Fermentation Basics
Understanding Specific Gravity
A practical introduction to gravity readings, hydrometers, temperature correction, and fermentation progress.
What specific gravity means
Specific gravity (SG) compares a liquid’s density with water at a reference temperature. Dissolved sugar and other extract usually raise the reading; fermentation converts some extract to alcohol and carbon dioxide, lowering density.
An SG of 1.050 is denser than water, while a reading near 1.000 is close to water. Alcohol can pull density below water, so a dry fermented drink may finish under 1.000.
Taking a useful reading
Use a clean sample jar, remove foam and bubbles from the hydrometer, and spin it gently to dislodge clinging bubbles. Read at eye level at the instrument’s calibration temperature.
Correct for temperature when needed, sanitize sampling equipment, and avoid returning a sample to the batch. Keep readings in a batch log so you can compare changes over time.
Gravity is a process measurement
A gravity reading does not identify which sugars or acids are present. Fruit, malt, honey, and additives can contribute different extract, and a refractometer reading after fermentation needs alcohol correction.
Use repeated, consistent measurements rather than a single number to judge progress. If gravity does not change as expected, investigate temperature, yeast health, and measurement technique before making an addition.