In hydraulic system design and fluid mechanics, two fundamental properties—specific gravity and density—govern pump selection, pipe sizing, and pressure loss calculations. Specific gravity (SG) is the ratio of a fluid's density to the density of water at a reference temperature (typically 4°C, where water density is 1000 kg/m³). Density (ρ) is mass per unit volume. Converting between them is straightforward: ρ (kg/m³) = SG × 1000. This article provides a practical reference for common fluids encountered in industrial, commercial, and fire protection applications, along with conversion formulas and temperature considerations. For a broader foundation, see The Complete Guide to Hydraulic Calculations for Engineers and Designers.