Attenuation involves the removal of sugars and production of alcohol by yeast during fermentation such that the wort becomes less dense and viscous and is thus thinned-out or attenuated. The use of the hydrometer or saccharometer for measurement of attenuation during a fall in specific gravity of fermenting wort was introduced to brewing in late 18th-century England.  Determination of temperature, brought in a few years earlier, and gravity measurement were the first quantitative process control measures made available to the brewer.

The extent of attenuation achieved in fermentation of wort to beer is often given as the difference between the starting gravity of the unfermented wort and the gravity of the finished beer, expressed as a percentage of the starting gravity. Thus, if the wort has a starting gravity of 15°Plato and the finished beer a gravity of 3°Plato, then the percent attenuation would be (15 – 3)/15 x 100 = 80%.

Because alcohol produced during fermentation has a lower density then that of water, measurement of the beer gravity overestimates the extent of sugar removal from the wort and the calculation above yields what is termed the “apparent attenuation.” To determine the “real attenuation,” it is necessary to remove the alcohol by distillation before measuring the beer gravity and use this figure in the determination of percent attenuation of the wort. Real attenuation is about 80% of the measured apparent attenuation in normal circumstances.