Specific gravity (SG) is a measurement that compares the density of a liquid to the density of water.

  • Water has a specific gravity of 1.000 (at a standard temperature, usually ~60°F / 15.6°C).

  • If a liquid has an SG greater than 1.000, it’s denser than water (contains dissolved substances like sugars, minerals, etc.).

  • If it’s less than 1.000, it’s less dense (like hard alcohol or some solvents).

In simple terms:

Specific gravity tells you how “heavy” a liquid is compared to water.

In brewing / wine-making:

SG is hugely important because it reflects how much sugar is in the liquid.

  • Original Gravity (OG): SG before fermentation → indicates starting sugar content

  • Final Gravity (FG): SG after fermentation → indicates remaining sugar


As yeast ferments sugar into alcohol:

  • Sugar (dense) → Alcohol (less dense)

  • So SG drops over time

Why it matters:

  • Estimate alcohol content (ABV) is calculated off of change in gravity

  • Tracks fermentation progress

  • Diagnose issues (stuck fermentation, etc.)

Quick example:

  • OG = 1.080

  • FG = 1.010

That drop tells you:

  • Sugar was consumed

  • Alcohol was produced

  • Fermentation likely completed