The specific weight (also known as the unit weight) is the weight per unit volume of a material. The symbol of specific weight is γ (the Greek letter Gamma ). A commonly used value is the specific weight of water on Earth at 4°C, which is 9.807 kN/m 3 or 62.43 lbf /ft 3 . 2 depending upon the API gravity degrees of the material in question. Specific gravity test results for asphalts are reported by the refinery or Department chemists. These specific gravities may be converted to API values by use of the following formula: API Gravity (degrees) = 141.5 - 131.5(G) G Where: G = The specific gravity of the material. The apparatus constant is determinate by measuring permeability of reference cement of known specific surface. The procedure repeats three times for three samples of cement bed. For each sample is calculated constant K from formula: where S0 is the specific surface of the reference cement [cm 2.g-1] Specific Gravity is the ratio of the weight of a given volume of aggregate to the weight of an equal volume of water. Water, at a temperature of 73.4°F (23°C) has a specific gravity of 1. Specific Gravity is important for several reasons. Some deleterious particles are lighter than the good aggregates.