Mulch, soil and gravel calculator
Enter the size and depth of a garden bed or path to get cubic metres, bags to buy and, for gravel and sand, the weight in tonnes.
Bagged mulch directions give about 50 mm for a new bed and 25 mm to top up.
Volume needed
- Volume needed
- 0.5 m³500 L
- Bags to buy
- 1050 L bags
- Area covered
- 10 m²
Bag size and other assumptions
Bag sizes from an Australian maker. For gravel and sand, choose Other size and type the weight on the bag.
Optional margin for uneven ground or material that settles.
Estimate only. Check product labels and the building rules in your state or territory before you buy or build.
Quick answer
Multiply the area in square metres by the depth in metres to get cubic metres; one cubic metre is 1,000 litres. A 4 × 2.5 m garden bed mulched 50 mm deep needs 10 × 0.05 = 0.5 m³, or 500 litres, which is 10 bags of 50 litres. Divide the litres by the size on the bag and round up.
How to use the mulch calculator
- Choose the material: mulch, garden soil, compost, gravel or sand.
- Pick the shape: length and width for a garden bed or path, the diameter for a circle around a tree, or a known area if you have already added up several beds.
- Enter the depth in millimetres: 50 mm for a new bed of mulch, 25 mm to top up an old layer. The cubic metres, litres, bags and area update as you type.
- Open ‘Bag size and other assumptions’ to pick a 30 or 50 litre bag, type the size on your own bag, add a margin for settling, or set the weight per cubic metre your supplier uses for gravel and sand.
How do you calculate how much mulch you need?
Mulch, soil and gravel are measured by volume: the area of the bed times the depth of the layer. Work in metres, so 50 mm is 0.05 m, and the answer is in cubic metres, the unit bulk landscape supplies are sold in. Multiply by 1,000 for litres, the unit printed on bags of mulch and soil.
Divide the litres by the bag size and round up to whole bags. Gravel and sand are also sold by the tonne, so the calculator multiplies their volume by a weight per cubic metre. It starts at 1.6 t/m³, a neutral value rather than an Australian standard figure, so replace it with your supplier’s number.
area (m²) = length × width, or 3.1416 × (diameter ÷ 2)² for a circlevolume (m³) = area × depth (mm) ÷ 1,000litres = m³ × 1,000bags = litres ÷ bag size (L), rounded upweight (t) = m³ × weight per cubic metre (t/m³)How much does a bag of mulch or soil cover?
Divide the bag size in cubic metres by the depth in metres: a 50 litre bag is 0.05 m³, so at 50 mm it covers 0.05 ÷ 0.05 = 1 m², the figure printed in the maker’s directions.
| Depth | 30 L bag | 50 L bag | 1 m³ |
|---|---|---|---|
| 25 mm | 1.2 m² | 2 m² | 40 m² |
| 50 mm | 0.6 m² | 1 m² | 20 m² |
| 75 mm | 0.4 m² | 0.67 m² | 13.3 m² |
| 100 mm | 0.3 m² | 0.5 m² | 10 m² |
Compressed mulch does not follow this table. One 80 litre bag of sugar cane mulch is labelled to cover up to 23 m² at 20 mm, which is 460 litres once spread, so use the coverage printed on such bags.
Worked examples
Mulch for a 4 × 2.5 m garden bed
The bed covers 4 × 2.5 = 10 m². At 50 mm deep the mulch fills 10 × 0.05 = 0.5 m³, which is 500 litres.
In 50 litre bags that is 500 ÷ 50 = 10 bags. Topping up an existing layer by 25 mm takes half as much: 0.25 m³, or 5 bags.
Garden soil for a 2.4 × 1.2 m raised garden bed
The bed covers 2.4 × 1.2 = 2.88 m². Filled 300 mm deep it holds 2.88 × 0.3 = 0.864 m³, or 864 litres of soil.
In 30 litre bags that is 864 ÷ 30 = 28.8, so buy 29 bags. In 50 litre bags it is 17.28, so 18 bags. From a landscape supplier, order 0.864 m³ or the next amount they sell.
How deep should mulch be?
Spread mulch about 50 mm deep on a bare bed and 25 mm when topping up an old layer, as the directions on two Australian bagged mulches say. For coarse mulch, the Western Australian Department of Water and Environmental Regulation suggests a layer of 50 to 100 mm (5 to 10 cm) to reduce evaporation.
Leave a gap of 5 to 10 cm between the mulch and plant stems; the same directions say not to place mulch against plant and shrub stems. For a ring around a tree, measure the outer diameter: the small bare circle in the middle barely changes the number of bags.
Common questions
How many bags of mulch do I need per square metre?
One 50 litre bag of mulch covers 1 m² at 50 mm deep, so a new bed needs one bag per square metre. For a 25 mm top-up, one bag covers 2 m². These are the figures in the directions on Australian bagged mulch, and they follow from 0.05 m³ ÷ 0.05 m = 1 m². A 10 m² bed takes 10 bags.
How many square metres does a cubic metre of mulch cover?
One cubic metre of mulch covers 20 m² at 50 mm deep, 13.3 m² at 75 mm and 10 m² at 100 mm. Divide 1 by the depth in metres for any other depth. A cubic metre is 1,000 litres, the same as 20 bags of 50 litres. At the 50 to 100 mm a WA government factsheet suggests for coarse mulch, plan on 10 to 20 m² per cubic metre.
How many 30 litre bags of soil make a cubic metre?
It takes 33.3 bags of 30 litres to make 1 m³, so buy 34 bags; with 50 litre bags it is exactly 20. A cubic metre is 1,000 litres and bagged soil is labelled in litres, so divide 1,000 by the bag size. For a large raised bed, compare the price of bags with a bulk load from a landscape supplier.
How many tonnes is a cubic metre of gravel?
The calculator starts at 1.6 tonnes per cubic metre, but treat that as a placeholder: we found no Australian standard or manufacturer figure to quote. Landscape suppliers sell gravel and sand by the cubic metre or the tonne, so ask yours what a cubic metre of that product weighs. At 1.6 t/m³, a path 5 m long, 1 m wide and 50 mm deep needs 0.25 m³, or 0.4 t.
How much mulch do I need around a tree?
A circle 2 m across, mulched 50 mm deep, takes 0.157 m³ of mulch, which is 157 litres or 4 bags of 50 litres. The circle covers 3.1416 × 1 × 1 = 3.14 m², and 3.14 × 0.05 = 0.157 m³. Choose Circle in the calculator and enter the diameter. Keep the mulch 5 to 10 cm away from the stem.
Sources
- Searles, Searles Cypress Mulch (checked 19 October 2026)50 litre bags; spread about 50 mm deep, 25 mm to re-mulch; a 50 L bag covers 1 m² at 50 mm and 2 m² at 25 mm; leave 5 to 10 cm from stems.
- Searles, Searles Tea Tree Mulch (checked 19 October 2026)50 litre bags, the same 50 mm and 25 mm directions and 5 to 10 cm gap from stems.
- Searles, Searles Garden Soil Mix (checked 19 October 2026)Garden soil in 30 and 50 litre bags.
- Searles, Searles Premium Compost Mix (checked 19 October 2026)Compost in 30 litre bags.
- Searles, Searles Sugar Cane Mulch (checked 19 October 2026)80 litre bag covers up to 23 m² at 20 mm depth.
- Department of Water and Environmental Regulation, Western Australia (Be Groundwater Wise), Planning a waterwise garden (factsheet) (checked 19 October 2026)Apply a 5 to 10 cm layer of coarse mulch to reduce evaporation.
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