Topsoil depth guide
Last reviewed 1 July 2026
Short answer
100–150 mm (4–6 in) for a new lawn, 200–300 mm (8–12 in) for ornamental beds, 300–400 mm (12–16 in) for vegetables, and 10–15 mm for a top dressing. What matters as much as the depth is what is underneath — soil laid on compacted, unbroken subsoil will waterlog whatever depth you use.
Work it out for your project with the topsoil calculator →
Depth by use
| Use | Metric | Imperial | Why |
|---|---|---|---|
| Lawn top dressing | 10–15 mm | ½ in | Levels minor dips without smothering grass |
| New lawn from seed or turf | 100–150 mm | 4–6 in | Enough rooting depth to survive a dry summer |
| Flower border | 200–300 mm | 8–12 in | Suits most ornamental planting |
| Vegetable bed | 300–400 mm | 12–16 in | Root crops need the depth |
| Raised bed | Full internal depth | Full internal depth | Fill to the level you want after settlement |
| Levelling | Average fill depth | Average fill depth | Measure the average, not the deepest point |
| Around new trees | Match existing level | Match existing level | Never bury the root flare |
Why 100 mm is the lawn minimum
Grass roots will go deeper than 100 mm given the chance, but 100 mm is roughly the point below which a lawn stops being able to survive a dry spell without irrigation.
Thinner layers dry out from below as well as above, and the grass browns off in the first hot week. Thinner layers over compacted subsoil are worse still, because the roots hit a pan and stop.
Above about 150 mm the returns diminish quickly for an ornamental lawn — you are paying for soil the grass will not use.
What is underneath matters more than the depth
- Remove or kill existing weeds first. Perennial weeds will come straight through new soil.
- Break up the subsoil to at least 150 mm, particularly if machinery has been over it. A compacted pan under new topsoil creates a bathtub.
- Do not create a sharp interface between a light topsoil and a heavy clay subsoil. Blend the boundary by forking the two together.
- Firm the topsoil in layers by treading, not by rolling it into a slab. Firm enough to walk on without leaving deep prints, no more.
- Rake to level and let it settle for a week before seeding or turfing if you can.
Important: Never raise soil against a building
Levelling: measure the average
For a levelling job, the depth to use in the calculation is the average fill depth, not the depth of the deepest hollow.
Run a string line at the finished level, measure down to the existing ground at a grid of points, and take the mean. On an uneven lawn that is often half what people estimate by eye.
For anything more than about 50 mm of fill over an existing lawn, lift the turf rather than burying it. Buried turf rots and leaves a soft, uneven layer.
Safety notice: Material planning only