A tonne is not a quantity of anything useful
Topsoil, compost and bark are sold interchangeably by weight and by volume, and their densities differ by more than four times. That single fact is behind most of the ordering that goes wrong.
- Screened topsoil, about 1.5 tonnes per m³, so a tonne is 0.67 m³
- Soil and compost blend, about 0.9, so a tonne is 1.1 m³
- Compost, about 0.5, so a tonne is 2.0 m³
- Bark mulch, about 0.35, so a tonne is 2.9 m³
Order two tonnes of topsoil and 1.3 cubic metres turns up. Order two tonnes of bark and five and a half cubic metres lands on your drive. Same number, four times the material, and nobody has made a mistake.
So this tool works in volume, which is what a bed needs, and converts to weight and bags afterwards. Starting from a tonnage somebody quoted you is starting from the wrong end.
A bulk bag is not a cubic metre
The bag everybody calls a tonne bag or a dumpy bag holds around 0.6 m³ filled to the seam. Merchants fill them by weight rather than to a mark, so two bags are neither two cubic metres nor reliably two tonnes.
This matters most when comparing prices, because two suppliers quoting "£X a bag" may be selling meaningfully different amounts. Ask what theirs holds. The bag size is an input here for exactly that reason.
Order more than you need, because it settles
Loose material drops once it is spread, watered and walked on. Ordering the finished volume is the commonest way to end up a bag short and with a bed that has sunk below its edging over the first winter.
Around 20% is the usual working allowance. It is a trade convention rather than a standard, which is why it is editable: a light compost settles more than a screened soil, and material tipped from height has already been compacted by the fall and will settle less.
Depth is the number people cut, and the one that decides the job
The area is fixed by the garden. The depth is the number that feels negotiable when the quote comes back. Get it wrong and nothing grows properly.
- Under new turf: 100mm. Less and it dries out in the first hot spell.
- Borders and beds: 300mm. Roots want more than people give them; 150mm is a covering rather than a planting depth.
- Vegetables: 450mm, because root crops need somewhere to go.
- Mulch: 75mm. Below about 50mm it does not suppress weeds, it only looks like it should.
For levelling, remember the depth to enter is the average across the area. Measure the deepest point and the shallowest and halve the difference, rather than working off the worst spot.
Wet material is heavier, so buying by weight after rain buys water
The densities here are the usual trade approximations rather than a standard, and moisture moves them a long way. Wet topsoil is considerably heavier than dry topsoil, which means a tonne of it contains less soil.
If you are ordering by the tonne rather than the cubic metre, it is worth asking whether the quoted density is wet or dry, and worth preferring volume where the supplier offers it.
The bit that catches people out on a new bed
If the bed is being dug out before it is filled, the spoil has to go somewhere, and excavated soil swells: it takes up noticeably more space loose than it did in the ground.
That means a skip or a grab lorry, and on a decent sized bed it is frequently the larger cost of the two. Worth pricing before the soil is ordered rather than after it has been dug and is sitting on a tarpaulin across the lawn.
Common questions
How much does a tonne of topsoil cover?
About 0.67 cubic metres, which at 150mm deep covers roughly 4.5 square metres. But the question is a trap, because a tonne of bark mulch is nearly 2.9 cubic metres. These materials are sold interchangeably by weight and their densities differ by more than four times, so ordering "two tonnes" of one and "two tonnes" of another gets you wildly different amounts.
Is a bulk bag a cubic metre?
No. The bag everybody calls a tonne bag or a dumpy bag holds around 0.6 cubic metres filled to the seam, and merchants fill them by weight rather than to a mark. Two bags are neither two cubic metres nor reliably two tonnes. Ask what a supplier's bag holds before comparing their price with anyone else's.
Why order more than I need?
Because loose material settles once it is spread, watered and walked on. Ordering the finished volume is how a bed ends up sitting below its edging by spring. Around 20% extra is the usual working allowance, though a light compost settles more than a screened soil and anything tipped from height has already been compacted by the fall.
How deep should topsoil be?
Deeper than most people order. Around 100mm as a rootzone under new turf, 300mm for borders and beds where things have to grow, and 450mm for vegetables because root crops need somewhere to go. Depth is the number people cut to save money, and it decides whether the job works, since the area is fixed and the depth is not.
How deep does bark mulch need to be?
At least 50mm and preferably 75mm. Below about 50mm it does not suppress weeds, it just looks like it should, and you will be doing it again next year. Bark is also the lightest material here by a distance, so the volume looks alarming next to the weight.
Should I buy loose or in bags?
Small bags make sense for topping up pots and for anything you have to carry through a house. Past about twenty of them, loose delivered or bulk bags are almost always cheaper per cubic metre and a great deal less work. The tool shows all three so you can compare properly.