What this calculator works out
Ordering topsoil comes down to three questions: how big is the area, how deep does the layer need to be, and how heavy is that once it's real soil rather than a shape on paper. This tool takes an area's shape and size, a depth, and the state the soil is likely to arrive in, and turns that into a volume and a weight, then rounds the weight up into a practical order: UK bulk ("tonne") bags, small 25L bags for a top-up, or a loose tonnage figure to ask a supplier about.
It sits alongside the turf calculator, the mulch calculator and the gravel calculator as one of the site's material-ordering tools, but topsoil has its own quirk: unlike gravel or sand, its weight per cubic metre genuinely shifts with how wet and how settled it is, so this page asks for that as a separate input rather than assuming one fixed figure.
How the maths works
Area comes first, from whichever shape matches the space. A rectangle is length times width; a circle uses its diameter, halved to a radius and squared, times pi; and if you've already worked out the area some other way, you can type the total straight in and skip the shape step.
Depth turns that flat area into a volume, once both are measured in the same unit of length. A 50 square metre area, topped up 15cm (0.15m) deep, is a 7.5 cubic metre volume.
Weight then comes from the volume multiplied by the soil's density, which is where the soil state selector matters: freshly delivered, loose and moist topsoil weighs less per cubic metre than the same soil once it has settled or been walked on, and a blended mix with added compost is lighter again. Working the default example through: 10m by 5m is 50 square metres; at 15cm deep that's 7.5 cubic metres; at the as-delivered density of 1.3 tonnes per cubic metre, that's 9.75 tonnes; divided by a 0.75 cubic metre bulk bag and rounded up, that's 10 bulk bags.
As a US aside, the same maths works entirely in cubic yards if you'd rather skip weight altogether: a 20ft by 10ft bed, 6 inches deep, is 100 cubic feet, which is 100 ÷ 27 = 3.70 cubic yards, since a cubic yard is exactly 27 cubic feet.
Choosing a depth
Rolawn's own guidance for calculating topsoil requirements sets out a fairly clear ladder by use: a top dressing over an existing lawn needs only a thin 25mm or so; a new lawn's rootzone, the layer new turf or grass seed actually roots into, wants 100mm to 150mm; general planting and borders want more again, typically 150mm to 200mm; and root vegetables such as carrots and potatoes, which need real depth to grow down into, want 300mm to 400mm. This calculator's guidance chips use 100 to 150mm for a new lawn and a slightly wider 200 to 300mm band for beds and borders being dug over for planting, which matches the same order of magnitude as Rolawn's figures and the wider horticultural convention that a border being properly prepared benefits from more depth than the bare minimum for grass.
Thin, patchy topsoil under a lawn tends to show up later as scorched grass in a dry spell or a lawn that never quite thickens up, even when the turf or seed itself was good quality; skimping on depth to save on the order is one of the more common regrets after a new lawn goes in.
Soil state and density
Topsoil is not a fixed-density material the way stone or sand is, and two published UK supplier figures show the spread clearly. Rolawn's calculating-requirements guide states that one tonne of standard topsoil is about 0.67 cubic metres, which works out to a density of roughly 1.5 tonnes per cubic metre. Topsoil Shop's topsoil calculator states that one cubic metre of moderately damp, freshly dug topsoil weighs 1.3 to 1.7 tonnes depending on how tightly packed it is, and separately notes that a lighter blended topsoil runs closer to one cubic metre to the tonne, around 1.0 tonnes per cubic metre. This calculator uses those figures directly: 1.3 t/m³ for loose, as-delivered moist topsoil (the middle of both suppliers' lower range), 1.5 t/m³ for topsoil that has settled or been lightly firmed (matching Rolawn's implied figure and the top of Topsoil Shop's range), and 1.0 t/m³ for a blended or compost-lightened mix (Topsoil Shop's own figure for that case).
The state you pick only changes the weight shown, never the volume; the cubic metres and cubic yards you need for the job stay the same whichever soil state is selected, since that's set by the area and the depth alone.
Bulk bags, small bags and loose tonnes
A UK "bulk bag", also called a tonne bag or jumbo bag, is this calculator's default ordering unit, sized at 0.75 cubic metres (750 litres), the size Rolawn itself uses for its own Blended Loam Topsoil bulk bag product. At the as-delivered density this calculator defaults to, a full 0.75m³ bag holds close to a tonne of soil, which is where the "tonne bag" nickname comes from, even though the bag is really sold and sized by volume rather than by a guaranteed weight. Small 25L bags, the kind stacked at garden centres and DIY stores, suit a top-up or a small container job; carrying dozens of them for a job the size of a new lawn is rarely worth the trip.
For larger orders, most topsoil suppliers can also deliver loose, an unbagged tipped load quoted by the tonne, which usually works out cheaper per tonne than bulk bags once the quantity is large enough, provided there's room to tip a load and it gets spread reasonably promptly. Bulk bags cost more per tonne but need only ordinary driveway access and can be opened and used gradually rather than committing to a whole load at once. Neither is universally cheaper; it depends on quantity, access and how quickly the soil will get used, which is why this calculator shows both figures side by side rather than picking one.
Will topsoil settle after it's spread?
Yes, and it's worth planning for. Freshly tipped topsoil is loose and full of small air gaps; rain, watering and ordinary foot traffic gradually press those gaps out, so a layer measured at 150mm on delivery day commonly reads a little shallower a few weeks later once it has bedded in. This is the moist-versus-settled distinction the soil state selector reflects: the volume ordered doesn't change, but the depth on the ground can drop slightly as the same tonnage settles into a smaller, denser layer. Lightly firming new topsoil with the back of a rake, rather than leaving it fully loose, helps it settle evenly, and checking the depth again a few weeks after laying, particularly under a new lawn, catches any low spots worth topping up.
Screened versus unscreened topsoil
Screened topsoil has been passed through a mesh screen to remove stones, roots, clay lumps and other debris, leaving a finer, more consistent soil that's ready to rake level and plant or turf straight into; it costs more for that reason. Unscreened topsoil is soil dug and delivered largely as it came out of the ground, cheaper but coarser, and more likely to contain stones or the odd root that needs picking out by hand. Unscreened topsoil suits bulk fill and levelling, building up a low patch of ground or filling in under where a screened top layer will go. For the final 100 to 150mm under a new lawn, or a border being planted directly, screened topsoil is the one worth paying for.
Questions people ask
How much topsoil do I need?
Work out the area, multiply by the depth you need to get a volume, then multiply by the soil's density to get a weight. This calculator does all three steps at once and rounds the result up into bulk bags, small bags, or a loose tonnage figure.
Should I order bulk bags or small bags?
Small 25L bags suit a top-up or a container job, typically under about half a cubic metre before the trip count gets silly. Beyond that, a 0.75m³ bulk bag holds the equivalent of 30 small bags in one delivery, and for a job the size of a new lawn or a large border, asking a supplier about loose delivery by the tonne is usually worth comparing too.
How deep should topsoil be for a new lawn?
Rolawn's own guidance puts a new lawn's rootzone at 100mm to 150mm of reasonable topsoil, levelled and lightly firmed rather than compacted hard. That depth gives new turf or grass seed somewhere to root into; thinner cover tends to show up later as a lawn that scorches quickly in dry weather.
Does topsoil settle once it's laid?
Yes. Freshly tipped topsoil is loose and full of small air gaps that rain, watering and foot traffic gradually press out, so a layer can read a little shallower a few weeks after laying than it did on delivery day. The soil state selector on this page reflects that difference in weight; checking the depth again after a few weeks catches any low spots worth topping up.
What's the difference between screened and unscreened topsoil?
Screened topsoil has been sieved to remove stones, roots and clumps, giving a finer soil that's ready to plant or turf into directly. Unscreened topsoil is cheaper and coarser, better suited to bulk fill and levelling under a final layer than to the top few centimetres a lawn or border actually roots into.
How much does a bulk bag of topsoil weigh?
This calculator's default bulk bag is 0.75 cubic metres (750 litres), matching Rolawn's own bulk bag product. At an as-delivered density of around 1.3 tonnes per cubic metre, a full bag weighs close to a tonne, which is why it's commonly nicknamed a "tonne bag", even though it's really sold and sized by volume rather than a guaranteed weight.
This calculator uses topsoil density and bulk bag figures checked against Rolawn's calculating-requirements guide and Topsoil Shop's topsoil calculator, both published UK topsoil suppliers, current as of the sources cited. Real density and bag size vary a little by supplier, moisture content and how tightly a load has settled in transit, so treat the figures here as a solid ordering estimate and confirm the exact bag size and weight with your chosen supplier before placing a large order.