Subbase

Aggregate is sold by weight, not volume, and a sub-base compacts so you order more than the hole holds. Subbase turns the area and depth into tonnes, bulk bags and a loose volume to order, from the material's bulk density. For MOT Type 1 and gravel through to sand and slate.

The area

Give a length and width, or type the area if you have it. For an odd shape, split it into rectangles and add them up. A driveway sub-base is usually 100 to 150 mm; a decorative gravel topping 40 to 50 mm.

The material

The density is what turns volume into tonnes. These are typical figures; confirm the density on the supplier's own product page, especially for Custom. Moisture changes it, so a wet load weighs more.

Allowances

A sub-base is whacked down and loses height, so the compaction allowance orders extra loose material to reach your depth. Waste covers spillage and levelling. A dumpy bag is usually about 850 kg, but check the supplier's.

Why weight, not volume

Aggregate is delivered and priced by the tonne, but a job is measured in volume: the area you're covering times the depth you want. The bridge between the two is the bulk density, how heavy a cubic metre of that material is. A cubic metre of MOT Type 1 weighs about two tonnes; the same cubic metre of light slate chippings is nearer 1.3, so the same hole needs very different tonnage. A sub-base adds a second step: it goes down loose and is then compacted with a plate, losing roughly a tenth of its height, so you order extra loose material to end up at the depth you want once it's whacked down. Decorative gravel you just rake level, so it gets no compaction allowance. Subbase works out the finished volume, the loose volume to order, the tonnes, the bulk bags and a waste margin, and shows the coverage per tonne so you can check it against a supplier's own figure. These are planning numbers: densities shift with moisture and the exact stone, so use the supplier's density when you place the order.