A site engineer measures a layer in cubic metres. A supplier in Maharashtra quotes per brass. A yard further north sells by the cubic foot. The weighbridge prints tonnes, and the tipper driver counts trips. They are all describing the same heap of stone, and the conversions between them start more billing arguments than the stone ever does.
Converting aggregate from cubic metres, CFT or brass to tonnes takes two numbers: a unit conversion, which is fixed, and a bulk density, which depends on the product, the rock and how wet it is. This page gives both, and says how far the second one moves.
The unit conversions: cubic metre, CFT and brass
These are exact and have nothing to do with the material.
| Unit | Cubic feet | Cubic metres | Brass |
|---|---|---|---|
| 1 cubic metre (m³, cum) | 35.315 | 1 | 0.353 |
| 1 cubic foot (cft) | 1 | 0.0283 | 0.01 |
| 1 brass | 100 | 2.832 | 1 |
Brass is a trade unit of volume, used mostly in Maharashtra and western India, where state notifications also quote royalty per brass. It is not a weight.
Bulk density: 20mm aggregate density in kg/m³, and the rest
Bulk density is the weight of a known volume of loose material, voids included. It is far below the density of the rock, because close to half of a tipper load of single-sized stone is air.
The figures below are typical for crushed rock with a specific gravity of 2.6 to 2.9, loose as loaded and surface dry. They are a planning range, not a specification. For comparison, IS 875 Part 1, the Indian code for unit weights of materials, gives 1,600 to 1,870 kg/m³ for broken stone ballast shaken down and 1,540 to 1,600 kg/m³ for clean dry sand; loose-tipped single sizes sit a little under the first of those.
| Product | Loose bulk density | Tonnes per m³ | Tonnes per brass (100 cft) |
|---|---|---|---|
| 40mm aggregate | 1,400–1,550 kg/m³ | 1.40–1.55 | 4.0–4.4 |
| 20mm aggregate | 1,450–1,600 kg/m³ | 1.45–1.60 | 4.1–4.5 |
| 10mm aggregate | 1,450–1,600 kg/m³ | 1.45–1.60 | 4.1–4.5 |
| Crusher dust, dry | 1,600–1,800 kg/m³ | 1.60–1.80 | 4.5–5.1 |
| GSB and WMM, loose | 1,700–1,900 kg/m³ | 1.70–1.90 | 4.8–5.4 |
Typical ranges, not test results. Basalt and other dense rock sit at the top of each range, sandstone and lighter granite at the bottom. Measure your own source before a figure goes into a contract.
Three things move a product inside its range. Rock: basalt at a specific gravity near 2.9 weighs about a tenth more than stone near 2.6, size for size. Grading: a single size has nothing to fill its voids, so 20mm weighs less per cubic metre than GSB made from the same rock. Shape: flaky stone packs badly and weighs light in the body, one of the quieter costs of a high flakiness index.
Worked conversions: 1 brass in tonnes, CFT to tonnes
1 brass = 100 cft ÷ 35.315 = 2.832 m³
Bulk density = 1.45 to 1.60 t/m³
Weight = 2.832 m³ × 1.45 to 1.60 = 4.1 to 4.5 t
One brass of 20mm aggregate is about 4.1 to 4.5 tonnes
Volume = 600 cft ÷ 35.315 = 16.99 m³
Weight = 16.99 m³ × 1.70 t/m³ = 28.9 t
About 29 tonnes at a mid-range density
Volume = 25 t ÷ 1.80 t/m³ = 13.9 m³
In cft = 13.9 m³ × 35.315 = 491 cft
In brass = 491 cft ÷ 100 = 4.9 brass
A 25 tonne load of GSB is about 14 m³, or just under 5 brass
What a particular tipper can legally and physically carry is a separate sum, set out in tipper capacity in tonnes and cubic metres.
Loose in the tipper, compacted in the road
Rolling drives the air out. Crushed-stone GSB compacted to specification typically comes out at 2.2 to 2.3 tonnes per cubic metre and WMM at 2.2 to 2.4. The stone has not got heavier. The same tonnes now fill less space.
The ratio between the two states is what an estimator needs. MoRTH measures both layers for payment as finished work in cubic metres, so the drawing gives compacted volume. The MoRTH Standard Data Book's analysis of WMM takes 2.2 tonnes per compacted cubic metre and allows 1.32 m³ of loose aggregate for each one.
Tonnage = 1,000 m³ × 2.20 t/m³ = 2,200 t
Loose volume = 2,200 t ÷ 1.70 t/m³ = 1,294 m³
Ratio = 1,294 m³ ÷ 1,000 m³ = 1.29
Roughly 1.3 m³ in the tipper for every 1 m³ in the road
Order in tonnes from the compacted volume and the lab density, and the loose volume looks after itself. Order in loose cubic metres or brass against a compacted drawing quantity and you will be about a quarter short. A whole section is worked through in how much aggregate a kilometre of highway needs.
Moisture and bulking
Water does two different things. In coarse aggregate it only adds weight: stone that has been washed or rained on carries surface water, usually a per cent or two, and the weighbridge counts it.
In dust and sand it also adds volume. Damp fines are held apart by films of water and take up more room than the same material bone dry or flooded. This is bulking, and IS 2386 Part 3 gives the method for measuring it. Sand commonly bulks by 20 to 40 per cent at around 4 to 6 per cent moisture, the finer the sand the more, and crusher dust does the same. At 25 per cent bulking, a brass of damp dust holds a fifth less dust than a brass of dry dust. Anyone buying fines by volume in the monsoon is paying for air.
Why highway supply is billed by the weighbridge tonne
Volume depends on who measures it, and when. A tipper body is heaped or struck level. The load settles on a rough haul road. Fines bulk or they do not. Weight has one answer: a loaded pass and an empty pass over a weighbridge give a net tonnage that both sides can read off a printed slip.
A weighbridge is also regulated in a way a tipper body is not. One used for trade has to be verified and stamped by the state Legal Metrology department under the Legal Metrology (General) Rules, 2011, ordinarily every twelve months. Royalty is a separate matter. Maharashtra, for one, notifies it per brass, and other states per tonne or per cubic metre, so any conversion between volume and weight on a royalty or transit pass is whatever the current state notification says. Check it.
Every load from our plants goes over a calibrated weighbridge and leaves with a slip. The dispatch weight is the billed weight.
Measure your own density, then order in tonnes
For a contract quantity, measure. Fill a container of known volume the way the tipper is loaded, without shaking it down, weigh it and divide; IS 2386 Part 3 describes the test. Better still, reconcile a week of weighbridge slips against the body volumes of the tippers that carried them. Then convert the drawing to tonnes once and order in tonnes. Send us the layer, the compacted quantity and the delivery point and the rate comes back per tonne delivered, as set out on the aggregate supply page.