The specific gravity stands for a dimensionless unit that defines the ratio of density of a substance to the density of water. The density of water is 1000 kg/cubic meters at 4 Celsius. In physics, the weight of the substance differs from its mass. The weight is the gravitational force that pulls any object to Earth. The specific weight corresponds to the weight per unit of the volume and can be calculated from the specific gravity. Knowing the specific weight, you can easily compute the weight of any amount (volume) of a substance.

Multiply the specific gravity by 1,000 to calculate the density of the substance in kilograms per cubic meter (kg/cubic meters) units. For example, the specific gravity of 0.84 corresponds to the density of 840 (0.84 x 1000) kg/cubic meters.

Multiply the density by the acceleration of gravity (9.81) to calculate the specific weight. In our example, the specific weight is 840 x 9.81 = 8,240.4.

Measure or obtain elsewhere the volume of the substance.

Convert the volume to the cubic meter unit. If the volume is given in liters, then divide it by 1,000. If it is measured in gallons, then multiply the value by 0.003785. For example, 5.2 liters will convert to 0.0052 (5.2 / 1,000) cubic meter.

Multiply the specific weight of the substance by the volume to calculate the weight. In this example, the weight is 8240.4 x 0.0052 = 42.85 newtons. Note that "newton" is a unit of force in physics.