Mulch and Soil Quantity Calculator — How Much Do You Need?
Most gardeners either over-order and end up shifting leftover bags for months, or under-order and face a second delivery fee. Getting the quantity right comes down to one simple formula — and knowing the recommended depth for the material you're using.
This guide walks through the calculation step by step, covers recommended depths for every common landscape material, compares buying in bulk versus bags, and provides 2026 Australian price ranges so you can budget before you order.
Last updated: May 2026.
The formula
The core calculation for any landscape material is:
Area (m²) × Depth (m) = Volume (m³)
That's it. Measure the length and width of the area you want to cover, multiply them together to get the area in square metres, then multiply by your target depth in metres (not millimetres — convert first).
Worked example
You have a garden bed that is 5 m long by 4 m wide, and you want to apply mulch at the standard 75 mm depth.
- Area: 5 m × 4 m = 20 m²
- Depth in metres: 75 mm ÷ 1,000 = 0.075 m
- Volume: 20 m² × 0.075 m = 1.5 m³
So you need 1.5 cubic metres of mulch for that garden bed.
Use the Leadkit mulch and soil calculator to run this instantly without doing the arithmetic yourself — enter your dimensions and target depth, and the calculator returns the volume plus an estimated bag count.
Recommended depths by material
Depth is where most ordering errors happen. Apply mulch too thin and it won't suppress weeds or retain moisture effectively. Apply top soil or sand too deep without compaction and it will settle unevenly. The table below gives the standard recommended depth for each material and the reason behind it.
| Material | Recommended depth | Why |
|---|---|---|
| Wood chip / bark mulch | 75 mm | Minimum for effective weed suppression; below 50 mm weeds push through |
| Pine bark | 75 mm | Same suppression principle; coarser texture means less compaction |
| Sugar cane mulch | 75–100 mm | Finer material compacts more; slightly deeper recommended |
| Lucerne (alfalfa) mulch | 50–75 mm | Dense, nutrient-rich; thinner layers to avoid matting |
| Top soil / garden mix | 100 mm | Standard top dressing depth for new beds; 150 mm for new lawn preparation |
| Compost | 50–75 mm | Incorporated or surface applied; thicker layers can cause nitrogen draw-down |
| Washed sand | 50 mm | Lawn top dressing or levelling; deeper applications require compaction |
| Gravel / river pebbles | 50–75 mm | Purely decorative; 50 mm covers ground without shifting excessively |
| Decomposed granite | 50–75 mm | Similar to gravel; needs edging to stay in place |
Key terms explained:
- Weed suppression depth — the minimum material depth at which light is blocked sufficiently to prevent most weed seeds from germinating. For organic mulches this is 50–75 mm.
- Top dressing — the practice of applying a thin layer of top soil, compost, or sand over an existing lawn or bed to improve soil structure without burying existing plants.
- Compaction factor — organic materials like sugar cane and compost compress after application, often by 10–20%. Order a small surplus (around 10–15%) to account for this.
Bag equivalents
If you're buying in bags rather than bulk, you need to know how many bags equal one cubic metre. The standard 40L bag is the most common retail size at Bunnings and Flower Power, but 25L bags are also widely sold.
| Material | 40L bags per m³ | 25L bags per m³ |
|---|---|---|
| Wood chip mulch | 25 bags | 40 bags |
| Pine bark mulch | 25 bags | 40 bags |
| Sugar cane mulch | 25 bags | 40 bags |
| Top soil / garden mix | 25 bags | 40 bags |
| Compost | 25 bags | 40 bags |
| Washed sand | 25 bags | 40 bags |
| River pebbles / gravel | 25 bags | 40 bags |
The maths is consistent because 1 m³ = 1,000 litres, so 1,000 L ÷ 40 L = 25 bags for any material. The actual weight per bag will differ — a 40L bag of sand is significantly heavier than a 40L bag of sugar cane mulch — but the volume conversion remains the same.
Bulk delivery vs bagged: cost crossover
For volumes around 1 m³ or more, bulk delivery from a landscape supplies yard is almost always cheaper per cubic metre than buying bags at a hardware store. The cost crossover point typically sits at approximately 0.8–1 m³, depending on your location and the delivery fee.
The table below compares estimated costs for 1 m³ of wood chip mulch across common purchasing options.
| Purchase method | Estimated cost for 1 m³ |
|---|---|
| 40L bags (Bunnings, 25 bags) | $95–$150 |
| Bulk bag (1 tonne bag, hardware/garden centre) | $80–$130 |
| Bulk loose delivery (local landscape yard) | $60–$110 |
This is a price indication only. Supplier pricing varies.
A bulk bag (also called a 1 tonne bag or bulka bag) is a large cubic metre-capacity woven bag delivered on a pallet. It's a halfway option — you get bulk pricing without needing a tip-truck delivery. Bunnings, Flower Power, and some Barbeques Galore garden centres stock bulk bags, though availability varies by store.
The rule of thumb: if you need less than half a cubic metre, buy bags for convenience. If you need 1 m³ or more, call your local landscape supplies yard and ask for a loose delivery price — you'll almost always save money, and the quality of material from a specialist yard is generally superior to retail-bagged products.
For larger landscaping projects, the Leadkit landscaping quote calculator can help you estimate the full job cost including materials and labour.
Material types and 2026 price guide
Prices below are per cubic metre for bulk loose delivery from a landscape supplies yard. Retail bagged pricing will be higher.
These are price indications only. Supplier pricing varies by state, season, and supplier. Prices were compiled from typical AU landscaping supplier rates as at May 2026.
| Material | Bulk price per m³ | Key characteristics |
|---|---|---|
| Wood chip mulch | $60–$90 | Coarse texture, long-lasting, good weed suppression; typically recycled arborist chips |
| Pine bark mulch | $70–$110 | Uniform appearance, acidic pH suits camellias and azaleas, slow to decompose |
| Sugar cane mulch | $55–$80 | Fine texture, breaks down quickly to improve soil, popular for vegetable beds |
| Lucerne (alfalfa) mulch | $70–$100 | High nitrogen content, excellent for vegetable gardens, compacts over time |
| Top soil / garden mix | $50–$90 | Quality varies significantly — ask suppliers about composition and pH |
| Washed sand | $65–$100 | Used for levelling, lawn top dressing, and under pavers |
| River pebbles | $90–$150 | Decorative use in garden beds and paths; heavier than organic mulches |
| Decomposed granite | $80–$130 | Permeable path and driveway surface; natural appearance, low maintenance |
Brand note: For bagged mulch and soil products, common Australian brands include Richgro, Baileys Fertiliser, Osmocote, and Scotts. Product quality from these brands is generally reliable for garden beds. For bulk supply, source from a specialist landscape yard rather than a hardware retailer for better value and more consistent material quality.
Methodology: Ranges are drawn from typical rates charged by landscape supplies yards across NSW, Victoria, Queensland, and South Australia as at May 2026. Local availability and seasonal demand affect pricing.
Common ordering mistakes
1. Ordering too little
The most common error. It's tempting to round down to the nearest bag count, but mulch settles and compresses — especially organic materials like sugar cane and compost. Order 10–15% extra to account for the compaction factor and to top up thin spots after settling.
2. Applying at the wrong depth
Applying wood chip mulch at 25–30 mm (a single bag spread too far) looks covered but provides minimal weed suppression. Weeds emerge through thin mulch within weeks. Stick to the recommended 75 mm depth for organic mulches.
3. Butting mulch against plant stems
Mulch piled against tree trunks and shrub stems traps moisture and causes collar rot. Leave a 100–150 mm gap around the base of all plants — the "volcano mulching" look is one of the most common garden mistakes in Australia.
4. Not accounting for irregular-shaped beds
The area × depth formula assumes a rectangle. For curved or irregular beds, break the shape into approximate rectangles, calculate each section separately, and add them together. When in doubt, add 10% to your total.
5. Ignoring delivery access
Bulk loose delivery arrives by tip-truck. If your property has a narrow driveway, steep access, or limited street frontage, discuss this with the supplier before ordering — some jobs suit a bulk bag on a pallet instead.
How to use the Leadkit mulch and soil calculator
The Leadkit mulch and soil calculator is free and takes about 30 seconds to use:
- Select your material — mulch, top soil, sand, gravel, or compost
- Enter your area dimensions — length and width in metres (or total area if you already know it)
- Select or enter your depth — the calculator pre-populates recommended depths for each material
- View your result — cubic metres required, bag count equivalent, and an estimated material cost range
Leadkit uses data from real Australian suppliers to generate cost estimates. The result gives you a realistic budget figure before you call a landscape yard or head to Bunnings.
Frequently asked questions
How do I calculate how much mulch I need?
Multiply the area in square metres by the depth in metres. For example, a 10 m² garden bed at 75 mm depth needs 10 × 0.075 = 0.75 m³. That's approximately 19 bags of 40L mulch, so most people would round up to 20 bags or order 0.75–1 m³ bulk.
How many 40L bags of mulch do I need for 1 m³?
Exactly 25 bags. One cubic metre equals 1,000 litres, and 1,000 ÷ 40 = 25. This applies to any material regardless of whether it's mulch, soil, sand, or gravel — the volume maths is the same even though the weight will differ.
Is bulk mulch cheaper than bags?
Yes, for most quantities above 0.8–1 m³. A cubic metre of wood chip mulch in 40L bags from Bunnings costs roughly $95–$150. The same volume bought loose from a landscape yard costs $60–$90 delivered. The savings increase with volume.
What depth of mulch is best for weed suppression?
The minimum effective depth for weed suppression in organic mulches is 50 mm, but 75 mm is the standard recommendation. Below 50 mm, light penetrates enough for most weed seeds to germinate. Applying at 75 mm also gives you a buffer as the mulch naturally breaks down and settles over time.
Can I use a soil calculator for gravel and sand as well?
Yes — the same area × depth formula applies to any material. The only variable is the recommended depth, which differs by material type (50 mm for sand and gravel vs 75–100 mm for mulch and top soil). The Leadkit calculator covers all common landscape materials in one tool.
How much does a cubic metre of soil weigh?
Loose top soil weighs approximately 1,300–1,500 kg per cubic metre (1.3–1.5 tonnes). Washed sand is heavier at 1,600–1,800 kg/m³. Organic mulches are lighter — sugar cane mulch runs around 200–400 kg/m³ depending on moisture content. Weight matters for delivery access and if you're loading into a trailer yourself.
Get an accurate quantity and cost estimate
Getting the quantity right before you order saves money and the frustration of a second delivery. The formula — area × depth — takes 30 seconds with a tape measure and a calculator.
For anything more than a simple rectangular bed, or when you're combining multiple materials across a landscaping project, use the Leadkit mulch and soil calculator to get an accurate volume and indicative cost without the manual arithmetic.
If you're planning a broader landscaping project, the Leadkit landscaping quote calculator connects you with local landscapers who can quote the full job — supply, delivery, and installation included.
All prices in this guide are indicative estimates based on typical AU landscaping supplier rates as at May 2026. Actual prices will vary by supplier, location, and project specifics. Always obtain quotes from your local landscape yard before ordering.