Start with the compacted volume
Compacted volume = area × compacted depth. Use metres to get cubic metres. Take the base depth and stone type from the plans. This sum does not choose a base specification.
The stone comes off the truck loose. It packs down when rolled. How much it packs down depends on the stone and how it is laid. Ask the supplier for the right factor. A blanket 20–30% allowance can give the wrong order.
Work it out by hand
For an example 4 m × 3 m base at 100 mm compacted thickness:
- Convert the thickness: 100 mm = 0.1 m.
- Calculate the final volume: 4 × 3 × 0.1 = 1.2 m³.
- For this sum only, say the supplier gives a factor of 1.25.
- Loose quantity: 1.2 × 1.25 = 1.5 m³.
The 1.25 factor is made up for this sum. It is not a value to use on your job. Ask how much you can buy and how much spare to allow.
If the stone is sold by weight, ask how to turn your volume into tonnes. Do not use a density from a different product. Check whether the density is for loose stone or stone that has been packed down.
Check the base on site
Measure the base and its levels. Split odd shapes into boxes and count each part once. Check the depth of each layer and how it must be rolled. Enough stone is only one check. The base must also carry the load on it.
Calculation basis
Area times thickness gives volume. The loose-volume factor and density must come from the chosen material’s supplier or approved specification. No unverified bulking table is used as a universal rule here.