Measuring and Calculating Alcohol Using a Hydrometer

A hydrometer is must-have equipment in any brewer’s or distiller’s list of essential equipment. It is not only important to have one, knowing how to use it will help you get accurate alcohol content of your drinks and ultimately get the quality and consistency right. Apparently, most consumers are concerned about how strong their drinks are, in other words, the alcohol content of their drinks. This is why it is important for any brewer/distiller to correctly calculate and indicate the alcohol content of their drinks on the label. You are not spared as a homebrewer. Drinkers still need to know and they will ask questions so get your details right and this is how.

What is a hydrometer? 

A sealed glass tube with a weight at the base and calibrations at the side which indicate specific gravity (SG). There are two types of hydrometers with specific calibrations for their purpose. 

  • The brewing hydrometer. Used for taking density measurements for the mash during fermentation. They measure the amount of sugar in the wort or mash. 
  • Distilling (spirit) hydrometer. Used for taking density measurements for spirits during distillation. Distillers would typically have both hydrometers given that the brewing hydrometer is not designed to be used in distillation. 

Measurement units explained 

Here are the common units of measurements used in the brewing/distilling/wine-making space. 

  1. Specific Gravity (SG). This unit refers to the density of a liquid expressed in relation to the density of water. The SG of water is 1.000. Liquids with sugar are denser than water and thus have a higher SG than that of water. 
  2. Plato Scale. This is the measure of the strength of dissolved solids in a liquid. It is a brewer’s measurement used to determine the concentration of sugar in the wort. 
  3. Potential Alcohol Scale. Is used to determine the potential alcohol in the mash. 
  4. Brix Scale. Like Plato is a brewer’s measurement, Brix is a winemaker’s measurement. It also refers to the concentration of sucrose in a solution. One degree Brix is equivalent to 1 gram of sucrose in a 100-gram solution. 

SG is the most common measurement thus we shall use this throughout the article. The brewer’s hydrometer is used to measure the concentration of sugar in your wort or mash and this should be done just before, through, and after fermentation. This measurement is what you’ll use to calculate the ABV (alcohol by volume) of your wort or mash. 

For example, when you dip your hydrometer in your wort before fermentation, because at this time sugar is present in high quantities, the hydrometer will float in the liquid. On the other hand, if you do the same after fermentation (This is after the sugars have been broken down into alcohol and are no longer present in your wort) your hydrometer will read the same or close to the reading it gives when dipped in water. 

At what point is the hydrometer used? 

There are three instances during brewing in which a hydrometer becomes useful in the brewing process. 

1. Before fermentation to determine the OG 

Once the wort has been prepared, i.e. the grain has been boiled and enzyme added to it, the initial reading is taken using a brewing hydrometer. This is before adding yeast to begin the fermentation process. This first reading is known as the original gravity (OG). As mentioned earlier, the hydrometer will float on the liquid since at this point the sugar content is still very high as it has not yet been broken down into alcohol. 

The importance of taking the OG is that it helps you determine the potential alcohol that your wort will produce. For instance, an SG 1.050 OG should yield 5% ABV.

To take the OG, 

  1. Fill the test jar with the liquid using your beer sampler. 
  2. Place your hydrometer inside the test jar and swirl it a bit so that it is not stuck on the wall of the test jar. The hydrometer will float in the liquid.
  3. Take the reading and write it down in your journal. 

What to do when your OG is too low?

Firstly, a low OG means that your ABV will also be lower. Here is what you can do:

  1. Proceed with the brewing and expect a low ABV beer 
  2. Add malt extract to enhance the gravity of your wort to yield a higher ABV. 
  3. Give your wort preparation a longer intense boil to evaporate most of the water for a more concentrated wort before fermentation. 

Most people have found adding malt extract to the wort to be a more effective option. On the other hand, you could also get a higher OG. You can dilute your wort or mash to lower the OG so that your final drink will not be too concentrated. This will also increase the quantity of your final drink. 

2. After fermentation to determine the FG

Another reading, the final gravity (FG) is taken after fermentation is complete. You’ll notice that when taking the final reading, your hydrometer will sink like it does when dipped in water to give you a specific gravity of between 1.000 and 1.020. Here the sugar density will be much lower because it will have been broken down into alcohol. In essence, the difference between the OG and FG should help you calculate the percentage of alcohol in your wash. 

A lower HG means that you have higher alcohol content in your drink with a thinner taste. On the other hand, a higher SG means the alcohol content is not as high and your drink will yield a sweeter taste. 

To take your FG,

  1. Fill the test jar with the liquid using your beer sampler. 
  2. Place your hydrometer inside the test jar and swirl it a bit so that it is not stuck on the wall of the test jar. The hydrometer will float in the liquid.
  3. Take the reading and write it down in your journal. You will notice that your reading has now dropped to 1.010 or lower. If you notice a higher reading than this, give your wash a few more days and take the reading again. 

3. During fermentation 

It is advisable to also take readings during the fermentation process. This helps you monitor the progress of the fermentation process. Brewers/distillers will mostly take two to three readings in the course of fermentation and they do this for various reasons. For some, it helps pick out any anomalies during the process while for others is to find out how efficient their brewhouse is in the process of developing consistent and predictable brewing, wine-making, or distilling procedures. 

How to take the hydrometer readings and adjust the temperature?

A hydrometer is normally calibrated to measure gravity at 60 °F (15.6 °C) or 68 °F (20 °C). If water is measured at sea level, it will read 1.000. 

The first thing you need to do after purchasing your hydrometer is take the gravity of water at the temperature rating on the hydrometer. A 1.000 reading means that your hydrometer is well calibrated. On the other hand, if it reads at 0.998 and below then it means for all the measurements you will be taking, you will be adding 0.0002 to make up for the difference. The readings could also be higher say 1.0002 in which case you will be subtracting 0.002 for all your measurements to make up for the difference. 

How to adjust the temperature?

There are two ways that you can use to improve the accuracy of your hydrometer readings.

  • Allow your sample to cool. Cooling your sample brings its temperature down closer to the temperature rating on the hydrometer. 
  • Use a calculator to adjust your reading to that of the calibration temperature of your hydrometer. If your mash sample is above the 60 °F (15.6 °C) or the 68°F (20 °C) on the hydrometer which is most likely the case since it will be coming from the boil, you can use an online calculator to convert these readings to the calibration temperature on your hydrometer. 

How to take readings on the hydrometer?

Most brewers/distillers/winemakers prefer to use the hydrometer with a test jar. Ideally, the test jar should be between 200mm (7.9 inches) and 356mm (14 inches) long with 35mm diameter and should be made of clear glass or plastic material. It should be placed on a flat surface. 

Fill the test jar with liquid and drop the hydrometer in the liquid ensuring that it is not stuck at the side of the jar. You’ll notice that the top of the liquid aligns with the mark on the hydrometer’s scale. Mark the reading at the point where the liquid crosses the scale on the hydrometer. 

It is important to note that because of surface tension, the liquid will often be curved up. Always take your reading from the lowest point of the curve known as the meniscus. Secondly, when taking the readings (the Specific Gravity a.k.a SG), look through from the side of the test jar at eye level. 

Calculating the alcohol content in your wash

By the time you are calculating the alcohol content in your wash, you will have taken the original gravity (OG) and final gravity (FG) readings before and after fermentation. These readings will help you calculate the alcohol content referred to as alcohol by volume (ABV). 

Here is the formula to use. 

  1. Subtract the final gravity from the original gravity
  2. Multiply the difference by 131.25 and you have your alcohol by volume as in the formula below. 

(OG – FG) x 131.25 = ABV


Hydrometers are important pieces of equipment for measuring gravity in beer, wine, spirits, or moonshine. They give accurate readings and are pretty easy to learn how to use for both homebrewers and commercial brewers and distillers. Consistently getting accurate readings will help you calculate the ABV. Ultimately, you should be able to predict the ABV of your final liquid before fermentation. This will help you produce consistent high-quality drinks.