Converting between weight and volume for bulk construction materials

2026-09-18T08:48:19Z

Wegravel: Created page with "== Introduction == Bulk construction materials such as aggregate, sand, topsoil and mulch are bought and sold in two incompatible ways. Quarri..."

== Introduction ==

Bulk construction materials such as [[Aggregate|aggregate]], [[Sand|sand]], [[Topsoil|topsoil]] and mulch are bought and sold in two incompatible ways. Quarries, recycling facilities and hauliers price and weigh material by mass, because a weighbridge is the only practical way to measure a lorry load. Designers, groundworkers and landscapers, by contrast, specify material by volume, because a drawing gives an area and a depth.

Converting between the two is therefore a routine calculation on almost every project that moves loose material. It is also a common source of over- and under-ordering, because the conversion factor is not a constant: it depends on the bulk density of the particular material, which varies with its composition, grading, moisture content and degree of compaction.

This article sets out the conversion method, the distinction between the different densities involved, the variables that shift the answer, and indicative bulk densities for materials commonly supplied loose.

== Bulk density, particle density and specific gravity ==

Three different densities are easily confused.

Particle density (or grain density) is the mass of the solid mineral divided by the volume of the solid alone, excluding any pore space between particles. For most natural aggregates derived from limestone, granite or basalt this falls in a narrow band of roughly 2,500 to 2,800 kg/m³.

Bulk density (or loose bulk density) is the mass of material divided by the total volume it occupies in a heap or a lorry body, including the voids between particles. Because a loose granular material is typically 30% to 45% voids, bulk density is considerably lower than particle density — commonly 1,400 to 1,900 kg/m³ for a natural aggregate.

Specific gravity is a dimensionless ratio of a material's density to that of water, and in aggregate testing is usually reported on a particle rather than a bulk basis.

It is bulk density that governs delivered quantities, because the customer is buying the heap, voids included. Quoting a particle density in a weight-to-volume conversion will substantially under-estimate the volume delivered.

== Loose and compacted volumes ==

A second distinction matters just as much. A material has one volume as it is tipped from the lorry and a smaller volume once it has been spread and compacted in place.

For granular sub-bases and crusher-run type materials, the reduction from loose to compacted volume is often in the region of 20% to 30%, though the figure is specific to the material, the layer thickness and the compaction plant used. A specification that calls for a finished 150 mm compacted layer therefore requires meaningfully more loose material than the finished geometry alone suggests.

Where compaction is significant, quantities should be derived from the compacted design volume and then increased by a compaction allowance, rather than by adjusting the density figure. Keeping the two corrections separate makes the assumptions visible and easier to check.

== The conversion method ==

The arithmetic is straightforward once the appropriate bulk density has been established.

In metric units, where bulk density is expressed in tonnes per cubic metre:

* volume (m³) = mass (tonnes) ÷ bulk density (t/m³)
* mass (tonnes) = volume (m³) × bulk density (t/m³)

In US customary units, where bulk density is expressed in short tons per cubic yard:

* volume (yd³) = mass (short tons) ÷ bulk density (short tons/yd³)
* mass (short tons) = volume (yd³) × bulk density (short tons/yd³)

The two unit systems are related as follows. One short ton is 2,000 lb, or 907.18 kg; one cubic yard is 0.7646 m³. A bulk density expressed in short tons per cubic yard is therefore multiplied by 1,186.55 to give kilograms per cubic metre. Conversely, one tonne per cubic metre is equivalent to 0.8428 short tons per cubic yard. Note also that the metric tonne (1,000 kg), the US short ton (907 kg) and the imperial long ton (1,016 kg) are three different units, and that "ton" is ambiguous without qualification.

== Worked examples ==

Calculating volume from a delivered weight. A 20 tonne delivery of 19 mm crushed stone with a bulk density of 1.60 t/m³ occupies 20 ÷ 1.60 = 12.5 m³ loose. The equivalent calculation in US units: a 20 short ton load at 1.35 short tons/yd³ occupies 20 ÷ 1.35 = 14.8 yd³.

Calculating weight from a design volume. A driveway 12 m long and 3 m wide is to receive a 100 mm compacted layer of granular sub-base. The compacted volume is 12 × 3 × 0.1 = 3.6 m³. Allowing 25% for compaction gives a loose volume of 4.5 m³. At a bulk density of 1.84 t/m³ the order is 4.5 × 1.84 = 8.3 tonnes.

A case where the distinction bites. Ten tonnes of shredded bark mulch at 0.36 t/m³ occupies about 28 m³, whereas ten tonnes of crushed concrete road base at 1.60 t/m³ occupies about 6 m³ — a difference of more than four to one in delivered volume for the same weight, and a correspondingly different number of lorry movements. Low density organic materials are for this reason frequently sold by volume rather than by weight.

== Indicative bulk densities ==

The values below are loose bulk densities for material as delivered, drawn from a published supplier conversion table for the US market and restated in both unit systems. They are indicative only. Actual densities vary with source rock, grading, particle shape and moisture content, and a supplier's own figures or a test to BS EN 1097-3 should be used where quantities are critical.

=== Crushed stone and gravel ===

Material kg/m³ t/m³ m³ per tonne short tons/yd³ yd³ per short ton

Crushed stone, 38–63 mm (US #4)

1,483

1.48

0.67

1.25

0.80

Crushed stone, 19–25 mm (US #57)

1,602

1.60

0.62

1.35

0.74

Crushed stone, 19 mm (US #67)

1,602

1.60

0.62

1.35

0.74

Railway ballast rock

1,543

1.54

0.65

1.30

0.77

Gabion stone

1,424

1.42

0.70

1.20

0.83

Pea gravel, 3–10 mm (US #89)

1,602

1.60

0.62

1.35

0.74

Rip rap / rock armour

1,483

1.48

0.67

1.25

0.80

Road base / crusher run, 50 mm to fines

1,839

1.84

0.54

1.55

0.65

Stone dust / screenings

1,602

1.60

0.62

1.35

0.74

=== Recycled and secondary materials ===

Material kg/m³ t/m³ m³ per tonne short tons/yd³ yd³ per short ton

Crushed concrete, 19–25 mm

1,424

1.42

0.70

1.20

0.83

Crushed concrete paver base

1,543

1.54

0.65

1.30

0.77

Crushed concrete road base

1,602

1.60

0.62

1.35

0.74

Recycled asphalt planings (RAP)

1,661

1.66

0.60

1.40

0.71

=== Sands ===

Material kg/m³ t/m³ m³ per tonne short tons/yd³ yd³ per short ton

Concrete sand (sharp sand)

1,602

1.60

0.62

1.35

0.74

Mason sand (building sand)

1,543

1.54

0.65

1.30

0.77

Fill sand

1,543

1.54

0.65

1.30

0.77

Beach sand

1,543

1.54

0.65

1.30

0.77

Play sand

1,543

1.54

0.65

1.30

0.77

=== Soils ===

Material kg/m³ t/m³ m³ per tonne short tons/yd³ yd³ per short ton

Topsoil

1,305

1.31

0.77

1.10

0.91

Fill dirt (subsoil fill)

1,483

1.48

0.67

1.25

0.80

Clay, red

1,543

1.54

0.65

1.30

0.77

Clay, sports surface (baseball clay)

1,543

1.54

0.65

1.30

0.77

Garden soil mix

1,009

1.01

0.99

0.85

1.18

Compost

712

0.71

1.40

0.60

1.67

=== Mulches and wood products ===

Material kg/m³ t/m³ m³ per tonne short tons/yd³ yd³ per short ton

Shredded bark mulch

356

0.36

2.81

0.30

3.33

Engineered wood fibre (playground surfacing)

356

0.36

2.81

0.30

3.33

Pine bark nuggets

261

0.26

3.83

0.22

4.55

== Factors that change bulk density ==

Moisture content. Water adds mass without proportionally adding volume, so a wet material weighs more per unit volume than the same material dry. For sands and fine-graded materials the difference between a dry stockpile and a saturated one after prolonged rain can be several percent, and material bought by weight in wet conditions delivers correspondingly less volume. Organic materials such as compost and bark are the most sensitive, as they absorb and retain water readily.

Compaction and handling. Material that has been stockpiled under its own weight, vibrated during transport, or loaded in layers will have a higher bulk density than freshly screened material tipped loosely. Density measured at the point of loading and density in the customer's heap are not necessarily the same.

Grading. A well-graded material containing a range of particle sizes packs more densely than a single-sized one, because the smaller particles occupy the voids between the larger. This is why crusher-run and road base materials, which run from a nominal top size down to fines, sit at the top of the density range, while single-sized decorative gravels sit lower.

Particle shape and angularity. Angular crushed rock interlocks and bridges, leaving a higher void ratio in the loose state than rounded river-worn gravel of the same mineralogy, though it achieves a higher density once compacted.

Source rock. Aggregate derived from a dense basalt will be heavier per unit volume than the same grading produced from a porous limestone. Recycled aggregates typically sit below primary aggregates because adhered mortar is less dense and more porous than natural rock.

== Practical implications for ordering ==

Several habits reduce the risk of a wrong order.

Establish which unit the supplier quotes in and which unit the specification uses, and convert once, explicitly, rather than assuming a shared convention. Confirm whether a stated density is loose or compacted. Ask the supplier for the bulk density of the specific product rather than relying on a generic figure, particularly for recycled and blended materials where the variation is widest. Derive quantities from the compacted design volume and apply a separate, stated compaction allowance. Finally, for high-value or difficult-access sites, consider the volume as well as the weight, since it is the volume that determines the number of lorry loads and therefore much of the delivery cost.

== Related articles on Designing Buildings ==

* [[Aggregate|Aggregate]]
* [[Recycled_aggregates|Recycled aggregates]]
* [[Secondary_aggregate|Secondary aggregate]]
* [[Sustainable_aggregates|Sustainable aggregates]]
* [[Subsoil|Subsoil]]
* [[Types_of_soil|Types of soil]]

== External references ==

* BS EN 1097-3: Tests for mechanical and physical properties of aggregates. Determination of loose bulk density and voids.
* [https://www.wegravel.com WeGravel], Tons to cubic yards conversion table for bulk construction materials — [https://www.wegravel.com/tons-to-cubic-yards https://www.wegravel.com/tons-to-cubic-yards] (source of the indicative density figures tabulated above).

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Converting between weight and volume for bulk construction materials