How deep should mulch be?
Depth is the number that changes everything, so pick it before you think about bags. Too thin and weeds push through and the soil bakes; too thick and water struggles to reach the soil, and fine mulches can go sour. These are the depths that work in Australian and New Zealand conditions:
| Mulch type | Recommended depth | Notes |
|---|---|---|
| Fine bark, pine bark mini nuggets | 50–75 mm | Good on ornamental beds; top up every 12–18 months. |
| Coarse wood chip, chunky bark, forest litter | 75–100 mm | Lasts longest, best under trees and on paths. |
| Sugar cane, pea straw, lucerne | 50–75 mm (settles fast) | Ideal for veggie beds; breaks down within a season. |
| Decorative pebbles or gravel | 30–50 mm | Inorganic – use the gravel calculator, sold by the tonne. |
| Around trees (keep clear of trunk) | 75–100 mm | Mulch a wide circle, 5–10 cm gap from the trunk. No "volcanoes". |
Water the soil deeply before mulching, and mulch when the soil is warm. Mulching cold, dry soil in early spring can keep it cold and dry for weeks.
The formula, and a worked example
bags = ⌈ volume (m³) × 1,000 ÷ bag size (L) ⌉
Say you are mulching a 6 m × 1.5 m border along a fence, plus a 2 m diameter circle under a lemon tree, both at 75 mm:
- Border: 6 × 1.5 = 9 m². Circle: π × 1² = 3.14 m². Total 12.14 m².
- Volume: 12.14 × 0.075 = 0.91 m³, which is 910 L.
- In 60 L bags: 910 ÷ 60 = 15.2, so buy 16 bags. In 70 L bags, 13.
- Bagged at $12 per 60 L bag: 16 × $12 = $192. Bulk at $75/m³ plus $70 delivery: 0.91 × $75 + $70 = $138. Bulk saves $54 and you skip lifting 16 bags.
Add 10 % if the ground is uneven or you are covering freshly dug soil – the mulch will settle into the low spots and you will be one bag short otherwise.
How much does a bag of mulch cover?
| Bag size | at 50 mm | at 75 mm | at 100 mm |
|---|---|---|---|
| 25 L | 0.5 m² | 0.33 m² | 0.25 m² |
| 50 L | 1.0 m² | 0.67 m² | 0.5 m² |
| 60 L | 1.2 m² | 0.8 m² | 0.6 m² |
| 70 L | 1.4 m² | 0.93 m² | 0.7 m² |
| 1 m³ (bulk) | 20 m² | 13.3 m² | 10 m² |
The rule of thumb many landscapers use: at 75 mm, one cubic metre does a bit over 13 square metres. If a supplier's coverage claim is much higher than that, they are assuming a thinner layer than you want.
Bagged or bulk mulch?
Mulch is where bulk delivery wins earliest, because bagged mulch is expensive per litre and bulk mulch is cheap per cubic metre. In most Australian metro areas bulk mulch runs roughly $50–$110 per m³ for the common grades (hardwood chip, pine bark, forest mulch) and delivery adds $60–$120 depending on distance and truck size. Bagged mulch at the hardware store commonly works out to $150–$250 per m³ equivalent. That means bulk starts winning at around 0.6–0.9 m³ – a single decent-sized bed.
Bags still make sense when you have nowhere to dump a pile, when you are topping up a few pots or a narrow strip, or when you want a specific product (a particular coloured bark, sugar cane in compressed bales) that the bulk yards do not stock. Some yards will also sell half-cubic-metre loads if you collect them in a trailer, which removes the delivery fee entirely.
For the full break-even maths see Bagged vs bulk: which is cheaper?
Choosing a mulch for Australian and NZ gardens
- Coarse organic mulches (wood chip, chunky bark, forest blend) let water through, last 2–3 years and are the default for ornamental beds and under trees.
- Fine organic mulches (sugar cane, pea straw, lucerne, fine compost) feed the soil as they break down and suit vegetable beds, but need topping up every season and can mat if laid too thick. The three straws are compared in pea straw vs sugar cane vs lucerne.
- Inorganic mulches (pebbles, scoria, crushed rock) do not break down, suit succulents and native gardens, and in a hot summer can heat the soil – use lighter colours in exposed positions.
- Look for AS 4454. Bagged composts, soil conditioners and mulches that carry the Australian Standard AS 4454 mark have been tested for contaminants, maturity and weed seeds. Uncertified "forest litter" can carry weed seed and, in some regions, fire-ant or pathogen movement restrictions – check your state's biosecurity rules before moving bulk mulch between properties.
Mulch questions we get asked
How many bags of mulch do I need for 10 square metres?
At 75 mm deep, 10 m² needs 0.75 m³ = 750 L, which is 13 × 60 L bags or 15 × 50 L bags. At 50 mm it drops to 500 L, or 9 × 60 L bags.
How many square metres does a cubic metre of mulch cover?
Divide 1,000 by the depth in millimetres: 20 m² at 50 mm, 13.3 m² at 75 mm, 10 m² at 100 mm.
Is mulch sold by the tonne or the cubic metre?
Organic mulch is sold by the cubic metre because it is light (roughly 0.3–0.5 t per m³ depending on moisture). Pebbles, scoria and crushed rock are sold by the tonne – use the gravel calculator for those.
Will a 6×4 trailer hold a cubic metre of mulch?
A standard 6×4 trailer (1.8 m × 1.2 m) holds about 0.6 m³ level with the sides and roughly 1 m³ if it has a cage and you heap it. Mulch is light enough that weight is rarely the limit; for soil or gravel it is. The trailer load calculator shows the safe load for each material.
Sources and method
- Standards Australia, AS 4454:2012 Composts, soil conditioners and mulches. standards.org.au
- Depth recommendations follow the ranges published by Australian state horticulture and water-authority "waterwise" guidance (50–75 mm for most organic mulches, up to 100 mm for coarse material), cross-checked against nursery-industry advice.
- Coverage figures are pure geometry (area × depth) and are reproduced in the tables above; price ranges are indicative for 2026 and vary by region.
Written by DueRiva · Last reviewed 23 September 2026 · Report an error