A Lesson In Calculating Alcohol Content

Half-pint, pint, liter, you say? When ordering one to drink, perhaps. But, when it comes to measuring beer, brewers have something else entirely in mind.

Specific Gravity

For reasons known only to history, the term ’specific gravity’ refers to a liquid’s density. By convention, pure water is assigned a specific gravity (SG) of 1.00 at 15.5C (60F) and is used as a standard. The 1.00 refers to the fact that the density of H2O, in metric units, is 1 kg/liter. So, if a liter of beer has an SG of 1.05 it will have a mass of 1.05 kg.

When used in brewing, specific gravity is broken down further into ‘Starting’ or ‘Original’ gravity (OG), which is a measure of SG before fermentation. The number helps brewers measure how much sugar is dissolved in the wort, the malty-liquid that is fermented to make beer.

Normal range for OG is from 1.020 to 1.160. Note that British charts often omit the decimal point. The added density is the consequence of sugars dissolved in the water, which will get converted to ethanol (the alcohol in beer) by fermentation.

Measurements are made of specific gravity after fermentation as well, where the number is called the ‘Final’ or ‘Terminal’ gravity (TG). The difference between the final number and the initial number provides a simplified method for estimating the amount of alcohol produced during fermentation. Fermentation converts maltose into ethanol and carbon dioxide, with most of the CO2 bubbling away during the process.

Since ethanol is less dense than the sugary water (wort) the Terminal Gravity is less than the Original Gravity. Pure ethanol has an SG of 0.79 kg/L. One side effect of this is that sweeter beers will have a lower OG, since the yeast doesn’t completely convert all the malt sugar available in that case.

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

Milds, Wheat Beers: 1.020-1.040 (OG)
Lagers, Stout, Porter, Pale Ale, Bitters: 1.040-1.050 (OG)
ESB, IPA, Oktoberfest: 1.050-1.060 (OG)
Strong Ales, Bocks: 1.060-1.075 (OG)
Barleywines, Belgian Trippels: > 1.075 (OG)

Alcohol Content

Calculating the amount of alcohol in the fermented brew is easy, with a little bit of help from simple chemistry. Skipping some technicalities, we find that the amount of CO2 produced for every gram of ethanol produced during fermentation is 1.05.

Suppose the OG is 1.06 and the TG 1.02. This means 0.04kg/L of CO2 has bubbled off. The alcohol content left behind is therefore:

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%.

These simple guidelines can help home brewers estimate how much alcohol is produced during fermentation. Or, you could just use the old-fashioned method and drink.