How this calculator works

We add up the area of each section you enter, multiply it by the depth in metres, and that gives the volume in cubic metres. We then add a settling allowance — 10% by default — because loose soil compacts once it has been rained on and walked over.

To turn that into a weight, we multiply by the density. Screened, moist UK topsoil is about 1.4 tonnes per cubic metre. Bulk bags are sold by weight rather than volume, so an 850kg bag is roughly 0.6m³ of soil — a fact that surprises a lot of people the first time they order.

In short: area (m²) × depth (m) = m³, then m³ × 1.4 = tonnes, then tonnes ÷ 0.85 = bulk bags.

Worked example: a 10m × 6m new lawn

A back garden lawn of 10m by 6m is 60m². For turf you want 150mm of decent topsoil, so that is 60 × 0.15 = 9m³. Add 10% for settling and you are at 9.9m³, which at 1.4 tonnes per cubic metre is 13.9 tonnes — seventeen 850kg bulk bags.

At about £70 a bag delivered, that is roughly £1,190 — which is exactly why most people reduce the depth to 100mm over a dug-over base, bringing it down to about eleven bags. At that volume it is also worth phoning a soil supplier for a loose tipped load, which can be half the price per tonne if you have somewhere for the lorry to tip.

Tips from the trade counter

• Buy the right grade. Screened topsoil for lawns, blended border or vegetable soil for planting, economy as-dug for bulk filling where looks don't matter.
• Order slightly over for raised beds. Soil drops noticeably in the first month. Fill beds proud and top up in spring.
• Don't lay topsoil on compacted ground. Break the base up with a fork or rotavator first, or you create a drainage pan and the lawn sits wet.
• Check where the lorry can tip. Loose loads are cheapest but need hard standing and space, and you'll be barrowing from wherever it lands.
• Wet soil weighs more. A tonne of sodden soil contains less actual soil than a tonne of dry soil — order after a dry spell if you can.