Wort is the sweet liquid extracted from malted grain during brewing. It contains water, sugars, proteins, and other dissolved compounds.

Because these materials make wort denser than pure water, brewers measure its specific gravity, which is the density of a liquid relative to water. Pure water has a specific gravity of 1.000.

Measuring Original Gravity (OG) tells brewers the density of the wort before fermentation begins and provides an indication of the fermentable sugars available to the yeast.