The formula
loose volume = length × width × depth × (1 + bulking)
Where it comes from
There are two volumes here and confusing them is the expensive mistake. The first is the hole that is left, called the bank volume, and it is simply length times width times depth. The second is what that same ground takes up once it is out: broken up it gains voids and grows by anywhere from 12 % to 50 % depending on what it is. That growth is called bulking, and it is the volume that has to be transported. A twenty-two cubic metre hole in ordinary soil sends twenty-eight away.
How to work it out by hand
- Multiply length by width by depth: that is the hole
- Apply the bulking factor for the type of ground
- That loose volume is what has to be carted away
- Divide by the lorry capacity for the number of loads
What is worth knowing
Bulking works the other way round when filling: soil put back into a hole compacts and takes up less than it did loose, so filling needs more material than it looks. For reference, sand bulks little — around 12 % — because it is already loose, stiff clay reaches 35 % and broken rock can pass 50 %. And a warning that is not theoretical: before digging, locate the buried services. Gas, water, electricity and telecoms. The sum on this page is arithmetic; breaking a gas main is not.