How deep should topsoil be?
The right depth depends on what will grow in it and what is underneath. These are the depths that work for common jobs in Australian and New Zealand gardens:
Before you order, peg a string line at the level you want the finished surface to be and measure down to the ground at several points. For new turf, the underlay should finish one turf-thickness below paths and edges so the lawn sits flush. Ask your turf supplier how thick their rolls are cut.
Topsoil, garden soil, turf underlay or fill?
Landscape yards use these names loosely, and one yard's "topsoil" can be another yard's fill. Ask what is in the product before you order a truckload.
- Turf underlay is a sandy loam, often around 80 % sand and 20 % soil. It drains fast, stays open under foot traffic and lets new turf roots knit in quickly. Use it for lawns, not vegetable beds.
- Topsoil is screened natural soil. Quality ranges from excellent loam to sandy subsoil, so squeeze a moist handful. Good topsoil crumbles; poor topsoil is either gritty and loose or sticky like modelling clay.
- Garden soil or garden mix is topsoil blended with compost and sometimes sand. It is the right product for new beds, and the better yards make it to AS 4419, the Australian Standard for soils for landscaping and garden use.
- Fill or clean fill is for building up levels under paving, sheds and paths. It has no organic matter and should never be the top layer anywhere you want plants to grow.
For raised beds, topsoil alone is too dense and too poor. Use a vegetable mix for the top layer and read the best soil mix for raised beds before you order.
Topsoil is heavy: plan the delivery and the barrow runs
A cubic metre of moist topsoil weighs about 1.3 tonnes, and sandy underlay about 1.4 tonnes. That is why the calculator shows weight and trailer loads next to the volume:
- Trailers. A standard 6×4 box trailer rated at 750 kg carries only about 500 kg once you subtract the trailer's own weight. That is under 0.4 m³ of topsoil, a little over a third of a cubic metre. The trailer load calculator works it out for your own trailer and car.
- Barrows. A builder's wheelbarrow holds about 80–100 L when heaped, so a cubic metre is 10–12 trips. Six cubic metres for a lawn is 60–70 barrow loads. Book a mini loader or a friend if the pile is far from the job.
- Where the truck tips. Tip on a tarp on the driveway, as close as the driver can safely reach. Soil left overnight in rain gets heavy and messy.
The formula, and a worked example
weight (t) = volume (m³) × density (t/m³) · bags = ⌈ volume × 1,000 ÷ bag litres ⌉
A new backyard lawn 12 m × 6 m, laid as instant turf on 100 mm of turf underlay:
- Area: 12 × 6 = 72 m². Volume: 72 × 0.1 = 7.2 m³.
- Add 15 % for rolling and settling: 7.2 × 1.15 = 8.28 m³. The yard will want a round figure, so order 8.5 m³.
- Weight at 1.4 t/m³: 8.28 × 1.4 = 11.6 tonnes, or 24 trips in a 6×4 trailer.
- In 30 L bags it would be 276 bags ($2,208 at $8). Bulk at $75/m³ plus $80 delivery is $701. Bulk is the only sensible option.
For the turf itself, including the number of rolls and the establishment watering, use the turf calculator. At lawn scale, bags never win. They only make sense for patching a few hollows or topping up a small bed. See bagged vs bulk for the break-even maths.
Preparing the ground before the soil goes on
- Kill or remove weeds first. Couch and kikuyu runners grow straight up through 100 mm of new soil. Dig them out or treat them and wait until they are dead before you spread anything.
- Loosen compacted ground. Rip or rotary-hoe hard ground to about 100 mm so the new soil and the old soil join up. If you skip this, water perches at the boundary and roots stop there.
- Deal with clay. If the base is dispersive clay, fork gypsum into it before the topsoil goes on. The clay soil guide shows how to test for it and how much to use.
- Spread in layers and settle. Spread about 50 mm at a time, rake it level, then water or roll it and fill the low spots again. For turf, finish with a light roller so footprints are no deeper than a few millimetres.
Topsoil questions
How much topsoil do I need for 100 square metres?
At 100 mm deep, 10 m³ – about 13 tonnes of moist topsoil or 14 tonnes of sandy underlay. Add 10–15 % if it will be rolled or compacted, so order 11–11.5 m³. At 150 mm it is 15 m³.
How much does a cubic metre of topsoil weigh?
About 1.2 tonnes dry, 1.3 tonnes moist and 1.4 tonnes for sandy turf underlay. Soil soaked by rain can be a fifth heavier again. Ask your yard for their own conversion; most print it on the price list.
Can I lay turf straight onto clay?
It will survive, but it roots shallowly and struggles in dry spells and wet winters. Loosen the clay, add gypsum if it is dispersive, and spread at least 80–100 mm of underlay on top. That is what turf growers base their establishment advice on.
How many bags of topsoil are in a cubic metre?
40 × 25 L, 34 × 30 L or 25 × 40 L bags. A 30 L bag of moist soil weighs 20–25 kg, so a cubic metre in bags is roughly 40 heavy lifts. See how many bags in a cubic metre.
Sources and method
- Turf underlay depth: Australian turf growers' soil preparation advice, for example Lawn Solutions Australia – How to lay turf, part 2: preparation (at least 80–100 mm of quality soil; 100–150 mm is common advice across growers).
- Standards Australia, AS 4419:2018 Soils for landscaping and garden use. standards.org.au
- Densities (1.2–1.4 t/m³) are typical values from Australian and New Zealand landscape-supply price lists. Trailer payload: a 6×4 box trailer with a 750 kg ATM and 200–275 kg tare, as listed in manufacturers' specifications.
- Volumes are geometry only. Bag counts round up, and prices are illustrative.
Written by DueRiva · Last reviewed 23 September 2026 · Report an error