How the topsoil calculator works
Topsoil is measured by volume. Bulk bags are sold by the litre (usually 500 or 730 litres), and loose loads by the tonne or cubic metre. The calculator works out the volume of the area, adds a bit for settling, then converts it into bags and weight:
volume (m³) = length × width × (depth in mm ÷ 1,000)
with settling = volume × (1 + extra %)
bulk bags = volume × 1,000 ÷ bag size in litres, rounded up
tonnes = volume × 1.5
For weight we use about 1,500kg per cubic metre, based on Rolawn's guide that one tonne of standard topsoil is roughly 0.67m³. Soil weight varies a lot with how wet and how compacted it is, and loose, screened soil in a bag can weigh less. Materials Market, for example, lists its 500 litre bulk bag of topsoil at 1,100kg per m³. So if you're buying by the tonne, ask your supplier how many cubic metres a tonne of their soil makes.
The 10% extra follows supplier advice to order at least 10% more than the measured volume, because loose topsoil settles once it's raked, firmed and watered.
How deep should topsoil be?
Turf and topsoil supplier Rolawn recommends:
| Use | Topsoil depth |
|---|---|
| New lawn from turf or seed | 100 to 150mm |
| Ornamental beds and borders | 150 to 200mm |
| Salads, beans and courgettes | 150 to 200mm |
| Root vegetables such as carrots and potatoes | 300 to 400mm |
Raised beds benefit from deeper soil for drainage and root growth. If you're filling a raised bed, measure the inside depth of the frame and enter that.
How much does a bulk bag of topsoil cover?
A 500 litre bag is 0.5m³. Before allowing for settling, it covers:
| Depth | 500L bag covers | 730L bag covers |
|---|---|---|
| 50mm (topdressing or levelling) | 10m² | 14.6m² |
| 100mm | 5m² | 7.3m² |
| 150mm | 3.3m² | 4.9m² |
| 200mm | 2.5m² | 3.7m² |
| 300mm | 1.7m² | 2.4m² |
Worked example: topsoil for a new lawn
You're turfing a garden 5m long and 4m wide and want 150mm of topsoil underneath.
- Area: 5 × 4 = 20m²
- Volume: 20 × 0.15 = 3m³
- Add 10% for settling: 3 × 1.1 = 3.3m³
- 500 litre bulk bags: 3,300 ÷ 500 = 6.6, rounded up to 7 bags
- Weight: 3.3 × 1,500 = 4,950kg, or about 4.95 tonnes
In 25kg bags, that would be 198 bags, so bulk bags or a loose tipper load make far more sense for a whole lawn.
Raised bed example
A raised bed 2.4m × 1.2m filled 300mm deep holds 0.864m³. With 10% extra that's 0.95m³: two 500 litre bulk bags, or about 58 bags of 25kg (1.43 tonnes).
Tips for buying topsoil
- Pick the right grade. Many suppliers sell different blends for lawns, for beds and borders and for vegetables. Look for screened topsoil with stones removed.
- Prepare the ground first. Clear weeds and break up compacted subsoil before spreading, or roots and water won't get through.
- Spread in layers and firm each one by treading it down before raking level. This reduces settling later.
- Keep bags covered if they're delivered before you're ready. Wet soil is heavy to move and hard to spread.
Laying a gravel path next to the new lawn? The gravel calculator works out tonnes and bulk bags, and the square metre calculator helps you measure curved or L-shaped beds. Building a deck as well? Try the decking calculator.
Frequently asked questions
How much topsoil do I need for a new lawn?
Rolawn recommends 100 to 150mm of topsoil under turf or seed. At 150mm, each 10m² of lawn needs 1.5m³ of soil, or about 1.65m³ with 10% extra, which is four 500 litre bulk bags.
How many bulk bags of topsoil do I need?
Divide the volume in litres by the bag size. A 20m² area at 150mm deep with 10% extra needs 3.3m³ (3,300 litres), which is 7 bags of 500 litres or 5 bags of 730 litres.
How much does a tonne of topsoil cover?
Rolawn says one tonne of standard topsoil is about 0.67m³. That covers about 6.7m² at 100mm deep or 4.5m² at 150mm deep.
How heavy is a cubic metre of topsoil?
Roughly 1.1 to 1.5 tonnes, depending on moisture and how compacted it is. We use 1,500kg per m³ to match Rolawn's guide.
How many 25kg bags of topsoil make a bulk bag?
At 1,500kg per m³, a 500 litre bulk bag of soil weighs about 750kg, which is the same as 30 bags of 25kg. Lighter, drier soil means fewer bags.
Last reviewed October 2026 using 2026/27 figures. This calculator gives an estimate for guidance only.