Original Gravity (OG), sometimes called original extract, is a measure of the solids content originally in the wort, before alcoholic fermentation has commenced to produce the beer. OG is one of the major measurements used by brewers to determine the future alcohol content of a beer fermented from a particular wort. It is measured by a saccharometer, hydrometer, or refractometer as the density of the wort at standard temperature and pressure (STP; usually 20°C and 760 mm) at the final collection point before the yeast is added. Original gravity is expressed as the density above that of distilled water and in the UK is called the excess gravity. Water is deemed to have a density at STP of 1.000. If the wort density is 1.048, it will have 48° of excess gravity and an OG of 48.