Topsoil Calculator — Bags Needed for Lawns and Garden Beds
Whether you are topping up a lawn, filling raised beds, or levelling a garden, knowing how much topsoil to order saves you from expensive over-ordering or a frustrating second delivery. Enter your dimensions and target depth to get the volume and bag count.
How we calculate
Volume = length × width × depth (m) × (1 + waste %). Bags = ⌈volume ÷ bag volume⌉.
Worked example
5 m × 4 m at 100 mm depth: 5 × 4 × 0.1 = 2 m³. With 10% waste: 2.2 m³. At 35 L/bag (0.035 m³): ⌈2.2 ÷ 0.035⌉ = 63 bags.
Common mistakes
- Underestimating depth — a shallow layer of topsoil over poor subsoil still produces poor growth.
- Not testing soil pH before adding topsoil — mixing incompatible soils can harm plants.
- Ordering bags when a bulk tonne bag or tipper load is much cheaper for large areas.
- Forgetting that topsoil settles by 10–20% after watering — add extra depth to compensate.
Frequently asked questions
How deep should topsoil be for a new lawn?+
Grass roots need at least 100–150 mm of good topsoil. Less than that and the lawn will struggle in dry spells. For a vegetable garden or flower bed, aim for 200–300 mm to support deeper root systems.
How many bags of topsoil do I need per square metre?+
At 100 mm depth, you need 0.1 m³ (100 L) per square metre. A standard 35 L bag covers 0.35 m² at that depth — roughly 3 bags per square metre. Use the calculator to adjust for your depth and bag size.
Is it cheaper to buy bags or bulk topsoil?+
For areas over about 5–10 m², bulk topsoil delivered by tipper truck or bulk bag is almost always cheaper per cubic metre than individual bags. Bags are convenient for small top-ups but expensive at scale.
Does topsoil settle after laying?+
Yes. Freshly laid topsoil typically settles 10–20% after the first watering and rain. If laying a lawn, overfill slightly so the settled level matches surrounding areas. If planting immediately, account for settling in your depth calculation.