How much mulch do I need?
Mulch is sold by the cubic metre, so the sum is area times depth. The part worth thinking about isn't the arithmetic — it's the depth you choose, because that one number decides whether the mulch does its job or causes a problem.
Depth is the decision
75mm is the standard for a garden bed. That's deep enough to block the light weed seeds need to germinate and to slow evaporation off the soil, without going so deep that the bottom of the layer goes airless and sour.
| Depth | Use it for |
|---|---|
| 25–50mm | Topping up an existing bed, or decorative pebble over a weed mat |
| 50mm | Veggie beds and anything you'll plant seedlings through |
| 75mm | Standard garden bed — the default for weed suppression |
| 100mm | Coarse bark on an exposed bed, or where weeds keep coming back |
| Over 100mm | Only with a coarse, chunky mulch. Fine mulch this deep can go anaerobic — it smells like vinegar or ammonia and it can burn what you spread it around |
One cubic metre covers 13.3 m² at 75mm, 20 m² at 50mm, or 10 m² at 100mm. Handy for a quick sanity check when the supplier is on the phone.
A worked example
Front garden beds adding up to 50 m², going in at 75mm with bark chip:
- Volume: 50 × 0.075 = 3.75 m³
- Rounded to how it's sold: 4 m³
- Weight, at bark's 0.35 t/m³: 3.75 × 0.35 = 1.31 tonnes
- If you bagged it instead, at 50L a bag: 3750 ÷ 50 = 75 bags
Seventy-five bags is the whole argument for bulk in one line. Bagged mulch makes sense for a 12 m² courtyard bed — that's 0.9 m³, or 18 bags — and stops making sense fast after that.
Bulk densities, and why light mulch is sold by volume
| Mulch | Bulk density | One tonne is | Lasts about |
|---|---|---|---|
| Pea straw | 0.15 t/m³ | 6.7 m³ | One season |
| Sugar cane mulch | 0.20 t/m³ | 5.0 m³ | One season |
| Bark / wood chip | 0.35 t/m³ | 2.9 m³ | 2–3 years |
| Compost / fine mulch | 0.60 t/m³ | 1.7 m³ | 6–12 months |
| Decorative pebble | 1.50 t/m³ | 0.67 m³ | Permanent |
A tonne of pea straw is nearly seven cubic metres — it would fill a truck. That's why the light organic mulches are quoted by the cubic metre and the heavy stone is quoted by the tonne. If a supplier gives you a price per tonne for bark, ask them what that is per cubic metre before you agree to it, because those are very different numbers.
The other side of that coin: mulch is one of the few landscape materials you can genuinely put in a trailer. Half a cubic metre of bark is about 175kg. Half a cubic metre of pebble is 750kg, and that's a different conversation with the towball.
Pick the mulch for the job, not the price
- Bark and wood chip — ornamental beds, around shrubs and trees, anywhere you don't want to be back next year. Chunky grades last longest.
- Sugar cane and pea straw — veggie gardens. They break down fast, which is the point: they feed the soil. Budget on topping up every season.
- Compost and fine mulch — good soil conditioner, average weed suppressant. Fine material can set into a mat that sheds water instead of letting it through.
- Decorative pebble — low-maintenance strips, side paths, anywhere a mower won't go. It doesn't feed anything and it doesn't break down, so put a weed mat under it.
Keep it off the trunk
Mulch piled up against a trunk or a stem holds moisture right where the plant least wants it, and collar rot follows. Pull the mulch back 50 to 100mm clear of every trunk and leave the root flare visible. It takes two minutes with a rake at the end and it's the single most common landscaping callback there is.
Two more while you're there: water the bed before you mulch, not after — dry mulch over dry soil keeps the rain out. And rake fine mulch over lightly if it's been sitting a while, so it doesn't set into a crust.
Quoting a mulching job
Price the material off the cubic metres, then price the labour off the barrow runs. Fifty square metres of bed is four cubic metres of mulch and somewhere around forty barrow loads if the truck can't get near it. Access is the number that moves a mulching quote, not the mulch.
Worked out the mulch. Now price the job.
Yamate turns your metres and hours into a quote the customer can accept on their phone, then an invoice, then a payment — with GST handled and the follow-ups chased for you.
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How much mulch do I need for 50 square metres?
At the standard 75mm depth, 50 m² needs 3.75 cubic metres — so you'd order 4. In bark that's about 1.31 tonnes. Change the depth above to see it at 50mm or 100mm.
How deep should mulch be?
Seventy-five millimetres for a normal garden bed. Fifty for veggie beds you'll plant through, a hundred where weeds keep coming back. Deeper than 100mm with a fine mulch risks the bottom of the layer going airless and sour.
How many square metres does 1 cubic metre of mulch cover?
13.3 m² at 75mm deep, 20 m² at 50mm, or 10 m² at 100mm. Divide 1 by the depth in metres and you have it.
How many bags of mulch in a cubic metre?
Twenty bags if they're 50 litres, about seventeen if they're 60. That's why anything past a couple of cubic metres is cheaper delivered in bulk.
What does a cubic metre of mulch weigh?
Depends entirely on the mulch. Pea straw is around 150kg, sugar cane 200kg, bark chip 350kg, compost 600kg and decorative pebble 1500kg. Check yours against the table above before you load a trailer.
How often do I need to top up mulch?
Sugar cane and pea straw go through in a season. Compost and fine mulches last six to twelve months. Coarse bark holds up two to three years. Pebble stays put — you just weed it.
Should mulch touch the trunk of a tree?
No. Keep it 50 to 100mm clear of every trunk and stem so the collar can breathe. Mulch banked against a trunk traps moisture and rots it.
Other calculators you'll want
General information for Australian trades, and an estimate only — not tax, financial or legal advice. It is not engineering, building or design advice either: a calculator works out a quantity from the figures you enter, and anything that needs sign-off still needs your engineer, certifier or building surveyor. Bulk densities are typical figures from Australian landscape suppliers and vary with moisture, grade and how long the material has been sitting in the stockpile — confirm with your own supplier before ordering. This calculator is a materials estimate to help you order and quote, not a horticultural specification.