How To Use a Hydrometer To Measure The Alcohol Content of Your Homebrew

homebrew hydrometer

Whenever someone samples my homebrew, it seems the first question is, “what’s the alcohol content”?” It makes sense: some people can’t handle a high volume of alcohol, so being able to know how to use a hydrometer has always put me in a great position to be able to answer that question.

But whether you really care about the alcohol content or not, understanding how to use a hydrometer is more than just knowing how much alcohol is in your beer.  It will help improve the consistency and quality of your home brew, by knowing if the yeast is doing its job or not.

While looking at a hydrometer with all those numbers and lines might look confusing, it really isn’t. Using a hydrometer is easy, and in this article, we’re going to walk through how to use one, what it’s for, and everything you need to know about measuring the ABV of your homebrew.

Table of Contents

What Is a Hydrometer?

A hydrometer is a basic tool that is used to measure the relative density of liquids. More specifically the density between water and other liquids like beer and wine.  In homebrewing, it is a way to monitor the fermentation process and ultimately measure the alcohol by volume (ABV) of your finished beer.

Hydrometers are not hard to use, but for any beginner, understanding how to accurately read one is the key to success. It can bring you from basic brewer to an advanced brewer in no time at all – which is the goal if you love beer!

It might look like a thermometer, but it a density scale. The tube is marked in such a way for accurate hydrometer readings. If you want to get to know the ABV of your home brew, then a hydrometer will be the most valuable tool in your arsenal for brewing. It’s a great one to use when you are experimenting with new and different recipes, and it will offer you some insight on the final product.

Do You "Need" To Take a Gravity Reading?

The very first time you brew your own beer, it should be as stress-free as possible. It’s for this reason that you really don’t need to worry about a gravity reading. However, it does give a good indication of the fermentation of the beer and the final alcohol by content reading of the beer, too.

If you’re a first timer, or you are not concerned with the alcohol content, you can just bypass this. Why? Well, when you take two hydrometer readings as a newbie brewer, you’re simply adding steps that won’t do much more than confirm the final ABV. 

When you follow instructions that come in an extract recipe kit, you shouldn’t need to worry about alcohol content readings, as the approximate ABV will be met through the recipe if you follow the directions carefully. It also doesn’t really matter if you don’t hit it the first time: it’s all about experimentation!

This is YOUR brew, and if you like the taste, that’s what counts the most. Eventually, you can introduce taking alcohol content readings for the specific gravity and final gravity, as this will hone your brewing craft. As with anything, it just takes practice to get it right, so start off with brewing by following the recipes, and then move up to perfecting the final gravity readings on future brew days.

Often times, it’s not always enough to see if fermentation has begun, is stuck, or finished just by looking at the activity in the airlock. A lack of bubbles in the airlock does not necessarily mean that fermenation is over.  But opening the lid to see what is happening during fermentation is not always recommended, but taking a gravity reading can tell you if the fermentation process has started, and when it has ended.

How To Measure The ABV of Your Beer

The calculation of ABV of you beer can be done very simply, and it goes something like this.

Use a sterilized wine thief or turkey baster to take a sample of the wort after it’s cooled and transferred the wort to the fermenter, but before you pitch the yeast.  If you have a spigot on your fermenter, you can get by without the use of a wine thief.  Either way, use a test tube jar, and fill with the un-fermented beer.

Put the hydrometer into the test jar with the weighted end down and allow it to float, making sure it is not touching the sides and able to float freely in the jar.  Also make sure you are measuring on a flat surface.

Read the original gravity, which is based on the type of beer you are brewing and that particular recipe.  Make sure you are reading the actual liquid line and not the bubbles. You can spin the hydrometer which can sometime eliminate the bubbles and make it easier to read.  

This number is your original gravity or OG.  Write this number down in your brewing notes. It is recommended that you do not add the test sample back to the fermenter.

After fermentaion is complete, take another sample using the same process as before.  This will be your final gravity, or FG reading. Again, write this number down in your notes too.

If your eyesight isn’t so great anymore, you can get a magnifying glass or use your phone to take a picture and then zoom in to get a more reliable number.

How To Do a ABV Calculation of Your Homebrew

Finding the alcohol by volume in your homebrew is simple.

The ABV calculation is like this:

(Original gravity – Final gravity) x 131.25 = ABV %

For example, if you were making a beer that had an OG of 1.040, and a FG of 1.002 the ABV would be 4.98%

How Do You Calibrate a Hydrometer?

If for some reason your OG and FG readings are varying wildly, compared to what you were expecting, it might be time to make sure your hydrometer is properly calibrated. Every hydrometer sold should come already calibrated to measure liquid at the temperature marked on the tube.

If you want to calibrate yourself to ensure accurate readings, check the gravity of water at the desired temperature. Almost all hydrometers on the market are calibrated between 60-68F. If your current plain water reading sits at 0.998 or lower, then you know it’s off just a few tenths.

A perfectly calibrated hydrometer should read 1.000, and there are ways to correct it if you don’t want to constantly add to the low numbers to make it 1.000.  But to be honest, a few points one way or the other probably won’t make that much of a difference.

You can choose to add mass or file away at the tube to calibrate it properly, but it’s not recommended. You’re working with fragile equipment, and if you try to alter it, you can cause more issues than it’s worth. It’s usually better to just use it as you can, just be mindful of how it’s calibrated from the beginning.

If your hydrometer is wildly off base, then the best thing that you can do is buy a new one that is appropriately calibrated to save yourself the mathematical headache!

Different Types of Hydrometers

Many different kinds of hydrometers are used in various industries. There are specially designed hydrometers for breweries that are on the market, and it’s essential that you buy the right one for your home brewing adventures!

Triple Scale Hydrometer

This is the most common type of hydrometer for home brewing. Almost every starter kit for home brew comes with a triple scale hydrometer, and they are around 9-12 inches long.

A triple scale hydrometer measures the potential alcohol content, the specific gravity of beer and wine. Although they are not very expensive, they are made of glass and can easily break if not taken care of.

The triple scale hydrometer will meet the needs of most homebrewers.


The thermohydrometer has a thermometer on the inside of the tube, and it measures the specific gravity of the brew as well as its temperature. It makes it so much easier to get the correct temperature of the liquid.

This is the hydrometer that will ensure that you maintain a consistent temperature throughout brewing. They’re not massively expensive, and they are an excellent option for your home brewing.

Precision Hydrometer

These hydrometers give a precise reading – as the name suggests! When you want to measure a more accurate specific gravity in beer, this is the hydrometer that can do it.

Precision hydrometers have numbers that are spaced out on a much smaller scale. This allows for a more easy and accurate reading. If you want the most precise readings, these are the best option.

Narrow Range Hydrometer

For a specialized tool that measures a specific gravity with the most accuracy, you want a Narrow Range Hydrometer. You need to know when fermentation has stopped, as this will allow you to move to a secondary fermenter or get ready to bottle.

The narrow range of specific gravity between 0.990 and 1.020 can let you see exactly when the fermentation process has finished. Professional breweries use these more than home brewers do, so don’t worry if you can’t find one.

Another Option - Refractometer

A refractometer is a very simple instrument that allows you to measure the brew you are making. Compared to the others, it sounds more science-y, but it’s not as complicated as you think! It only needs a few drops of the liquid, and it is used across many different industries.

As you may already know from science class, refraction occurs when light enters a liquid and changes direction. Refractometers measure the degree to which the light changes direction when it enters the liquid, and they take those angles and put them into the correct refractive index values. You can then determine the concentration of a type of solution – like your beer!

When the solution is a low concentration solution, the prism’s refractive index is bigger than that of a sample. This then leads to a bigger refraction angle and a subsequent low reading. With a higher concentration, the reverse occurs.

Why a Hydrometer Is an Important Part of Homebrewing

An elegant and fancy looking tool, a hydrometer, is useful for measuring the gravity of the beer. They are easy to use, reliable, and are a big part of the brewing world.

They’re not just used for home brewers, but for professional settings, too. When you handle and read your hydrometer correctly, you can hone your whole process, making your beers taste delicious every single time.