Topsoil calculator for lawns and beds

How the topsoil calculator does the sums

Topsoil is measured as a volume and usually sold as a weight. The calculator takes the area times the depth to get cubic metres, adds a margin for settlement (the Allowance box, 10% to start), then converts to tonnes at 1.5 tonnes per cubic metre. Bags come from dividing the tonnes by the bag weight, or the cubic metres by the bag size in litres, always rounded up.

The 1.5 comes from Rolawn, the turf and topsoil grower, whose guide to calculating topsoil requirements says one tonne of standard topsoil is about 0.67 cubic metres. One divided by 0.67 is 1.49. Pick sharp sand from the material list instead and the calculator switches to 1.7, the top of M&P Moran's bag figures, for top dressing a lawn.

Here is a new lawn on a cleared plot: 10m by 5m, 100mm of topsoil to turf onto, with the default 10% margin.

• Area: 10 × 5 = 50m².
• Volume: 50 × 0.100 = 5.00m³. Plus 10% = 5.50m³.
• Weight: 5.50 × 1.5 = 8.25 tonnes.
• 800kg bags: 8,250 ÷ 800 = 10.3, so 11 bags.
• 750 litre bags: 5.50 ÷ 0.75 = 7.3, so 8 bags.
• Loose, tipped: 9 tonnes.

How deep for lawns, beds and borders

Depth is the setting that changes the answer most, so start with what is going in the soil. Grass roots stay fairly shallow. Shrubs and vegetables need more. Rolawn's guide gives the ranges below, and the Turfgrass Growers Association's specification for laying cultivated turf asks for at least 100mm of topsoil under new turf.

Those depths are for the finished topsoil layer. If the ground underneath is compacted builder's rubble or clay, the soil you add sits on a hard pan. Break the ground up before you spread anything, or the new lawn or bed will struggle to drain.

Beds take far more soil per square metre than a lawn. A new border along a fence, 7m by 1.5m and filled 200mm deep for shrubs, is 10.5m² × 0.200 = 2.10m³, or 2.31m³ with the 10% margin. At 1.5 tonnes per m³ that is 3.47 tonnes: 5 bags of 800kg, or 4 of 750 litres. It is a fifth of the lawn's area but needs over 40% of its soil.

Tonnes, litres and bulk bags

Suppliers sell topsoil three ways, and it is easy to compare the wrong numbers. Loose loads are priced by the tonne. Bulk bags are sold either by weight, such as Breedon's screened topsoil at about 800kg, or by volume, such as Rolawn's 500 and 750 litre bags in its product calculator. Small retail bags are sold in litres.

Litres are the easy conversion: 1,000 litres is one cubic metre, so a 750 litre bag is 0.75m³. Tonnes depend on the soil. Suppliers' own calculators use anything from 1.4 tonnes per m³ (George Davies Turf) through 1.6 (British Sugar Topsoil) to 1.8 (Tillers Turf). Damp soil weighs more, and a blended topsoil with compost in it weighs less. The calculator uses Rolawn's 1.5. If your supplier gives a figure, use theirs.

To see what a quantity covers, divide its volume by the depth in metres.

• One tonne (about 0.67m³) covers about 6.7m² at 100mm, or 4.5m² at 150mm.
• A 500 litre bag covers 5m² at 100mm, or 10m² at 50mm.
• A 35 litre retail bag covers 0.35m² at 100mm. For anything bigger than a planter, bulk bags are the sensible buy.

What BS 3882 means on the bag

BS 3882:2015 is the British Standard for topsoil that is bought and sold. It does not cover soil left where it is on site. Ground Materials (GMAT), an aggregates and soil supplier, summarises it as two families: multipurpose, suitable for most gardening and landscaping, and specific purpose, for jobs needing acidic, calcareous or low fertility soil, such as a wildflower meadow.

To claim the standard, a supplier has to test for fertility, soil structure and classification, and contaminants, and declare the results, including where the soil came from and when it was sampled. For a lawn or a border, multipurpose is the grade to ask for.

The label alone tells you little. Ask the supplier for the current analysis and check the sampling date. Soil stripped from a building site can be cheaper, but it usually comes without that analysis, so you are taking it on trust.

Raising levels and settlement

Loose soil settles once it has been walked on and rained on. The RHS, in its guide to laying a lawn from turf, warns that a new lawn on disturbed ground such as an old flower bed will drop a little over time, and suggests finishing slightly high to meet an existing lawn. It also says to leave prepared ground to settle for at least a few days, ideally five to six weeks.

That is what the margin in the calculator is for. It is set to 10% because no supplier we found publishes a settlement figure, so treat 10% as a starting point, not a standard. Raise it for deep fills or soil with a lot of compost in it, which shrinks as it rots down.

Fill a deep hollow in layers and firm each one, rather than tipping it all in at once. Keep soil below the house's damp proof course, and away from airbricks.

What the calculator leaves to you

It gives you a quantity to order. It does not check your ground, your levels or your drainage, and it does not price the soil, the delivery or a grab lorry for the spoil.

Laying turf on the new soil? The turf calculator counts the rolls and adds the 100mm soil bed for you. If you are pricing the job for a client, put the soil, the turf and the labour on the quote template.

• The weight is an estimate. Moisture and the soil type change it, so a delivery note may not match to the kilo.
• The margin is your call. Use more on a deep fill.
• Access matters. A bulk bag needs a solid, level spot at the kerb, and a loose load needs room for a tipper.

Common questions