Quick answer: Multiply the length by the width by the depth, all in metres, to get the cubic metres of topsoil you need. A 40 m² lawn topped with 10 cm of soil needs 40 × 0.10 = 4 m³, or about 4.4 m³ once you add 10% for settling. That is very roughly 5 to 7 tonnes.
Topsoil is sold by volume (litres, cubic metres or bulk bags) and sometimes by weight, so the job is really a volume calculation with a little allowance on top. This page gives you a calculator, a depth chart and a worked example for a typical UK garden; for related jobs such as gravel, fencing and lawns, see our garden and outdoor measuring guides.
How do you calculate how much topsoil you need?
Volume in cubic metres = length (m) × width (m) × depth (m). The only common slip is the depth: soil depth is usually quoted in centimetres, so divide by 100 first. A 15 cm layer is 0.15 m, a 5 cm layer is 0.05 m.
- Measure the area. Use a long tape to take the length and width in metres. If you are unsure of the markings, our guide to reading a tape measure covers metric and imperial scales.
- Choose the depth. Use the chart below, or measure how far below the finished level the ground currently sits.
- Multiply. Length × width × depth in metres gives cubic metres (m³). Multiply by 1,000 if you need litres for bagged soil.
- Add an allowance. Loose soil settles as it is firmed and watered, so order roughly 10 to 20% more than the calculated volume.
Topsoil calculator
m m cm %Area: m² → topsoil to order: m³ ( litres), roughly to tonnes
The weight range uses 1.2 to 1.5 tonnes per cubic metre, a range commonly used for estimating loose topsoil. Damp, heavy or clay-rich soil sits at the top of that range, so if your supplier quotes a density, use theirs.
How deep should topsoil be?
The RHS advises at least 10 cm (4 in) of topsoil for a lawn and about 20 cm (8 in) or more for new beds. Raised beds are usually filled to their full depth, although the bottom can be bulked out with subsoil or rubble-free fill if the bed is very deep.
| Job | Typical depth | Litres per m² | Area covered by 1 m³ |
|---|---|---|---|
| Levelling hollows, top dressing | 2.5 cm (1 in) | 25 L | 40 m² |
| Patching thin areas of lawn | 5 cm (2 in) | 50 L | 20 m² |
| New lawn, seed or turf (RHS minimum) | 10 cm (4 in) | 100 L | 10 m² |
| New lawn on poor ground | 15 cm (6 in) | 150 L | 6.67 m² |
| New flower or shrub border | 20 cm (8 in) | 200 L | 5 m² |
| Vegetable bed or shallow raised bed | 30 cm (12 in) | 300 L | 3.33 m² |
One cubic metre of topsoil spread 10 cm deep covers 10 square metres. Every centimetre of depth over one square metre adds 10 litres, which makes quick mental checks easy: a 20 m² border at 20 cm needs 20 × 200 = 4,000 litres, or 4 m³.
What does a worked example look like?
A rectangular lawn 8 m by 5 m, covered to 10 cm, needs 4.4 m³ of topsoil with a 10% allowance. The working is:
- Area: 8 m × 5 m = 40 m²
- Volume: 40 m² × 0.10 m = 4.0 m³
- With 10% extra: 4.0 × 1.10 = 4.4 m³ (4,400 litres)
- Approximate weight: 4.4 × 1.2 = 5.3 tonnes up to 4.4 × 1.5 = 6.6 tonnes
A raised bed is the same sum. A timber bed 2.4 m long, 1.2 m wide and 30 cm deep holds 2.4 × 1.2 × 0.30 = 0.864 m³, or 864 litres. For pots and containers, work from the pot’s internal size instead; our plant pot size guide shows where to measure.
Odd-shaped areas
Split an irregular lawn into rectangles and triangles, work out each area and add them together. A triangle is half the base times the height, and a circle is π × radius². A round bed 3 m across (radius 1.5 m) has an area of about 7.07 m², so at 20 cm it needs about 1.41 m³. The area converter helps if some measurements are in feet.
How do you work it out in feet and inches?
Multiply length (ft) × width (ft) × depth (in) ÷ 12 to get cubic feet, then divide by 27 for cubic yards. A 20 ft by 12 ft lawn at 4 in deep needs 20 × 12 × 4 ÷ 12 = 80 cubic feet, which is 2.96 cubic yards or about 2.27 m³. One cubic yard is 0.7646 m³, and one cubic metre is about 1.31 cubic yards.
How is topsoil sold in the UK?
UK suppliers sell topsoil in litre bags, in bulk bags and as loose loads priced by the cubic metre or tonne. Retail bags state their volume in litres, so divide your total litres by the bag size. Bulk bag volumes differ between suppliers, often somewhere around a cubic metre or a little less, so check the stated volume rather than assuming one bag equals 1 m³.
Cubic metres are the standard trade unit, but you may still hear older landscapers talk in cubic yards, and loose loads delivered by tipper are often priced by the tonne. Convert between them with the volume converter. For quality, look for soil that conforms to BS 3882, the British Standard specification for topsoil, which sets limits for properties such as texture, pH, nutrients and contaminants.
Common mistakes to avoid
- Leaving depth in centimetres. 8 × 5 × 10 gives 400, not 4. Always convert to metres.
- Forgetting settlement. Freshly spread soil can sink noticeably once firmed and rained on.
- Measuring from the wrong level. If you are raising a lawn to meet a patio, measure the gap between the existing ground and the finished level at several points and average them.
- Ignoring access. A loose load is tipped where the lorry can reach. Plan for barrowing it to the back garden.
Frequently asked questions
How much topsoil do I need for 10 square metres?
At a depth of 10 cm, 10 m² needs 1 m³ of topsoil, or about 1.1 m³ with a 10% allowance for settling. At 5 cm it needs half that.
How deep should topsoil be for a new lawn?
The RHS recommends a layer of at least 10 cm (4 in) of topsoil for a new lawn, whether you sow seed or lay turf. Use more on very poor or stony ground.
How much does a cubic metre of topsoil weigh?
Roughly 1.2 to 1.5 tonnes, depending on how wet it is and how much clay, sand and organic matter it contains. Use your supplier’s figure if they give one.
How many square metres does a tonne of topsoil cover?
At 1.3 tonnes per cubic metre, a tonne is about 0.77 m³. That covers around 7.7 m² at 10 cm deep, or about 15.4 m² at 5 cm.
How much extra topsoil should I order?
Add around 10 to 20% to the calculated volume to allow for settling, uneven ground and spillage. Use the higher figure for deep beds and very loose soil.
Sources
- Topsoil: buying, Royal Horticultural Society
- BS 3882:2015 Specification for topsoil, British Standards Institution
- National Physical Laboratory, for SI units of volume and the metre
More Garden and Outdoor guides
- How to Measure for a Fence Panel (UK): Panels and Posts
- How to Measure for Artificial Grass: UK Roll Guide
- How Much Gravel Per Square Metre? UK Calculator
- How Much Topsoil Do I Need? UK Calculator and Guide
- How to Measure Lawn Size for Turf (UK Calculator)
- How to Measure a Garden in Square Metres (UK Guide)