Converting Tonnes and Cubic Metres: Bulk Density and Sand Bulking

Materials are bought and calculated sometimes in tonnes and sometimes in cubic metres. If the relationship is unclear, estimating an order and checking a delivery both become hard. There is no fixed conversion, because the volume of a tonne depends on the material, its compaction and its moisture.

What bulk density is

Bulk density is the mass per unit volume of a material including the voids between particles, measured by ASTM C29. It differs from relative density, which is defined for the particle itself. Loose and compacted material also have different bulk densities.

As a rough picture, the loose bulk density of many sands and gravels is about 1.4 to 1.7 tonnes per cubic metre. For project calculations, take the real figure for the material from testing.

Bulking of damp sand

A little moisture forms a film of water around sand particles and pushes them apart. At about 5 percent moisture, sand volume can increase by 20 to 30 percent. This is called bulking. With more water the effect falls away. A volume batch of damp sand therefore holds less solid material, which makes weighing more accurate.

Worked example

This example is hypothetical. For a layer 100 m by 6 m with a compacted thickness of 0.2 m, the compacted volume is 120 cubic metres. At a compacted density of 2.2 tonnes per cubic metre, about 264 tonnes are needed. An allowance for waste is added according to site conditions.

Checking deliveries

• Make the contract unit clear: tonnes or cubic metres, loose or compacted.
• Match the weighbridge ticket with the order.
• With volume, allow for material settling in the truck.

Read ordering notes in How to order aggregate. For the effect of moisture on mix design, see Moisture and absorption.