How this mulch calculator works
The maths is simple: length × width × depth = volume in cubic metres. For a rectangular bed that's 4 m long by 2 m wide, mulched to 75 mm (0.075 m) depth, you need 4 × 2 × 0.075 = 0.6 cubic metres.
For circular beds, the formula is π × radius² × depth. The calculator handles that automatically — just enter the diameter.
We add a 10% waste margin to the recommended order quantity. That covers the reality of Australian mulch delivery: bulk loads settle during transport, some mulch compacts when you walk on it, and you'll always over-apply slightly near the edges of the bed. It's better to have half a bag left over than to run out on a Saturday afternoon.
How deep should mulch be?
For most established Australian garden beds, 75 mm is the sweet spot: deep enough to suppress weeds and hold moisture, shallow enough to breathe. Go deeper (100 mm) if you're fighting heavy weed pressure or planting in full sun in QLD, WA or NT. Go shallower (50 mm) for delicate groundcovers or young plantings.
Keep mulch 50–100 mm away from plant stems and tree trunks. Piled up against stems, it holds moisture against the plant and invites rot.
Choosing a mulch type
- Woodchip — all-purpose, chunky, long-lasting. Best for pathways and large garden beds.
- Bark mulch — finer than woodchip, tidier appearance. Favoured for formal gardens.
- Pine bark — lightweight, naturally acidic. Ideal for azaleas, camellias, and most Australian natives.
- Eucalyptus / hardwood chip — dense, great weed suppression, native-garden friendly.
- Cypress / red chip — naturally termite-resistant. The pick near buildings.
- Sugar cane — light, breaks down fast, adds nutrients. Best for vegetable gardens.
- Tea tree — dark, fragrant, native-look. Slow-breakdown decorative mulch.
- Pea straw / lucerne — nitrogen-rich, breaks down fast. Best for veggie patches and productive gardens.
Bag vs bulk: which is cheaper?
For anything over about 0.5 m³, bulk delivery is significantly cheaper than buying bags — often half the per-cubic-metre cost. But bulk usually has a minimum delivery fee ($40–80) and needs a spot a truck can reach. For small garden beds (under 0.3 m³) or jobs where a truck can't access, bags from Bunnings or your local hardware store make sense.
A 6×4 trailer holds about 0.7–1 m³ when loaded level. A tandem 8×5 holds 1.5–2 m³. Most landscape yards will load a trailer for you at no extra cost.
Want more detail?
Read the full guide: How much mulch do Australian gardens actually need? — covers mulch types, depth recommendations, bulk vs bags, and when to mulch in Queensland.
Mulch calculator — frequently asked questions
How much mulch do I need per square metre?
At the standard 75 mm depth, you need 0.075 cubic metres per square metre of garden bed. A 10 m² bed needs 0.75 m³. At 50 mm depth it's 0.05 m³ per square metre; at 100 mm it's 0.1 m³.
How many bags of mulch make a cubic metre?
Most bagged mulch in Australia comes in 50 or 60 litre bags. You need about 20 of the 50 L bags or 17 of the 60 L bags to make one cubic metre — which is why bulk delivery is far cheaper for any bed larger than about 0.5 m³.
How much does a cubic metre of mulch weigh?
Between 150 kg and 450 kg depending on the type. Pea straw is the lightest; cypress chip is the heaviest. Wet or freshly chipped mulch weighs more than dry, so factor that in if you're towing it home in a trailer.
Will a cubic metre of mulch fit in my trailer?
A standard 6×4 trailer holds roughly 0.7 to 1 cubic metre loaded level — so one cubic metre is a full, heaped load. Check your trailer's ATM rating against the mulch weight before you load, especially for heavier types like hardwood chip or cypress.
When is the best time to mulch in Queensland?
Late spring — after the soil has warmed but before the summer heat sets in. Mulch damp soil to lock moisture in for the hot months. Get it done before the build-up if you're in Queensland or northern Australia; mulching bone-dry soil in mid-summer traps heat rather than keeping it out.
Should I remove old mulch before adding new?
Usually no. If the old layer has broken down, top up over it — the decomposed material is feeding your soil. Only remove old mulch if it has matted into a hard water-repellent crust, or if you suspect disease or pests underneath. Rake it loose first and you'll likely need less new mulch than the calculator suggests.