Enter the area you want to cover and how deep you want the topsoil, and this calculator works out the volume in cubic metres, the weight in tonnes, and how many bulk bags to order. It is built for UK screened topsoil at about 1.5 tonnes per cubic metre.
How much topsoil do you need?
Typical depths: 25 to 50 mm to top-dress an existing lawn, 100 to 150 mm for a new lawn or turf, 300 to 450 mm for beds and borders.
The figures below include a 5% allowance so you are not caught short by settling, spillage and uneven ground. Topsoil compacts as it is walked in and watered, so ordering a little over is normal.
How it is calculated
The maths is the same every time. Work out the volume of soil you are spreading, then convert that volume to a weight and to bags.
Step 1, volume. Multiply the area by the depth, with the depth in metres so the units match:
Volume (m3) = area (m2) x depth (mm) / 1000
If you measured length and width instead of a total area, the area is simply length x width.
Step 2, allowance. Add 5% so you are not short once the soil settles and you lose a little to spillage:
Volume with allowance = volume x 1.05
Step 3, weight. Convert the volume to tonnes using the density of screened topsoil, which is about 1.5 tonnes per cubic metre:
Tonnes = volume with allowance x 1.5
Step 4, bags. A standard UK bulk bag (also called a dumpy bag or tonne bag) holds roughly 0.8 tonnes of topsoil, so:
Bulk bags = tonnes / 0.8, rounded up
Figures used
- Density: 1.5 tonnes per cubic metre for screened topsoil. Unscreened or peaty soil can be lighter, around 1.3 t/m3.
- Bulk bag: about 0.8 tonnes per bag. Bags are often sold by volume rather than an exact weight, so check your supplier's figure.
- Wastage allowance: 5%, to cover settling and spillage.
Worked example
Say you are laying a new lawn over 20 m2 at 100 mm deep.
- Volume = 20 m2 x 100 mm / 1000 = 20 x 0.1 = 2 m3.
- With a 5% allowance = 2 x 1.05 = 2.1 m3.
- Weight = 2.1 x 1.5 = 3.15 tonnes.
- Bulk bags = 3.15 / 0.8 = 3.94, rounded up to 4 bags.
As a quick sanity check, 10 m2 at 100 mm is 1 m3, which weighs about 1.5 tonnes, or roughly 2 bulk bags. Scale that to your own area to gut-check any topsoil calculator before you order.
Frequently asked questions
How does this topsoil calculator work in m2?
Enter your area in square metres (m2) and the depth in millimetres. The calculator multiplies the two to get the volume in cubic metres, adds a 5% allowance, then converts that to tonnes and bulk bags. If you only know the length and width, switch to the length x width option and it works out the area for you.
How much does 1 m3 of topsoil weigh in tonnes?
About 1.5 tonnes for screened topsoil. Lighter, unscreened or peaty soil can be nearer 1.3 tonnes per cubic metre, so 1 m3 typically lands between 1.3 and 1.5 tonnes. This calculator uses 1.5 t/m3 as a sensible UK default.
How many bulk bags of topsoil do I need?
A bulk bag (dumpy or tonne bag) holds roughly 0.8 tonnes of topsoil. Divide the total tonnes by 0.8 and round up. For example, 3.15 tonnes works out at four bulk bags. The calculator does this for you and always rounds up so you do not run short.
How much topsoil do I need for a new lawn?
For a new lawn or fresh turf, aim for 100 to 150 mm of topsoil so the grass has enough depth to root. Enter your lawn area and that depth above. As a guide, 100 mm over 20 m2 needs about 3 tonnes, or four bulk bags including the allowance.
How deep should topsoil be for turf or seeding?
For laying turf or sowing seed, 100 to 150 mm of good topsoil is the usual target. To top-dress and level an existing lawn, 25 to 50 mm is plenty. Raised beds and borders usually want 300 to 450 mm so plants can root deeply.
How do I convert topsoil from cubic metres to tonnes?
Multiply the volume in cubic metres by the density. For screened topsoil that is 1.5, so tonnes = m3 x 1.5. To go the other way, divide tonnes by 1.5 to get cubic metres. The calculator shows both the volume and the weight so you can order in whichever unit your supplier uses.
Is bagged topsoil or loose topsoil cheaper?
For small jobs, sealed bags from a builders' merchant are convenient but cost more per tonne. For anything above a tonne or so, loose topsoil delivered in bulk bags or tipped loose is usually cheaper. Use the bulk-bag figure above to compare quotes on a like-for-like basis.
Should I order extra topsoil?
Yes, a small margin is sensible, which is why this calculator already adds 5%. Topsoil settles once it is walked in and watered, and you always lose a little to spillage and uneven ground. If your ground is rough or you are filling hollows, consider rounding up to the next full bulk bag.