Topsoil Calculator with Depth Guide

How the Topsoil Calculator Works

The calculator converts your area and depth measurements into a volume, then translates that volume into a weight in tonnes and a tonne bag count you can quote directly when placing an order with a landscape yard.

It starts by multiplying your length by your width to produce the surface area. That area is then multiplied by the depth you enter — converted from millimetres to metres — to give a volume in cubic metres. A 10% waste factor is applied by default to account for compaction during raking, material that falls into hollows in the subsoil, and the inevitable spillage at the edges of the prepared area.

Because screened topsoil is sold by weight for bulk deliveries, the calculator multiplies volume by an average density of 1.4 tonnes per cubic metre. That density represents a typical multipurpose-grade product at normal moisture content. Freshly screened material in dry conditions can be lighter (around 1.2 tonnes per cubic metre), while damp product straight off the screen after rain can reach 1.6–1.8 tonnes per cubic metre. The 1.4 figure sits in the middle and gives a reliable planning estimate that most grab lorry operators recognise as standard.

This same area-times-depth approach is used by our raised bed soil mix planner, which then splits the total volume into component ratios. The litre output restates the same volume in a unit that maps to retail sacks. Most garden centre products are sold in 25-litre or 40-litre bags. If the calculator shows 3,960 litres, dividing by 40 reveals you would need 99 small bags — a comparison that quickly shows why a tonne bag delivery makes sense for anything beyond a single raised bed. The tonne bag count rounds your cubic metre figure up to the nearest whole bag, because running short mid-project means a second delivery charge that wipes out any saving from ordering tight.

Making Sense of the Output

Each output figure maps to a different stage of the ordering and delivery process.

The volume in cubic metres is the figure landscape yards quote prices against. Phone your local supplier, give them the cubic metre number, and they will return a delivered price. Knowing this before you call stops you relying on a salesperson's estimate, which may not reflect your specific depth requirement.

The litre figure helps with smaller jobs. If you only need 670 litres for a border top-up, picking up 17 bags of 40-litre product may work out cheaper than paying a minimum delivery charge. Most yards have a minimum of 1–2 cubic metres before they will dispatch a grab lorry.

The weight in tonnes is the figure that drives delivery logistics. The same weight-awareness matters for indoor coverage estimates where bags rather than bulk deliveries are the norm. A standard grab lorry carries 15–20 tonnes of payload, so a 5.55-tonne order fits on a single trip. Larger orders may need two dispatches or a bigger vehicle. Weight also determines whether the lorry can safely use your access road — a fully loaded 20-tonne grab lorry with the vehicle's own mass can exceed 32 tonnes gross, which some residential roads and driveways cannot handle. Check with the haulier if you have any doubt about weight restrictions on your street.

The tonne bag count is the practical ordering unit. Four tonne bags for a new lawn tells you exactly what to request. Each bag occupies roughly a metre square on the ground, so you need about 1.2 metres of width per bag if the driver lines them up side by side on your driveway. Plan your drop zone before the lorry arrives — moving a full tonne bag after placement is not realistic without machinery.

Common Mistakes to Avoid

Before placing an order, check your vehicle access. A standard grab lorry is about 2.5 metres wide and needs at least 3 metres of clear width to pass, including wing mirrors. The grab arm has a reach of 5–6 metres, so the driver can place tonne bags over a fence or wall as long as the lorry can park within that range. If your garden is accessed through a narrow side passage, bags dropped on the driveway and barrowed through may be the only practical route.

Assess your existing ground condition before deciding how much to order. Dig a test hole 300 millimetres deep in two or three spots across the area. If the subsoil beneath is heavy clay, compacted rubble, or builder's fill, you will need the full 150-millimetre depth to create a viable growing layer. If the existing ground is reasonable but just thin or uneven, a shallower 50–75-millimetre skim raked level may be enough. This simple test can halve your order on a decent site.

Always specify screened product when ordering for lawns or turfing. Screening passes the material through a mesh — typically 10-millimetre or 20-millimetre grade — to remove stones, roots, and rubble. Unscreened product is cheaper but contains lumps that make levelling difficult and leave bumps under turf. For borders and general planting, unscreened or general-purpose grade is acceptable because debris sits below the surface and does not affect root establishment. Our guide to raised bed soil mixes explains how screened grades compare when blending custom growing media.

Ask your supplier whether their product carries a BS 3882 certificate. BS 3882 is the British Standard that classifies screened topsoil into three grades based on laboratory analysis of texture, organic content, stone count, and contaminant levels. Multipurpose grade has the strictest limits and is the right choice for lawns and raised beds. General-purpose grade permits more stones and suits planting borders. Economy grade is only for landscaping fill where nothing will grow directly in the material. A supplier who cannot produce a BS 3882 test report may be selling subsoil, excavation spoil, or ungraded fill rather than genuine screened product.

Real-World Adjustments

The depth you enter has the biggest effect on your total order, so getting it right avoids both overspending and a second delivery charge.

New lawns need 100–150 millimetres of screened product over prepared subsoil. The higher end of that range is worth the cost if your existing ground is heavy clay or compacted builder's rubble, because turf roots need at least 100 millimetres of quality growing medium to establish properly. If you are turfing over decent existing ground that just needs levelling, 50–75 millimetres raked smooth is sufficient. Once your soil is in place, the lawn seed rate calculator or turf roll estimator covers the next step.

Border top-ups sit at the shallow end. Established beds where the level has dropped over several years of planting typically need only 25–50 millimetres to bring them back flush with the lawn edge. A layer of bark or gravel mulch on top of the fresh soil helps preserve that new level by slowing erosion and suppressing weeds. Applying more than 50 millimetres over existing planting risks burying the crowns of perennials and suffocating shallow roots.

New-build gardens change the calculation dramatically. If the construction contractor stripped all growing material and left compacted clay or hardcore, you may need 200–300 millimetres of depth across the entire plot to create a viable growing layer. At that depth, a 6 by 4 metre area requires over 7 cubic metres — a substantial delivery that needs careful driveway planning.

Wet weather increases the weight per cubic metre. The calculator uses 1.4 tonnes as a mid-range density, but product delivered after prolonged rain can weigh up to 1.8 tonnes per cubic metre. If you are taking delivery in winter or after a wet spell, increase the weight estimate by 20–25% when checking vehicle access limits. Wet material is also harder to rake level, so aim to schedule delivery during a dry window if your timeline allows it.

Clay-heavy gardens sometimes need less imported material than you expect. If your existing clay has reasonable fertility but poor structure, mixing in sharp sand and organic amendment — a technique called soil improvement — can open up the drainage without replacing the entire layer. Spreading 50 millimetres of fresh screened product mixed with 25 millimetres of sharp sand and forking the combination into the top of the clay often produces better long-term results than layering 150 millimetres of new material on top of untreated clay, because the boundary between the two layers creates a perched water table that stays waterlogged.