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Calculating Alcohol By Volume (ABV) with a Hydrometer or Refractometer

Aussie Brewmakers sell Home Brew Starter Kits for beer, cider, ginger beer and wine that include everything you need to start brewing. An important component in these kits is a hydrometer.

Surprisingly, many home brewers decide not to use a hydrometer, mainly due to not knowing how to use it properly. In addition, many home brewers will break their hydrometer and not replace it. Given they are a fragile glass instrument they do break easily, especially if you are brewing in a cramped area with fermenters and bottles bashing into each other in a frenzy of activity. In this instance a refractometer might be better suited to your needs (but more on that a bit later).

Hydrometers and refractometers are a valuable tool for the home brewer and perform two main tasks:

  1. They let you know when primary fermentation is complete and it is time to bottle
  2. They allow you to calculate the estimated ABV (alcohol by volume) of your home brew

What does a Hydrometer or Refractometer do?

A hydrometer or refractometer measures the density of a liquid relative to the density of water. This measurement is done on a scale called specific gravity (SG). The measurement of SG by hydrometers is also done at a set temperature, typically calibrated to 20 degrees Celsius.

For the home brewer, when they add malts or sugars to water (the wort) there is an overall increase in the SG which means there is a physical increase in weight over what water weighs (water has a SG of 1.0000). For example, if a 23L wort has a SG of 1.040 and water has a SG of 1.0000 then the wort will weigh 1.040 times as much as the equivalent amount of water.

When yeast is added to the wort the fermentation process converts the sugars to carbon dioxide and alcohol which lowers the overall SG or overall weight / density of the wort. The SG will continue to decrease until the fermentation stops.

Basically, the more sugar in the wort, the higher the reading. The more alcohol there is in the wort the lower the reading.

Should I use a Hydrometer or Refractometer?

Hydrometers and refractometers perform the same function. It is a personal choice which to use.

Hydrometers are a fragile glass tube that is suspended in a long tube or jar of the liquid being measured. They are relatively inexpensive but are prone to breakage. They also need to be read at 20 degrees Celsius or a compensating factor needs to be made (see below) to get a correct reading

Refractometers are a more robust alternative to the hydrometer but are about 3 times more expensive. They use a crystal prism and light to measure the specific gravity. They have the benefit of only needing a drop or two of the liquid rather than an amount sufficient to make a hydrometer float in a long tube or jar. They also have the benefit of being easier to read as well as temperature compensating.

For ease of use and longevity we like using refractometers however most home brewers still use a hydrometer as it is the most economical way to measure SG (as long as you don’t break more than 3 of them!)

Refractometers are also a bit gadgety – and we all like gadgets!

For the purposes of the rest of this article we will however, assume a hydrometer is being used as it is more common.

How to use a Hydrometer

Most home brewers use a hydrometer with a long clear jar or the tube it is shipped in. We do not recommend putting a hydrometer into your fermenter.

To use a hydrometer fill the jar or tube to about 35mm from the top with the liquid to be tested. Then suspend the hydrometer in the liquid. It will float. Twirl the hydrometer to ensure there are no CO2 or air bubbles attached to it and also make sure it is not touching the sides of the tube / jar.

You can then take a reading by looking through the side of the tube or jar.

The SG reading should be taken from the bottom of the meniscus (the lower of the two levels you see when looking at the side of the tube or jar) as per the below illustration.

Once a reading is taken do not return the test liquid back to the fermenter as you will run the risk of introducing bacteria and ruining your whole wort.

he hydrometer is calibrated to test liquid at a set temperature, normally 20 degrees Celsius. If we take a measurement at a higher or lower temperature then an incorrect reading will be result. This is because the absolute density of a liquid will change with temperature shifts but the SG does not change. If you are taking a SG reading at a temperature other than 20 degrees Celsius then the below table details the adjustments that need to be made to get a correct reading.

Temperature (C)Correction

10-0.0025

15-0.0014

20 0.0000

25+0.0028

30+0.0046

Using a Hydrometer to Calculate ABV

To calculate the alcohol content or alcohol by volume (ABV) of your home brew you will need to have measured the starting or Original Specific Gravity (O.S.G.) and the end or Final Specific Gravity (F.S.G.).

For example, if your brew starts off with an O.S.G. of 1.040 and an F.S.G of 1.010 when fermentation is complete then use this simple formula:

Temperature (C)Correction

1. Remove the decimal point to make life easier.

2. Subtract the F.S.G. from the O.S.G. i.e. 1040 – 1010 = 30

3. Divide the result by a factor of 7.36 i.e. 30 / 7.36 = 4.1

(4.1% is the alcohol content generated in the primary fermentation in your fermenter)

4. If you are adding sugar to each bottle to carbonate your brew (secondary ferment / bottle condition) you need to add a factor of 0.5 for the alcohol generated by the secondary fermentation.

i.e. 4.1 + 0.5 = 4.6%

In this example the final alcohol content in the bottles will be approximately 4.6%.

It is as easy as that!

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