# Specific gravity

Specific Gravity (SG) is a special case of relative density.

It is defined as the ratio of the density of a given substance, to the density of water (H2O). Substances with a specific gravity greater than 1 are heavier than water, and those with a specific gravity of less than 1 are lighter than water.

$\mbox{SG} = \frac{\rho_\mathrm{substance}}{\rho_\mathrm{H2O} }$ [1]

SG is by definition dimensionless and therefore not dependent on the system of units used (e.g. slugsft−3 or kgm−3), insofar as the units are consistent. Based on the SG-value of a given substance, the density of that substance can be calculated.

## References

1. The density of the denominator, or reference, is $\rho_\mathrm{H2O}$ = 1000kgm-3 (at 4°C/39.2°F) in SI units.
• Fundamentals of Fluid Mechanics Wiley, B.R. Munson, D.F. Young & T.H. Okishi
• Introduction to Fluid Mechanics Fourth Edition, Wiley, SI Version, R.W. Fox & A.T. McDonald
• Thermodynamics: An Engineering Approach Second Edition, McGraw-Hill, International Edition, Y.A. Cengel & M.A. Boles