BULK DENSITY
Bulk density is the mass per unit volume of a bulk material including the voids between particles — ranging from 240 kg/m³ (wood chips) to 2880 kg/m³ (barite) — used directly in capacity and power calculations.
Bulk density (ρ_b) is the mass of a loose bulk material divided by the volume it occupies, including the air voids between particles. It is fundamentally different from the solid density of the underlying mineral or substance — coal has a solid density of about 1400 kg/m³, but a bulk density of only 720–900 kg/m³ as broken on a belt because of inter-particle voids. Bulk density is what conveyor designers care about, because it is what determines the mass per metre of belt at a given material cross-section.
Typical bulk densities span a wide range. Light materials: wood chips 240 kg/m³, fresh snow 160 kg/m³, dry sawdust 200 kg/m³. Medium-density bulks: coal 720–900, grain 700–820, cement 1400–1500. Heavy bulks: crushed limestone 1500–1700, iron ore 2300–2500, barite 2400–2880. Moisture significantly affects bulk density (wet sand is 1900 kg/m³ vs 1600 kg/m³ dry), as does compaction state — freshly poured material is less dense than the same material after settling under its own weight in storage.
Bulk density enters two key calculations directly. Mass flow rate = volumetric capacity × bulk density, used in the [belt capacity calculator](/tools/conveyor-belt-capacity-calculator) and indirectly in the [motor power calculator](/tools/conveyor-motor-power-calculator). Per-metre belt mass = capacity ÷ belt speed, used in belt sag and idler load sizing. The site's [bulk material properties](/tools/bulk-material-properties) reference lists ρ_b for over 40 commonly handled materials alongside angle of repose and abrasiveness.
Reference standards
- ISO 5048Continuous mechanical handling equipment — Belt conveyors with carrying idlers — Calculation of operating power and tensile forces
Uses bulk density directly in mass flow and per-metre mass calculations for Te and motor power.
Related engineering tools
Related terms
- Angle of Repose
The angle of repose is the steepest angle, measured from horizontal, at which a static pile of loose bulk material is stable without sliding; for most granular ores and aggregates it lies between 30° and 45°.
- Surcharge Angle
Surcharge angle is the angle of the material heap above the rim of a troughed conveyor belt during conveying, typically 5–25° lower than the static angle of repose due to belt vibration.
- Lump Size
Lump size is the maximum particle dimension of a bulk material — designated as 'sized' (uniform) or 'run-of-mine' (mixed) — and it drives idler spacing at load points, belt width sizing and impact rating.
- Abrasiveness Rating
Abrasiveness rating is a qualitative classification (Low, Medium, High, Very High) of how aggressively a bulk material wears belt covers and idler shells, used to specify cover grade and idler material.