Landscaping materials are sold two different ways — by volume and by weight — and mixing them up leads to ordering far too much or far too little. This guide covers gravel, road base and mulch in Australian terms, including how to convert cubic metres to tonnes.
Gravel, road base, sand and mulch are all spread over an area to a depth, so the calculation is the same as for concrete: area multiplied by depth, in metres, giving cubic metres.
Volume (m³) = Area (m²) × Depth (m)
A gravel driveway 10m long and 3m wide, spread 100mm deep, is 10 × 3 × 0.1 = 3 m³. A garden bed 8m × 2m mulched to 75mm is 8 × 2 × 0.075 = 1.2 m³. Our gravel calculator and mulch calculator handle the area and depth for you.
Depth is where landscaping estimates go wrong, because the right depth depends on the job.
Here is the trap. You calculate a volume in cubic metres, but many suppliers sell and deliver gravel by the tonne. To order, you have to convert, and that needs the material's density.
Gravel and road base sit around 1.5 tonnes per cubic metre, though this varies with the stone type and moisture. Our 3 m³ driveway therefore weighs roughly 3 × 1.5 = 4.5 tonnes.
Small quantities come in bags from a hardware store or garden centre. Larger quantities are far cheaper delivered loose, either tipped on your driveway or in a bulk (bulka) bag lifted off a truck.
Bagged product is convenient for a single garden bed but expensive per cubic metre. Bulk delivery wins decisively once you need more than about a cubic metre — which, as our examples show, most real jobs do. The saving usually more than covers a couple of hours with a wheelbarrow.
Mulch follows the same volume calculation, but two things set it apart. It is light and bulky, so it is almost always sold by volume (cubic metres or litres) rather than by tonne — a 50-litre bag is 0.05 m³, so our 1.2 m³ bed would need about 24 bags, or a single small bulk delivery.
And mulch breaks down. Unlike gravel, it decomposes into the soil and needs topping up, so a garden is an ongoing small order rather than a one-off. Organic mulches feed the soil as they break down; inorganic ones such as gravel don't, but last far longer.
Loose gravel and road base do not stay at their delivered volume. When compacted — by a plate compactor, or simply by vehicles driving over a driveway — the material settles and the depth reduces.
This means the volume you order should be slightly more than the finished volume you calculated, because some of it disappears into compaction. For a driveway base, an allowance on top of the calculated volume covers this settlement, so the finished surface still sits at the depth you designed.
Decorative gravel over a stable base settles far less, so the allowance is smaller. Structural road base under a driveway settles more and needs the larger margin.
Landscaping materials are priced per tonne or per cubic metre depending on the product, plus a delivery fee that often depends on distance and load size. Because rates vary widely by region and material, a local quote is the only accurate figure.
A few things to check before ordering: whether the price is per tonne or per cubic metre (and convert so you're comparing like with like), the delivery minimum and fee, and whether the truck can access where you want the material tipped. Use our gravel and mulch calculators to fix your volume, add around 5–10% for spread and compaction, then get a delivered quote.
How do I calculate how much gravel I need?
Multiply the area in square metres by the spread depth in metres to get cubic metres. A 10m by 3m driveway at 100mm (0.1m) deep is 10 × 3 × 0.1 = 3 cubic metres. Many suppliers then sell by the tonne, so you may need to convert using the material's density.
How many tonnes is a cubic metre of gravel?
Gravel and road base are around 1.5 tonnes per cubic metre, though it varies with stone type and moisture. So 3 cubic metres is roughly 4.5 tonnes. Always ask your supplier for the exact density of the product you're buying.
How deep should I spread mulch?
Around 50 to 75mm is ideal for garden mulch — deep enough to suppress weeds and retain moisture, but not so deep that it stops air and water reaching the soil. Decorative gravel over a stable base needs only about 50mm.
Is it cheaper to buy in bags or bulk?
Bulk delivery is significantly cheaper per cubic metre once you need more than about one cubic metre. Bags suit a single small garden bed, but for a driveway or several beds, loose or bulk-bag delivery usually saves enough to justify the wheelbarrow work.
How much mulch do I need for a garden bed?
Multiply the bed area by the depth in metres. An 8m by 2m bed at 75mm deep is 1.2 cubic metres, which is around 24 fifty-litre bags or one small bulk delivery. Remember mulch breaks down over time and needs topping up.
Does a driveway need one layer or two?
Usually two — a compacted road-base sub-layer for strength, then a finer or decorative top layer. Calculate and order each separately at its own depth, as they are different materials serving different purposes.