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Kilogram per cubic metre (kg/m³)

SI unit of density representing mass in kilograms contained in one cubic metre of volume; used in science and engineering to express material and fluid density.

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Overview

The kilogram per cubic metre is the International System of Units (SI) measure of density, expressing how much mass is present in a given volume. It describes the mass, measured in kilograms, that occupies one cubic metre of space. Written in unit form as kg/m³ or kg·m−3, this quantity is fundamental in physics, engineering, meteorology, and materials science.

Definition and notation

Density in SI base units is mass divided by volume, so the kilogram per cubic metre equals one kilogram of mass divided by one cubic metre of volume. Common notations include kg/m³, kg·m−3 and occasionally kg m−3 (here shown inline as kg·m−3). The concept is straightforward: a higher value means more mass is packed into the same volume.

Typical values and examples

Many everyday materials and fluids are described by their density in kg/m³. Typical approximate values include:

  • Air at sea level: about 1.2 kg/m³ (varies with temperature and pressure)
  • Fresh water (near 4 °C): about 1000 kg/m³
  • Aluminium: roughly 2700 kg/m³
  • Steel: roughly 7850 kg/m³
  • Mercury: around 13500 kg/m³

These examples illustrate how densities determine behavior such as buoyancy: an object with density lower than the liquid will float, while a denser object will sink.

Uses and importance

Density in kg/m³ is used across disciplines: in engineering it guides material selection and structural calculations; in fluid mechanics it appears in buoyancy, pressure and flow equations; in meteorology and oceanography it influences air and water circulation; in industry it is essential for quality control and shipping. Expressing density in SI units ensures consistency in calculations and reporting.

Closely related units and conversions are commonly used: 1 gram per cubic centimetre (g/cm³) equals 1000 kg/m³, so water at 4 °C is about 1.0 g/cm³ or 1000 kg/m³. Specific gravity is a dimensionless ratio comparing a substance's density to that of water. Specific weight (weight per unit volume) combines density with local gravitational acceleration and therefore has different units.

Historical and practical notes

The kilogram per cubic metre became the standard density unit as the metric system and SI units were adopted internationally, providing a uniform framework for science and industry. When reporting densities, it is important to state temperature and pressure where relevant, because the density of gases and many liquids depends strongly on those conditions.

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