Specific gravity
From BrewWiki
- Specific Gravity (SG) is a measure of the relative density of one material compared to another. For brewing, the reference material is water, so the specific gravity of wort or beer is simply the density of the liquid divided by the density of water. This results in a dimensionless number, which by brewing convention is usually listed to three decimal places. For example, water would have a specific gravity of 1.000, while an unfermented wort might have a specific gravity of 1.048. Finished beer would typically have a specific gravity in the 1.005-1.015 range. The specific gravity measure is somewhat ambiguous, since the density of water does vary with temperature, but by convention specific gravity in brewing is typically measured by a hydrometer calibrated to 60 F.