Topsoil Explained: Coverage, Depth, and How Much You Need

Topsoil is the upper layer of soil, dark with organic matter, where roots live and plants feed. It is bought by volume — cubic yards or cubic metres, or by the bag — and laid to a depth that depends on what is being grown: a thin topdressing for an existing lawn, a deeper layer for a new lawn, deeper still for a garden bed. Getting the quantity right matters because topsoil is heavy, it is delivered by the truckload, and a shortage means a second delivery while a half-laid bed dries out. This guide explains the coverage arithmetic, the depths for each use, the weight and bag conversions, and the difference between topsoil and the materials it is confused with — mulch, which stays with its own guide, compost, and fill dirt.

Coverage: area times depth

The calculation is one multiplication: the area to cover, times the depth to lay, gives the volume. Area in square metres times depth in metres gives cubic metres. Area in square feet times depth in inches, divided by 12, gives cubic feet; divide by 27 for cubic yards. The units must be consistent, and the depth must be in the same unit as the area’s linear measure, or the answer will be wrong by the conversion factor — the commonest error in the whole calculation.

Worked example: a bed 5 metres long and 2 metres wide, laid 10 cm deep. The area is 10 square metres. The depth is 0.1 metres. The volume is 1 cubic metre. At 15 cm, the same bed needs 1.5 cubic metres. At 5 cm — a topdressing over an existing lawn — it needs 0.5 cubic metres. The area is fixed; the depth decides the order, and a 5 cm error in depth on a large area is a truckload of difference.

Irregular areas need breaking into rectangles and circles, measured separately and added. An L-shaped bed is two rectangles. A circular bed is pi times radius squared. A slope needs its surface area, not its footprint, and a bed with a raised edge needs the depth measured to the finished level, not to the top of the edging. Measure the area to be covered, not the area of the garden, and measure the depth at several points, because ground is rarely level.

Try the method on your own figures with the Topsoil Calculator.

Depth: lawns, beds, and topdressing

For a new lawn on bare ground, the topsoil depth is commonly 10 to 15 cm (4 to 6 inches) of good topsoil over the subsoil, with the seed or turf laid on top. Less than 10 cm, and the roots run out of soil in a dry spell; more than 15 cm, and the cost rises faster than the benefit for most grasses. The subsoil beneath should be loosened and levelled first, so the topsoil sits on a base that drains and does not form a hard pan at the boundary.

For a new garden bed, 15 to 30 cm (6 to 12 inches) is the usual range, with the deeper end for vegetables and deep-rooted plants, and the shallower end for annuals and shallow-rooted shrubs. Existing beds being topped up need less: 5 to 8 cm of fresh topsoil, forked into the top of the old soil so the layers knit together rather than forming a distinct, water-shedding boundary. For topdressing an existing lawn, 5 to 10 mm — half a centimetre to a centimetre — worked into the sward, is enough; more smothers the grass.

Compaction changes the effective depth. Loose topsoil as delivered settles by 10% to 20% when watered and walked on, so a bed laid to 15 cm loose finishes at 12 to 13 cm settled. Order for the settled depth wanted, and allow for settlement in the calculation: multiply the loose volume by 1.1 to 1.2 for the delivered volume, or lay, water, and top up. The calculator’s waste and settlement allowance covers this, and it should not be skipped to save a bag.

Weight, bags, and delivery

Topsoil is sold loose by the cubic yard or cubic metre, and bagged by volume or weight. A standard bag is commonly 40 litres, or about 1.4 cubic feet, and a cubic metre needs 25 such bags. A cubic yard, 0.76 cubic metres, needs about 19 bags of 40 litres. Bags suit small jobs and precise topping up; loose delivery by the truck suits anything over a cubic metre, and it is far cheaper per unit volume.

Weight governs delivery and handling. Loose topsoil weighs roughly 1.1 to 1.4 tonnes per cubic metre, about 1.2 tonnes as a planning figure, and the figure rises with moisture: a load delivered after rain can be 20% heavier than the same load dry. A cubic yard weighs roughly 0.9 tonnes. Check the access before ordering loose: a truck needs a firm, wide approach, and a load tipped on a driveway or a lawn will need moving by barrow, which is slow, heavy work at a tonne per cubic metre. A bulk bag — a cubic-metre sack delivered by crane — suits sites where a tipper cannot reach, and it keeps the soil contained until use.

Order 5 to 10% more than the calculated volume, for settlement, uneven ground, and the shortfall that follows from measuring a depth that varies across the bed. A small surplus is useful: it tops up the low spots that appear after the first rain, and it stores under cover for repairs. A shortage, discovered with a bed half laid, costs a second delivery charge that exceeds the cost of the extra soil.

Topsoil, mulch, compost, and fill: telling them apart

Topsoil is the soil itself, the growing medium. Screened topsoil has had stones and large debris removed and is the right choice for lawns and beds. Unscreened topsoil is cheaper, contains stones and clods, and suits rough levelling under a finished layer. The quality to check is texture and smell: good topsoil is dark, crumbly, and earthy-smelling, not grey, sticky, or sour, and it should contain no glass, brick, or construction waste.

Mulch is a surface covering — bark, wood chip, straw, or gravel — laid on top of soil to suppress weeds, hold moisture, and moderate temperature. It is not a growing medium, it is not mixed into the soil in quantity, and its quantity is calculated by area and depth in the same way, but it is a different material with a different guide. Compost is decomposed organic matter, used to amend soil and feed plants: it is mixed into topsoil or spread thinly on top, and it is not a substitute for topsoil in bulk, because it shrinks as it continues to decompose.

Fill dirt, or subsoil, is the material beneath the topsoil: cheaper, heavier, low in organic matter, and poor for growing. It is the right material for raising levels under a finished topsoil layer, and the wrong material for the layer itself. A load described as topsoil at a price far below the market rate is usually fill dirt with a dark colour, and it will grow weeds and little else. Buy from a supplier who states the source and the screening, inspect a sample before a large order, and keep the delivery ticket with the quantity and the description.

Frequently asked questions