A Conversion Factor Has to Be Cubed

Bulk material is ordered in whole units

The answer of 1.48 is not what gets ordered. Aggregate, topsoil and ready-mixed concrete are sold in whole or half units, so this job is a delivery of one and a half or of two, and the difference between them is a quarter of the material. Round up rather than down, because a job that stops for want of a wheelbarrow of concrete costs a return visit and a cold joint. Add an allowance on top for what the ground absorbs, what compacts and what stays in the barrow — five to ten per cent is the usual habit, and on a slab poured onto uneven hardcore it is not enough.

Depth is where the quantity actually lives

The two horizontal measurements are usually known within a few centimetres, and the depth is the one that gets guessed. Taking the same 12 by 10 patch from four inches to six raises the requirement from 1.48 to 2.22 cubic yards — half as much again for a change nobody would notice on site. That sensitivity runs both ways: over-specifying the depth of a base by an inch across a large area buys a surprising amount of material that does nothing, and under-specifying it produces a slab that cracks. Measure the depth in several places rather than once, because the ground is rarely as flat as the plan assumes.

Frequently asked questions

How heavy is a cubic yard?

That depends entirely on what is in it, and the range is wide enough to matter for a vehicle. Dry sand, wet sand, topsoil, gravel and concrete all differ substantially, and the same material differs between dry and soaked — a load that was within a trailer's rating in August can be over it after rain. Volume is what is ordered and weight is what has to be carried, so where the transport is the constraint, ask the supplier for the density of the specific material rather than reasoning from a general figure.

Does the same rule apply to areas?

Yes, with the factor squared instead of cubed, and the error is just as common and half as visible. A square metre is ten thousand square centimetres, not a hundred; a square yard is nine square feet, not three. Because the mistake is smaller than the volume version, it survives longer — an area out by a factor of nine still looks like a number of the right kind, where a volume out by twenty-seven usually announces itself. The habit that catches both is to write the units into the calculation and cancel them, since square centimetres divided by centimetres cannot give metres.