Depth is the input that decides everything
Area is easy to measure and hard to get wrong. Depth is invisible, quoted in centimetres, and multiplies straight into the total. It is also the number where advice varies wildly, because a depth that is generous for mulch is nowhere near enough for a raised bed.
volume m³ = area m² × depth m
Keep the depth in metres, not centimetres. A 50 m² lawn at 5 cm is 50 × 0.05 = 2.5 m³. Entering 5 instead of 0.05 gives 250 m³ — a hundred times too much, and it is the single most common error in garden estimating.
Depth by use
| Use | Depth | Material | Why |
|---|---|---|---|
| Top dressing an existing lawn | 5-10 mm | Sandy loam / lawn dressing | Any deeper smothers the grass |
| Levelling before new turf | 25-50 mm | Topsoil | Enough to correct dips without burying the base |
| New lawn from seed or turf | 100-150 mm | Topsoil | Grass roots occupy the top 100 mm |
| Mulch on beds | 50-75 mm | Bark, wood chip, compost | Under 50 mm and weeds come through |
| Improving an existing border | 50-100 mm | Compost, dug in | Feeds the layer where roots already are |
| New planting border | 300-400 mm | Topsoil + compost | Shrubs need real root volume |
| Raised vegetable bed | 300-450 mm | Topsoil + compost mix | Roots and drainage both need the depth |
| Vegetable plot, ground level | 200-300 mm | Topsoil + compost | Deep-rooting crops go further than people expect |
| Tree or large shrub pit | 600 mm+ | Native soil + compost | Backfill with what is there; imported soil creates a bathtub |
| Under artificial grass | 50-75 mm | Sharp sand or grit | Compacted, not topsoil |
| Path or patio sub-base | 100-150 mm | MOT type 1 | Compacted in layers, not one lift |
Mulch never goes against tree trunks or plant stems. A ring of bare soil a few centimetres wide around each stem prevents rot. A cone of mulch piled against a trunk kills the tree slowly.
From volume to what you can actually buy
Garden material comes in three formats and they are priced completely differently. Knowing your volume first tells you which one you are in.
| Format | Typical volume | Covers at 50 mm | Sensible up to |
|---|---|---|---|
| Small bag, 40-50 L | 0.04-0.05 m³ | ~1 m² | 2 m³ / 40 bags |
| Bulk bag (dumpy bag) | 0.7-1.0 m³ | ~15 m² | 3-4 bags |
| Loose tipped load | 3 m³+ | 60 m²+ | no practical limit |
Small bags cost three to five times more per cubic metre than a bulk bag. The crossover is around one cubic metre — above that, bagged soil is an expensive way to buy dirt. Below it, a bulk bag you cannot use up is worse.
A bulk bag of topsoil weighs 800-1 000 kg. Check the delivery vehicle can reach where you want it: it is moved by crane off the lorry and cannot be repositioned by hand afterwards.
Weight, because it decides the logistics
| Material | Approx. weight per m³ |
|---|---|
| Screened topsoil | 1 200-1 400 kg |
| Compost / soil improver | 500-700 kg |
| Bark mulch | 300-400 kg |
| Wood chip | 350-450 kg |
| Sharp sand | 1 500-1 600 kg |
| Gravel 20 mm | 1 500-1 700 kg |
| MOT type 1 sub-base | 1 900-2 100 kg |
A builder's barrow holds about 80 litres, so a cubic metre is twelve to thirteen barrow loads. At topsoil density that is over a tonne moved by hand. Plan the route from the delivery point before the lorry arrives, not after.
Settlement: the reason you order more
Loose material compacts. Freshly spread topsoil settles by 10-20% in the first few weeks, more if it is watered or walked on. Compost settles more than that as it continues to break down. If you order the exact calculated volume, the finished level will be below where you wanted it.
- Topsoil: add 15% for settlement.
- Compost and soil improver: add 20%, and expect to top up again the following season.
- Bark and wood chip: add 20%, and plan to re-mulch annually — it breaks down, which is the point.
- Sand and aggregate under paving: add 20%, because it is compacted deliberately.
- Raised beds: add 25%. They settle more than anything else, and topping up a planted bed is far more awkward than filling an empty one.
A worked example
A new lawn area 8 × 5 m, needing 100 mm of topsoil, plus two borders totalling 12 m² getting 60 mm of bark mulch.
lawn = 8 × 5 × 0.10 = 4.0 m³
with 15% settlement = 4.6 m³
mulch = 12 × 0.06 = 0.72 m³
with 20% settlement = 0.86 m³
The topsoil is 4.6 m³ — about five bulk bags, or a loose tipped load, and roughly six tonnes. Loose delivery is clearly cheaper here. The mulch is 0.86 m³, which is one bulk bag with a little spare, or about eighteen 50 L bags at three times the price. One bulk bag.
Two materials, two formats, one job. This is normal — do not force both into the same purchase just because they arrive on the same day.
Practical notes
- Measure irregular beds by splitting them into rectangles and triangles. Getting within 5% is fine; the settlement allowance is bigger than your measuring error.
- Ask what grade of topsoil you are buying. Screened, general purpose and subsoil are different products at similar prices, and subsoil grows nothing.
- Do not import topsoil into a tree pit. Backfilling with rich imported soil creates a container the roots refuse to leave, and the tree stays small.
- Spread and rake in stages. It is far easier to add another barrow load than to shovel excess back off a levelled area.
- Order delivery for a dry day if you can. Wet topsoil is heavier, sticks to everything and compacts into a solid mass that will not rake level.