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How the topsoil calculator works
Topsoil is sold by the tonne, so the calculation turns an area and a depth into a weight. Multiply length by width to get the area, multiply that by the depth in metres to get cubic metres, then multiply by the bulk density of screened topsoil — about 1.5 tonnes per cubic metre. A raised bed 4 m × 1 m filled to 300 mm is 4 m² × 0.3 m = 1.2 m³, which is 1.8 tonnes before any allowance and 1.98 tonnes with the default 10% settlement allowance. That is three 850 kg bulk bags.
Topsoil is lighter per cubic metre than MOT Type 1 or sharp sand because it is full of organic matter and air, so a soil order always looks bulkier than the tonnage suggests.
The settlement allowance, and why you need it
Freshly delivered topsoil is fluffed up with air. Tip it into a bed, water it, and walk on it once, and it slumps by roughly a tenth — sometimes more on a deep fill. This is the most common reason a new raised bed finishes 30–50 mm below the rim.
So the calculator asks for the depth you want to end up with, then adds 10% on top. On a small 1.2 m³ bed that is only another 0.12 m³, but it is the difference between a level bed and a second trip. If you are filling a very deep bed in one go, or the soil arrives wet and heavy, nudge the allowance to 15%. If you are just topping up settled ground, 5% is plenty.
How deep should topsoil be?
For laying turf or sowing a lawn, 100 mm of prepared topsoil under the turf is the usual minimum, and 150 mm is better on poor ground. For a raised bed growing vegetables, roots want 200–300 mm of soil — shallower beds are fine for salads and herbs, deeper for carrots and parsnips. A planted border wants 300 mm or so. For levelling dips in an existing lawn, you are spreading a thin 5–10 mm dressing rather than filling a bed, so the tonnage is tiny.
One practical warning: never bury the crown of an existing shrub or the base of a tree trunk under a thick layer of new topsoil. Where you raise levels around established planting, keep the finished level just below the original soil mark on the stem.
Bulk bags, bags or loose
An 850 kg bulk bag holds about 0.57 m³ — a shade over half a cubic metre. A 25 kg bag is 25 kg, so it takes 34 of them to make up a bulk bag, and they cost far more per tonne. Loose loads are cheapest above about two tonnes, if a wagon can reach the spot and you have somewhere to tip it. For a back garden with no vehicle access, bulk bags on a pallet are usually the sensible compromise.
Topsoil FAQs
How many tonnes of topsoil do I need for a 4 m × 1 m raised bed at 300 mm?
About 1.98 tonnes. That is 1.2 m³ of soil plus the 10% settlement allowance, multiplied by 1.5 tonnes per cubic metre. In practice that is three 850 kg bulk bags, and it fills the bed to the depth you asked for once it has settled.
How much does a bulk bag of topsoil hold?
A standard 850 kg bulk bag holds roughly 0.57 m³ of screened topsoil — around 570 litres. That is enough for a raised bed about 1.9 m × 1 m at 300 mm, or a 5 m² area to 100 mm for turf.
Should I allow for settlement?
Yes — add about 10%. Loose topsoil is aerated when it is delivered and settles noticeably after the first good watering. Ordering to the exact finished volume is the classic mistake that leaves a new bed short. The calculator includes the allowance by default, and you can raise it to 15% for deep fills.
How much topsoil do I need to lay turf?
Plan on 100 mm of topsoil under the turf, or 150 mm on poor or compacted ground. For a 20 m² lawn that is 2 m³ before the settlement allowance, or about 3.3 tonnes — six bulk bags. Lay turf within 24 hours of delivery and water it daily for the first fortnight.
Is topsoil the same as compost?
No. Topsoil is mineral soil — the growing medium that gives roots depth and anchorage. Compost is decomposed organic matter used as a soil improver. For raised beds and borders, mixing roughly one part compost to three parts topsoil gives better structure, drainage and nutrients than either on its own.
How much does a tonne of topsoil cover?
One tonne is about 0.67 m³, so it covers roughly 6.7 m² at 100 mm, or 2.2 m² at 300 mm. Once the settlement allowance is added, plan on about 6 m² at 100 mm per tonne to be on the safe side.