The layers
A gravel path or driveway is usually built in layers: the ground dug out and levelled, a compacted sub-base of crushed stone for strength, a weed membrane, and decorative gravel on top. Paths that only take foot traffic need a thinner sub-base than a driveway that carries cars. Check with your supplier or a groundworker for the depth your ground needs.
How deep should the gravel be?
Decorative gravel is commonly laid about 40mm to 50mm deep. Too deep and it is hard to walk or drive on; too shallow and the membrane shows through.
Working out the weight
Suppliers sell by weight, so you need to turn the volume into tonnes. An 8m by 3m driveway with 50mm of gravel needs 1.2 cubic metres. At a typical 1,600kg per cubic metre, that is 1.92 tonnes, or three bulk bags of 850kg. The gravel calculator does the sums and lets you change the weight and bag size.
Bulk bags vary
Bulk bags are not a standard size. Some hold around 850kg, others close to a tonne. Always check the weight your supplier quotes.