How much aggregate, crushed rock or crusher dust do I need?

Calculate one layer at a time

Measure the area in square metres and use the finished thickness in metres. If the layer will be compacted, use its thickness after compaction.

Area × finished thickness = finished layer volume.

For this example, the plan specifies a finished layer of 15 cm over 20 m²:

20 m² × 0.15 m = 3 m³ of finished layer.

The 15 cm depth is an example input, not a recommendation for every garden or driveway. Calculate each material separately if the structure contains several layers.

Choose the product for the layer

Crushed rock, gravel and fine crushed products are not interchangeable simply because they are all aggregates.

Check the product’s grading and intended use. A material specified for a load-bearing layer has a different role from a surface finish or bedding directly beneath paving.

Names also vary by market. In Great Britain, Type 1 identifies a specified graded sub-base material, not any product sold as crusher dust. In parts of Australia, names such as cracker dust or quarry fines are used for fine quarry products. The name alone does not establish grading or suitability for the structure.

Convert the finished requirement into an order

Loose bulk density tells you the mass of one cubic metre of loose material. It does not, by itself, tell you how much to buy for one cubic metre of compacted layer.

Ask the supplier for either:

• tonnes required per cubic metre of finished compacted layer; or
• the loose volume needed to produce your finished layer.

If the supplier first confirms a loose volume, you can convert that volume to tonnes using the bulk density for the same loose product. Check the stated moisture and measurement basis.

For the example, ask:

“The area is 20 m² and the layer needs to finish at 15 cm after compaction, giving 3 m³. How many tonnes or loose cubic metres of this product should I order? What compaction assumptions does that amount include?”

Request a quote for the confirmed amount. Check that it identifies the product, grading and delivery unit.

Keep compaction and waste separate

The conversion from delivered material to a finished layer accounts for compaction. Waste, an uneven base and final adjustments may create a separate need for extra material.

Ask what the quoted amount already includes. Do not add a second compaction allowance or apply a standard percentage to every product.