The base is most of the job
A pedestrian patio wants roughly 100 mm (4 in) of compacted sub-base; a driveway that takes a car wants 200–300 mm (8–12 in), and more again on clay. On top of that sits 25–40 mm of sharp bedding sand, screeded but never compacted before the pavers go down. Aggregate compacts as you whack it, so you must order more loose material than the finished depth suggests — a compaction allowance of 20–30% is normal for MOT Type 1 or crusher run. Get the base wrong and no amount of care in the laying course will save the surface: it will rut, it will pond, and it will do so within a winter.
Aggregate is bought by weight, not volume
You calculate a volume — area × depth — and the supplier sells you tonnes, so you need the bulk density. Gravel and crushed stone run about 1.5–1.6 tonnes per cubic metre; MOT Type 1 sits nearer 2.0–2.1 t/m³; sand is around 1.6 t/m³ dry, and considerably heavier wet. In imperial, gravel is about 1.4 tons per cubic yard. Miss the density conversion and you can under-order by a third. Order in whole tonnes or whole bulk bags, because that is how it is delivered, and remember that the last 200 kg of a bulk bag is always the part you needed.
Pavers, joints and the cutting waste
Paver count is area divided by the coverage of one unit including its joint — a nominal 200 × 100 mm block with a 3 mm joint really covers 203 × 103 mm, and over a large patio that small difference is worth several units. Add 5% for a straight running bond, and 10–15% for herringbone, basketweave or any pattern with a diagonal edge, because every perimeter cut becomes an off-cut you cannot use. Then the joints themselves: polymeric sand runs roughly one 20 kg bag per 8–10 m² depending on joint width and paver depth, and it is the single component people forget to price until the pallet arrives.
Asphalt and retaining walls
Hot-mix asphalt is ordered by weight: area × compacted depth × a density of about 2.4 t/m³ (roughly 145 lb/ft³). A typical residential driveway is 50–75 mm of surface course over a compacted sub-base, and a resurfacing overlay is 40–50 mm — but asphalt must be laid and rolled while hot, so the tonnage has to arrive in one deliverable batch, not in a trickle. Segmental retaining walls are counted by block face area divided by the face area of one block, plus a buried base course, plus cap units by the linear metre. Anything above about 1.2 m usually needs geogrid reinforcement layers and, in most jurisdictions, an engineer — and every retaining wall, at any height, needs drainage stone and a pipe behind it, or hydrostatic pressure will eventually push it over.
