Gravity (alcoholic beverage)

When brewers and vintners talk about "gravity," they are referring to the specific gravity (SG) of their wort or must—that is, its density relative to water. This measurement is taken at different points during fermentation using tools like a hydrometer, refractometer, pycnometer, or an oscillating U-tube electronic meter. Since the density of the liquid depends largely on its sugar content, and yeast converts those sugars into carbon dioxide and alcohol, tracking the drop in SG over time tells the brewer how healthy and active the fermentation is. The process is considered complete when the gravity stops falling.

The starting measurement, taken before fermentation begins, is called the original gravity (OG). Once fermentation finishes, the reading is known as the final gravity (FG). For a typical beer, the OG might be 1.050 and the FG around 1.010. The difference between these two numbers indicates how much sugar has been turned into alcohol—the larger the gap, the stronger the beer. This is why strong beers are often called "high gravity" beers, while lighter "session" or "small" beers are referred to as "low gravity" beers, even though a strong beer’s final gravity can actually be lower than a session beer’s due to the higher alcohol content.

Several scales exist for measuring original gravity. The brewing industry historically uses the Plato scale (°P), which is essentially the same as the Brix scale used in winemaking. An OG of 1.050, for example, roughly equals 12 °P. The original extract (OE) is often called the "size" of the beer, and in Europe it may appear on labels as *Stammwürze* or simply as a percentage. In the Czech Republic, you’ll commonly see "10 degree beers" or "12 degree beers," which refer to the Plato gravity of the wort before fermentation.

Specific gravity itself is the ratio of a sample’s density to the density of water, and this ratio depends on temperature and pressure. In brewing, the standard pressure is 1 atmosphere, and the temperature is usually 20 °C (68 °F) for both sample and water, though some regions use different temperatures and hydrometers calibrated accordingly. The current ASBC table uses a 20 °C/20 °C reference, meaning the density is measured at 20 °C and compared to water at 20 °C. The true specific gravity is calculated as the density of the sample divided by the density of water.