
Most beds need 50–75 mm. Chunky bark and paths suit 75–100 mm, and fine or dense mulch should stay thinner. Depth decides volume directly, and sources disagree because mulch behaves differently by particle size. Fine mulch packs tightly and can shed water when thick. Chunky mulch lets water through and suppresses weeds better when deeper. The calculator's presets are 50 mm for beds, 75 mm for trees and shrubs and 75 mm for paths.
| Mulch type or purpose | Typical depth | Why |
|---|---|---|
| Garden beds and borders | 50–75 mm | Enough to block light, not enough to smother crowns |
| Trees and shrubs | 50–100 mm | Wide ring, kept clear of the trunk |
| Paths | 75–100 mm | Thickness keeps weeds down and mulch in place |
| Fine mulch (sugarcane, lucerne, shredded) | 50–75 mm | Mats and repels water if too thick |
| Coarse bark or chip | 75–100 mm | Stays loose, lasts longer |
| Gravel or pebbles | 40–50 mm | Heavy and does not break down |
These are typical ranges, so check the product label. Playground surfacing is a different job because depth depends on fall height and the surfacing material. Follow the supplier's specification or the relevant Australian Standard.
For the soil side of preparation, see our soil and fertilizer guide.
For an Australian horticulture reference on mulch depth and keeping mulch away from stems and trunks, see Gardening Australia's mulching guidance.
Step 2: Use the lookup table
Every 25 mm of depth needs 0.025 m³ of mulch per m². That means 1 m³ covers 40 m² at 25 mm and 10 m² at 100 mm.
| Depth | m³ per m² | 1 m³ covers | Coverage of one 50 L bag |
|---|---|---|---|
| 25 mm | 0.025 | 40 m² | 2.0 m² |
| 50 mm | 0.05 | 20 m² | 1.0 m² |
| 75 mm | 0.075 | 13.3 m² | 0.67 m² |
| 100 mm | 0.10 | 10 m² | 0.5 m² |
Bags per m³: 1 m³ is 1,000 litres, so you need 40 × 25 L bags, 34 × 30 L, 25 × 40 L, 20 × 50 L, 17 × 60 L or 15 × 70 L. Always round bag counts up, because a partial bag still has to be bought whole.
For a wider introduction to measuring, planting and routine garden tasks, see our beginner gardening guide for Australia.
Step 3: Measure the area
Break any shape into rectangles, circles and triangles, calculate each, and add them. The calculator's "Add another area" button does the adding.
- Rectangle: length × width
- Circle: π × (diameter ÷ 2)²
- Tree ring: outer circle area minus the trunk-gap circle
- Triangle: base × height ÷ 2
Keep every length in metres and the depth in mm. Mixing mm into a metre formula is the most common arithmetic error: 75 mm is 0.075 m, not 0.75.
For planning the surrounding space as well as the mulch layer, our garden design and landscaping guide covers broader layout considerations.
Worked examples
Border: 6 m × 1.5 m at 75 mm
The area is 9 m². 9 × 0.075 = 0.675 m³, which is 675 L.
That is 14 bags of 50 L (13.5 rounded up).
Tree ring: 1.5 m diameter at 75 mm
The area is about 1.77 m². That gives 0.133 m³, or 133 L.
That is 3 bags of 50 L. A small trunk gap reduces this slightly but does not change the bag count.
Top-up: a 20 m² bed with 30 mm already, topped up to 75 mm
Enter the difference as the depth: 45 mm. 20 × 0.045 = 0.9 m³.
With 10% settling that becomes 0.99 m³, so order about 1 m³. That is a bulk-delivery quantity.
For projects that include tree placement and ongoing care, see our Australian trees and shrubs guide.
For a tree-focused reference on mulch placement, trunk clearance and avoiding excessive mulch, see Colorado State University Extension's tree and shrub mulching guidance.
Bags or bulk?
Bags suit small jobs under about 0.5 m³. Bulk usually wins once you need around 1 m³ or more. Bagged mulch costs more per litre but needs no delivery fee, no tipping spot and no minimum order. Bulk costs less per m³ but often carries a delivery fee and a minimum load, commonly around 1 m³, so check your supplier.
| Bags | Bulk | |
|---|---|---|
| Best for | Under about 0.5 m³, pots, tree rings | About 1 m³ and above, large beds |
| Cost per m³ | Bag price × (1,000 ÷ bag litres) | Bulk price + delivery fee, spread over the load |
| Handling | Easy to carry and store | Needs a tipping spot and wheelbarrow work |
| Risk | Running short on a second trip | Over-ordering if you skip the settling allowance |
Break-even rule: work out the bag cost for your job (bags needed × price per bag). Compare it with the bulk price for your m³ plus delivery. Bulk wins when it is lower. The calculator's optional cost fields do this comparison and tell you which option is cheaper. Prices vary by supplier and region, so enter your own quotes.
Moving it: one m³ is about 12 wheelbarrow loads at 85 L. It weighs roughly 240–590 kg depending on mulch type and moisture, and wet mulch is heavier. A 6×4 trailer commonly takes around 0.5–0.7 m³ level and a ute tray around 0.5 m³, but check your own load limit and side height.
For practical upkeep once the material is down, see our low-maintenance garden design ideas.
Add a settling allowance
Add about 10% to your total. Fresh mulch compacts after spreading, and loads can settle in transit. Ticking "Add 10% for settling" in the calculator applies this. Skip it only if you want a thin-side estimate.
Avoid the common mistakes
| Problem | Why it happens | Fix | Prevention |
|---|---|---|---|
| Weeds still come through | Layer is under about 50 mm or patchy | Top up to 50–75 mm | Measure depth with a ruler in several spots |
| Water runs off the surface | Fine mulch laid too thick, or matted | Rake it loose, thin the layer | Use coarser mulch or keep fine mulch to 50 mm |
| Trunk or stem rot | Mulch piled against the trunk (a "volcano") | Pull it back into a doughnut | Keep a clear gap of roughly 75–150 mm around trunks |
| Ran out halfway | No settling allowance, bags not rounded up | Buy a few extra bags | Use the 10% option and round up |
For broader symptom diagnosis when garden problems appear, see our plant problems and troubleshooting guide.
Timing and safety in Australia
Mulch onto moist soil, ideally in autumn or spring. That locks in moisture before summer heat and avoids sealing in dry soil. In bushfire-prone areas, check your local council or fire authority's rules before placing combustible mulch near buildings. Gravel is a lower-fuel option for those zones.
For current Queensland guidance on combustible mulch and garden preparation before bushfire season, see the Queensland Government bushfire garden guidance.
For ongoing cutting and seasonal upkeep around mulched beds, see our pruning and maintenance guide.
FAQ
How deep should mulch be in Australia?
Mostly 50–75 mm for beds, 75–100 mm for paths and chunky bark. Keep it clear of stems and trunks.
How many 50 L bags are in a cubic metre?
20 bags. A 60 L bag gives about 17.
How much does a 50 L bag of mulch cover?
About 1 m² at 50 mm, 0.67 m² at 75 mm and 0.5 m² at 100 mm.
How many wheelbarrows is 1 m³ of mulch?
About 12 loads at 85 L each.
How heavy is 1 m³ of mulch?
Roughly 240–590 kg, depending on type and moisture.
How much mulch do I need per m²?
0.05 m³ at 50 mm, 0.075 m³ at 75 mm and 0.1 m³ at 100 mm.