The Hydrometer - How to Measure the Alcohol Content of your Cider

A hydrometer is a simple instrument that is used to measure the specific gravity (or relative density) of a liquid and allows a comparison between the density of the liquid to be compared to the density of water. A hydrometer is of great use to a home brewer as it permits the measurement of the density of pomace or wort before fermentation and after fermentation. During fermentation the yeast consumes the sugars held by the liquid being fermented, CO2 gas and ethanol are produced making the liquid less dense. Measuring this change in density allows the home brewer to calculate the theoretical amount of alcohol that fermentation has produced.

A hydrometer is usually made of glass with a thin cylindrical stem attached to a bulb at one end, which usually contains lead or mercury to weight it down and make the hydrometer float upright. The thin stem usually contains a scale, when the hydrometer is lowered into the liquid and allowed to float freely the number on scale at the point where the liquids surface touches the stem is read and is consider to be the reading. A variety of different scales are used depending on the purpose of the hydrometer. The most common scale used in home brewing has the specific gravity of water as 1.000 and everything is measured in comparison to the density of water. Many of my ciders for example will have a starting gravity (SG) of 1.045 and a final gravity (FG) somewhere around 1.005. Run this example through the formula used to determine the theoretical alcohol content and we get very respectable 5.3%.

The formula used to determine the alcohol content is-

((1.05  x  (SG  -  FG))  /  FG)  /  0.79  =  % Alcohol

Lets consider our example above where our cider had a starting gravity (or SG as it is often abbreviated) of 1.045 and a final gravity after fermentation of 1.005, so our formula looks like-

((1.05  x  (1.045  -  1.005))  /  1.005)  /  0.79  =  % Alcohol

So in our example the starting gravity is 1.045 and the final gravity is 1.005. So we start by subtracting the FG from the SG, 1.045 - 1.005, which gives a total of 0.04. Multiply 0.04 by 1.05 to give a total of 0.042. Dividing 0.042 by 0.79 give the grand answer of 0.053164 (and many more numbers) which we round off to be 5.3 percent alcohol.

Dont be daunted by the fact that you have to do a bit of maths, a calculator will take of that for you (the device you are reading this web page probably has one built into it) and you will be working out your alcohol content with ease after just a few goes. Start with a calculator and work out the above example till you get the same answer and take it from there. After all, your guests might ask you what the alcohol content of your cider is and it always looks better if you can provide an answer.