Mulch Calculator
Measure the bed, pick a depth, order once.
Work out how many cubic metres or bags of mulch you need for any garden bed, plus a bulk-vs-bags cost compare. Enter your bed size and depth and the calculator adds a waste allowance for settling and spillage, then shows the bulk load against the bag count.
Estimates from bed geometry and typical AU prices — confirm the volume and a delivered price with your mulch supplier.
How the mulch quantity is worked out
Volume = area × depth ÷ 1000, then add waste
The calculator takes your bed's plan area (length × width, times the number of beds) and multiplies it by the depth to get the volume: in metric, volume (m³) = area (m²) × depth (mm) ÷ 1000. It then adds a waste allowance — mulch settles and you'll spill some spreading it — so the mulch you actually order is volume × (1 + waste), about 10% on top. Coverage works the other way: 1 m³ covers 1000 ÷ depth(mm) square metres.
Worked example
A 20 m² garden bed at the standard 75 mm depth is 20 × 75 ÷ 1000 = 1.5 m³ before waste. Add the usual 10% and you order about 1.65 m³ — roughly $110 as bulk at $65/m³, or about 33 × 50 L bags at $9.45 (about $310). Depth drives the number directly: the same bed at 50 mm would need a third less mulch.
How deep, and how far it goes
Depth is the biggest lever on both quantity and coverage. Typical mulching depths are:
- 50–75 mm — most garden beds; enough to suppress weeds and hold moisture without smothering roots. 75 mm is a good all-round default.
- up to 100 mm — native gardens, slopes, or where you want maximum weed control.
Keep mulch 50–100 mm clear of plant stems and tree trunks so the collar doesn't stay wet and rot. Because coverage is 1 m³ ÷ depth, one cubic metre covers about 20 m² at 50 mm, 13.3 m² at 75 mm and 10 m² at 100 mm — deeper mulch covers less ground per cubic metre, which is why suppliers quote a range.
Bulk vs bags, and which mulch to use
Bulk or bags?
Bulk mulch — a loose cubic-metre load, delivered or trailer-collected — is the cheapest per cubic metre, around $40–90/m³, but you shovel and barrow it from a pile and pay a delivery fee. Bags cost roughly three times the price per litre but are clean, easy to carry to the back garden and store leftovers. As a rule, under about half a cubic metre bags are fine; more than that, order bulk and save real money.
Bags per cubic metre
Bag sizes run 25 / 40 / 50 / 70 L (0.025–0.07 m³ each), which works out to about 40 / 25 / 20 / 14 bags per cubic metre respectively. A 50 L bag is the common size.
Which mulch
Bark and woodchip last longest and are tidy for garden beds. Sugarcane and pea straw break down faster to feed the soil and are ideal for veggie patches, sold as bales rated by area. Whatever you pick, don't over-order — mulch left in a big pile heats up and can go anaerobic (sour-smelling) within weeks.
❓ Frequently asked Frequently asked questions
How much mulch do I need?
Multiply the bed length × width to get the area, then multiply by the depth in metres (75 mm = 0.075 m) to get the volume, and add about 10% for settling and spillage. A 20 m² bed at 75 mm is 20 × 0.075 = 1.5 m³, or roughly 1.65 m³ once you add the waste allowance.
How deep should mulch be?
For most garden beds, 50–75 mm is ideal — enough to suppress weeds and hold moisture without smothering roots. Go up to 100 mm for native gardens, slopes or maximum weed control. Keep mulch 50–100 mm clear of plant stems and tree trunks so they don't rot.
How many bags of mulch per cubic metre?
It depends on the bag size. A 50 L bag holds 0.05 m³, so you need 20 bags per cubic metre. A 25 L bag is 40 per m³, a 40 L bag about 25, and a 70 L bag about 14 bags per cubic metre.
Is bulk or bagged mulch cheaper?
Bulk is far cheaper per cubic metre — around $40–90/m³ versus roughly three times that once you buy the same volume in bags — but you shovel and barrow it from a pile and pay a delivery fee. Bags are clean and easy but only worth it for small top-ups, under about half a cubic metre.
How far does a cubic metre of mulch go?
Coverage is 1000 ÷ depth in millimetres. At 50 mm one cubic metre covers 20 m², at 75 mm about 13.3 m², and at 100 mm just 10 m². Deeper mulch covers less ground per cubic metre.
What type of mulch should I use?
Bark and woodchip last longest and are tidy for garden beds. Sugarcane and pea straw break down faster to feed the soil and are great for veggie patches, sold as bales rated by area. Recycled tree mulch is the cheapest bulk option but can be chunky. Pick for the job, then use your supplier's price in the calculator.
Where these figures come from
This is a geometry-based estimate — the mulch volume comes from your bed's dimensions, not from any external rate table. The guidance below reflects common Australian garden practice and typical supplier ranges.
- Volume and coverage — pure geometry: volume (m³) = area (m²) × depth (mm) ÷ 1000, and 1 m³ covers 1000 ÷ depth(mm) square metres (20 m² at 50 mm, 13.3 at 75 mm, 10 at 100 mm).
- Bag sizes — mulch is bagged in about 25, 40, 50 and 70 litre bags (0.025–0.07 m³), so roughly 40, 25, 20 and 14 bags per cubic metre; confirm the litres printed on the bag you buy.
- Prices — bulk bark/woodchip around $40–90/m³ and a 50 L bag around $9–10 are approximate 2026 retail and supplier ranges that vary with mulch type, region, load size and delivery; they are not a quote.
Last checked: July 2026. Prices are indicative and vary by supplier, mulch type and region. Always use your own supplier's price for a firm figure.
Select the question that matches where you are right now.
The headline number is the mulch to order: the bed's volume (area × depth) plus a waste allowance for settling and spillage. The breakdown also shows the coverage, the bulk cost and the bag count, and flags which is cheaper.
Order this many cubic metres of bulk from your supplier, or the bag count shown. Give them the mulch type you want — bark, woodchip or sugarcane — the volume is what you order, the type is a separate choice.
It's not a soil or fertiliser plan and it doesn't include edging, weed mat or delivery beyond the flat fee you enter. It sizes the mulch and compares bulk against bags — nothing more.
The volume maths (area × depth) is exact; real beds settle and you'll spill some, which is what the waste allowance absorbs. Confirm the bag litres and a delivered price with your supplier.
Four things move the mulch: the bed area, the depth, the waste allowance, and the prices you compare.
Volume scales straight with depth: a bed at 100 mm needs a third more mulch than the same bed at 75 mm, and twice as much as at 50 mm. Coverage moves the opposite way — deeper mulch covers less ground per cubic metre.
About 10% covers settling and the mulch you spill spreading it. Running short means a second delivery fee or trip, so it's worth ordering a touch over rather than under.
Bulk is cheapest per cubic metre but adds a delivery fee and the work of shovelling from a pile; bags cost about three times as much per litre but are clean and easy. The switch point is roughly half a cubic metre — set both prices to compare your own supplier's rates.
A few habits stop you over- or under-ordering.
Measure the actual length × width of each bed and add them up with the "number of beds" field. Pick your depth, then let the waste allowance cover settling and spillage.
A little spare is fine, but a big leftover pile heats up and can go anaerobic and sour within weeks. Order close to the number, and spread it soon after it lands.
Pull mulch back 50–100 mm from plant stems and tree trunks so the collar stays dry — piling it against the trunk invites rot. Depth in the open bed, a gap at the base.
Mulching is usually one line in a bigger garden or renovation job. Size the other materials too.