Density, Specific Weight and Specific Gravity

Overview

Density ($\rho$) is mass per unit volume and is a fundamental physical property used to characterize substances. At a given temperature and pressure, density is constant for a pure material, making it useful for identification and engineering calculations.

Specific Gravity (SG), also called relative density, is the dimensionless ratio of a substance's density to the density of water at a reference temperature — typically 4 °C where water reaches its maximum density of 1000 kg/m³.

Specific Weight ($\gamma$) is the gravitational weight per unit volume, linking density to the local acceleration of gravity.

Key Formulas

Density

$\rho = \frac{m}{V}$

Mass divided by volume. The SI unit is kg/m³; the Imperial unit is slugs/ft³ (though lbm/ft³ is common in practice).

Specific Gravity

$SG = \frac{\rho_{\text{substance}}}{\rho_{\text{H}_2\text{O}}}$

Because SG is dimensionless, its value is identical in SI and Imperial systems. For solids and liquids the reference is water; for gases the reference is typically dry air at NTP (20 °C, 1 atm), where $\rho_{\text{air}} \approx 1.204\;\text{kg/m}^3$.

Specific Weight

$\gamma = \rho \cdot g$

where $g$ = 9.807 m/s² (SI) or 32.174 ft/s² (Imperial).

Specific Volume

Specific volume is the reciprocal of density:

$\nu = \frac{1}{\rho}$

The SI unit is m³/kg. In Imperial engineering units it may be expressed as ft³/slug when density is in slugs/ft³.

Variables

Unit Conversions

• 1 slug = 32.174 lbm = 14.594 kg
• 1 kg = 2.2046 lbm = 6.852 × 10⁻² slugs
• Water at 4 °C: 1000 kg/m³ = 1.940 slugs/ft³ = 62.4 lb/ft³

Specific Gravity of Common Substances

Specific Weight of Common Materials

Restored Original Source Tables

The following tables are restored from the original source page to preserve the complete reference data.

Specific Gravities common Substances

Specific Weight common Materials

Engineering Notes

• Mass vs. weight: Density uses mass (kg, slugs); specific weight uses force (N, lbf). Conflating pound-mass (lbm) and pound-force (lbf) is a common source of error in Imperial calculations.
• Temperature dependence: Density changes with temperature. Water is an exception at 4 °C, where it reaches its maximum density. Always note the reference temperature when reporting SG or density.
• Pressure dependence: For liquids and solids the effect is small. For gases, density is strongly pressure-dependent — use the ideal gas law ($\rho = p / (R\,T)$) or real-gas models as needed.
• SG for liquids: Because the SG value equals density in g/mL (or Mg/m³), it provides a quick way to estimate density without unit conversion.
• Composites and alloys: Tabulated SG values are for pure materials. Alloys, polymers, and composites vary with composition and processing.

References

• Engineering ToolBox — Density, Specific Weight and Specific Gravity