Topsoil Calculator
Works out topsoil for a lawn, a bed or levelling an area. Gives cubic metres and cubic yards, which is how it is sold, plus the weight for anyone checking a delivery.
Includes the waste allowance — this is the figure to order.
Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.
The formula
volume = length × width × depth × (1 + waste) weight = volume × bulk density
The geometry is trivial; the bulk density is where the answer actually comes from. Topsoil runs about 1300 kg per cubic metre loose, so a cubic metre weighs roughly 1.30 tonnes. Suppliers quote one or the other, which is why both are given.
What to watch out for
Depth is the decision, and it depends on what is going in:
- New lawn from seed or turf: 100–150 mm. Grass roots are shallower than people expect.
- Beds and borders: 300–450 mm. Shrubs and perennials need the depth.
- Levelling an existing lawn: no more than 25 mm at a time, or the grass underneath dies.
Grade matters as much as quantity. Screened topsoil is sieved and consistent, and it is what you want near the surface. Unscreened is cheaper and full of stones and root fragments — fine for bulking out the bottom of a deep bed, frustrating to rake into a seedbed.
Fresh topsoil settles about 10–20% over the first season, so a bed filled flush to the rim will sink below it.
Frequently Asked Questions
How much topsoil do I need for a lawn?
Allow 100–150 mm. A 10 × 5 m lawn at 100 mm needs 5 m³, around 6.5 tonnes — well beyond bulk bags, so that is a loose load.
How many bulk bags of topsoil in a cubic metre?
Roughly two. A bulk bag of topsoil is about 730 kg, which is a little over half a cubic metre at typical moisture.
Should I use screened or unscreened topsoil?
Screened for anything you will seed, turf or plant into — it is sieved and consistent. Unscreened is cheaper and suits bulking out the bottom of a deep bed, where the stones do not matter.