Specific Gravity is the ratio between the density of a substance and the density of a reference substance. In the vaping community, as with almost all others, the reference substance for liquid is water. Therefore, the specific gravity of any liquid would be its density divided by the density of water.
You don't really need to know the specific gravity of your liquids to mix by weight, but it is necessary when you want to maintain a percent by volume in your mix. Unless you're mixing eLiquid outside of Earth's atmosphere, simply multiply the specific gravity by your desired volume to get the amount of grams you will need to weigh for your mix. Learn more about mixing by weight.
Below is a list of the Specific Gravity for all of our liquid products