Gravel, Soil & Mulch Calculator
Volume, weight and bag count for any loose material spread to a depth.
How it works
Volume is area times depth. Everything difficult about this calculation is that loose materials are sold by weight and spread by volume, and the number connecting them — bulk density — varies more than people expect.
Gravel is around 1600 kg per cubic metre, but that is loose and dry. The same gravel wet is 10–15% heavier, and compacted MOT type 1 goes to about 2000. Bark mulch is 350. Ordering by weight without knowing the density of what you are actually buying is how a driveway ends up half covered.
Depth is what people get wrong more often than area. A decorative gravel bed wants 50 mm — enough to cover the membrane, not so much that walking on it feels like wading. A driveway needs 50 mm of gravel over 100–150 mm of compacted sub-base, and skipping the sub-base is why gravel drives develop ruts and potholes within a season. Mulch wants 75–100 mm to actually suppress weeds; thinner than 50 mm and light reaches the soil and it does nothing.
The conversion trap worth naming: a cubic yard is 0.765 cubic metres, and a US short ton is 907 kg while a metric tonne is 1000. Yards and tons in the same sentence from an American supplier mean something quite different from metres and tonnes.
Order 5–10% over. Materials settle, the ground is never as flat as it looked, and a second delivery costs a full delivery charge for a fraction of a load.
Common questions
How deep should gravel be?
50 mm for a decorative bed over a membrane. For a driveway, 50 mm of gravel over 100–150 mm of compacted sub-base — the sub-base is the part that carries the car, and gravel laid straight onto soil will rut.
Bulk bag or loose?
A bulk bag holds roughly 850 kg, about half a cubic metre of gravel. Past three or four bags, loose delivery by the tonne is usually cheaper — provided you have somewhere it can be tipped.
How thick should mulch be?
75–100 mm to suppress weeds. Under 50 mm it does nothing. Keep it clear of stems and trunks; piled against bark it holds moisture there and rots it.
Why is my delivery lighter than ordered?
It probably is not. Loose material settles and compacts in transit, so it occupies less volume on your drive than it did on the weighbridge. Weight is what you paid for and weight is what turned up.