If you're planning a new lawn, the topsoil underneath matters as much as the turf on top. Too shallow a bed and grass roots struggle to establish, drainage suffers, and the lawn stays patchy however good the turf itself is. This guide covers how deep that bed should typically be, how to work out the volume, weight and bulk bags for your own lawn, and when your existing soil is already good enough that you don't need to buy any at all.
How deep should topsoil be for new turf?
Most trade advice puts the figure at 100 to 150mm of topsoil for a lawn built from scratch on bare or stripped ground, with 120mm a sensible middle point to plan around if nothing about your garden points strongly one way or the other. The reasoning has less to do with the turf itself than with what's underneath it: depth also decides how much water and nutrient the soil bed can hold in store for the grass above it - a skimpy few centimetres empties out fast after a dry spell and turns to sludge after heavy rain, with nothing in between, whereas a fuller bed buffers those swings and gives roots a steadier supply to grow into. A proper bed gives roots room to spread and holds onto moisture and nutrients through the first growing season, when a new lawn is at its most vulnerable. Heavy clay that needs organic matter dug in benefits from the deeper end of that range; soil that's already loose and free-draining can often make do nearer 100mm. Treat both figures as a starting point - a supplier or landscaper who's seen your actual ground can tell you more precisely.
Working out how much you need
Once you know your depth, multiply area by depth for the volume - just make sure the depth is in metres rather than the millimetres it's usually quoted in before you multiply the two figures together. For a 40m2 lawn at 120mm (0.12m): 40 × 0.12 = 4.8 cubic metres (m3) of topsoil. Measure your lawn as one rectangle if it's a simple shape, or split it into a few rectangles and add up the areas if it's irregular - the turf calculator does this automatically across up to three areas if you'd rather not do it by hand.
Converting cubic metres to tonnes
Topsoil is sold and delivered by weight, so the cubic metre figure needs converting to tonnes before you order. Density varies by supplier and how wet the soil is, typically 1.2 to 1.7 tonnes per cubic metre (t/m3), with 1.5 t/m3 a reasonable typical figure to plan around if your supplier hasn't quoted you one directly. Continuing the worked example: 4.8m3 × 1.5 t/m3 = 7.2 tonnes.
How many bulk bags is that?
A standard bulk bag - also sold as a jumbo bag or builder's bag - is the usual way topsoil arrives for garden-sized jobs, but most suppliers cap what goes into one at around 850kg. That's not the bag's physical limit; it's roughly the safe working load for a forklift tine or crane strap to lift without risk. At typical topsoil density, an 850kg fill works out to less than the bag's full 0.7m3 capacity, so a supplier quoting "0.7m3" and one quoting "850kg" for the same bag size aren't always describing an equal amount - worth clarifying before you order. Working from the 850kg figure: 7.2 tonnes ÷ 0.85 tonnes per bag = 8.5, rounded up to 9 bulk bags, since part-bags aren't sold. The topsoil calculator works this out automatically from your own area and depth, including a 25kg-bag option if you're buying smaller quantities from a garden centre instead.
Typical topsoil depths at a glance (40m2 lawn, at 1.5 t/m3 density)
| Job | Typical depth | Topsoil needed |
|---|---|---|
| New lawn, full topsoil bed | 100-150mm | 6.0-9.0 t |
| Topdressing an existing lawn | 25mm | ~1.5 t |
Topdressing an existing lawn vs a full new lawn
The depth you need depends heavily on what you're actually doing. A full new lawn from bare or stripped ground needs that 100-150mm bed described above. Topdressing an existing, reasonably healthy lawn to improve it - filling in low spots, improving drainage, encouraging thicker growth - needs far less, typically only 25mm spread evenly and worked in with a rake or the back of a spade. Using the same method at 25mm over the same 40m2 lawn: 40 × 0.025 = 1m3, around 1.5 tonnes at 1.5 t/m3 density - a noticeably smaller job than a full re-lay.
Is your existing soil good enough already?
Not every lawn needs imported topsoil. If the ground underneath is reasonably fertile, drains freely and isn't compacted, rotavating or forking over the existing soil to loosen it and raking it level is often enough before laying turf or seed - saving the cost and hassle of importing anything. Signs it's worth bringing in fresh topsoil instead: heavy clay that stays waterlogged, ground that's mostly rubble or subsoil left over from building work, or a lawn that's persistently patchy despite reasonable care. When in doubt, dig a small test hole - if you hit workable, dark topsoil within the first 100-150mm, your existing ground is probably a reasonable base to build on.
FAQ: topsoil depth for turf and grass
How much topsoil do I need for turf? For a new lawn from bare ground, 100 to 150mm is typical. Work out the tonnes and bulk bags for your own area and depth with the topsoil calculator.
How much topsoil for grass rather than turf? Sowing seed onto bare ground follows the same 100-150mm guidance as turfing, because what matters is the depth of workable soil under the grass, not whether it started as rolls or a bag of seed. Where seed differs is on a lawn that already exists: thickening up thin patches or overseeding after scarifying doesn't call for a fresh soil layer at all - a light rake to open the surface and good seed-to-soil contact does the job.
Can I just add a thin layer of topsoil over grass seed? That's a different job - a light covering after sowing, just enough to cover the seed, isn't the same as the 100-150mm structural bed described here. The deeper bed is for building up the ground itself before turfing or seeding a full new lawn, not for covering seed.