Beer Alcohol Content, Original Gravity and Specific Gravity

In relation to home beer making and calculating beer alcohol content, specific gravity is the measurement that home beer brewers use.

Beer Alcohol Content

Working out the amount of alcohol in the fermented brew is pretty easy, given that the amount of C02 produced per gram of ethanol produced during fermentation is 1.05.

For example, if the Original Gravity is 1.06 and the Terminal Gravity is 1.02, this means 0.04kg/L of CO2 has bubbled off. The alcohol content that has been left behind is thus:

1.05 x (1.06 - 1.02) = 0.042 kg/L. To calculate the percentage of alcohol is elementary from here. 0.042 / 1.02 = 0.041 or 4.1%.

This is the amount of alcohol by weight. The amount of alcohol by volume (the number you see quoted on cans and bottles) is a little larger. To convert one to the other is also easy. Just divide by the density of alcohol, 0.79 kg/L.

Alcohol by volume: 4.1% / 0.79 = 5.2%.

Such straightforward guidelines can assist home brewers estimate how much alcohol content is produced during fermentation. Or of course, you can use that tried and tested method of drinking it!

Specific Gravity

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"Specific gravity" relates to the density of any liquid. Conventionally, pure water is given an SG or specific gravity of 1.00 at 15.5C (60F); this is used as the standard. The 1.00 relates to the density of H2O, in metric units, being 1 kg per litre. Should a litre of beer have a SG of 1.05, it will have a mass of 1.05 kg.

For home brewing purposes, specific gravity breaks down further into the Starting or Original gravity (OG), a measure of Specific Gravity prior to fermentation. This assists brewers to calculate how much sugar has been dissolved in the wort (the malty liquid fermented for the purposes of making beer).

The usual range of OG is between 1.020 - 1.160. British charts often leave out the decimal point, resulting in OG figures like 1020 to 1160. The added density is due to the sugars dissolved in the water, which get converted to ethanol, beer alcohol content, via fermentation.

A specific gravity measurement is also taken after fermentation, herein the number is known as the 'Final' or 'Terminal' gravity (TG). The difference between the final number and the initial number gives the brewer an easy way of estimating the amount of alcohol produced during fermentation. The Fermentation transforms the maltose into ethanol and carbon dioxide, with the majority of the CO2 bubbling away in the process.

Here is a table with common ranges for different styles of beer:

  • Wheat beer and mild beers: 1.020-1.040 Original Gravity
  • Lagers, Bitters, Stout, Pale Ale: 1.040-1.050 Original Gravity
  • IPA, ESB, Oktoberfest: 1.050-1.060 Original Gravity
  • Bocks, Strong Ales: 1.060-1.075 Original Gravity
  • Barley Wines: > 1.075 Original Gravity

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