How To Measure Alcohol Content With A Hydrometer

The key to being able to enjoy your home brews without giving yourself a hangover or making yourself sick is to know how much alcohol content is in your beer before you start drinking it.

It’s about striking that perfect balance, so you only feel merry and not at all nauseous.

How To Measure Alcohol Content With A Hydrometer

And the good news is that there is an easy, straightforward way to measure the alcohol content in your home brews.

All you need is a hydrometer, a hydrometer tube, a thermometer, and a simple equation, and you’re good to go.

You will learn how much alcohol content is in every batch, so that you and your fellow drinkers can stay within their limits, keeping the experience the joy it is meant to be.

In this article, I’m going to explain how to use the hydrometer, and the equation I mentioned earlier. Once you know what you’re doing, you could even sell your beer on, complete with labels!

Quick note – this article assumes that you already understand the process of brewing beer.

So if you don’t already know the ins and outs, I recommend that you read our article titled “Learn to Brew Beer”.

We’ll kick things off with a quick glossary of useful terms.

Glossary Of Terms

  • Hydrometer – An instrument for measuring the relative density of liquids.
  • Wort – Wort describes the mixture following the first stages of brewing, before hops are added, when the sugars are steeped out of the malted barley through mashing. It contains a mix of both fermentable and non-fermentable sugars
  • ABV – ABV stands for Alcohol by unit Volume.
  • Specific Gravity (SG) – Specific gravity is the density of liquids compared to distilled water, which has a specific gravity of 1.000.
  • Original Gravity (OG) – This is the original specific gravity of your unfermented wort. It’s usually around 1.055.
  • Final Gravity (FG) – Final Gravity is the specific gravity measured at the end of the process, when fermentation is completed, and the specific gravity has dropped. It’s usually around 1.010.

Specific Gravity, OG, FG, And ABV

So, the reason I had you read all those different terms about the scale for measuring the density of liquids is because you need them to work out the alcohol content of your home-brewed beer.

Once you know the specific density of your wort (see above definitions) both before and after the fermentation process, then you can enter the figures into an equation that will give you the amount of alcohol present in the beer per unit of volume.

I’m going to lay out two different equations to help you work out how much alcohol you have brewed.

The first is the easiest one, which I call the “quick and dirty” calculation, because it gives you a good idea of the alcohol content very quickly, but is not perfectly precise.

The “Quick And Dirty” Alcohol Content Equation

ABV = (OG – FG) x 131.25

This easy equation will give you a rough idea of how much alcohol you’ve produced per unit volume.

But, if you’re something of a stickler for precision, and you don’t mind a little math, you may prefer to use the following equation for your alcohol content.

The “More Precise, Slightly More Complicated” Alcohol Content Equation

ABV = (76.08 x (OG – FG) / (1.775 – OG)) x (FG / 0.794)

So, as you can see, you don’t need some fancy online tool or app to work out how much alcohol is in your home brew, especially if you’re only after a rough idea of how much is in there.

(But I’m going to level with you and admit that I couldn’t work either equation out in my head, but if you’ve got a calculator app on your smartphone as most people do, then you can work it out there.)

And all you really have to do is take two hydrometer measurements before and after fermentation.

But since you don’t actually need a hydrometer for the home brewing process, you may have never used one before, so I’m now going to elaborate more on what a hydrometer is and how to use it for this purpose.

What Exactly Is A Hydrometer Anyway?

A hydrometer is basically just a device that measures the specific gravity of liquid samples.

You’ll find these devices in any decent kitchen shop. They come in various shapes and sizes, but they all look pretty similar.

They consist of a glass tube with a bulbous bottom section, where you put your sample, and a graduated cylinder attached to the top part of the tube.

As you fill up the graduated cylinder, the liquid rises until it reaches the top of the tube, where it stops rising. At that point, the specific gravity of the liquid is recorded.

You can also buy electronic versions of these hydrometers, but they are generally quite expensive, and you won’t be able to measure the specific gravity of every single batch of beer you make.

If you’re in the market for a hydrometer set, you might want to check out this link for a Brewer’s Elite Hydrometer, which I highly recommend.

It’s perfectly suited for measuring the specific gravity of lower alcohol content mixtures such as beer. It comes with a handy protective storage case and an easy-to-follow set of instructions.

It even has a scale for potential alcohol, so if you didn’t want to use the equation, you could go with that instead. You can even buy it with a test jar included in the price.

How To Take A Hydrometer Sample

Please do not put your hydrometer directly into the fermentation vessel. Because if your hydrometer is not perfectly sanitized, you can risk the whole batch of beer being ruined.

How To Measure Alcohol Content With A Hydrometer

Anything that comes into contact with the wort once it has boiled MUST be properly cleaned and sanitized. Otherwise, any new bacteria brought into the mix will ruin it completely, and it will be undrinkable.

This is why you should get yourself a test jar, and take a small sample of the wort in a jar which is large enough to float the hydrometer.

How to take a sample depends on what vessel you are using. With a fermentation bucket (whether plastic or stainless steel) you can simply open the tap (spigot) and run a sample directly into the test jar.

If you are brewing in a glass carboy, however, it is best done using a 100% sanitized turkey baster or wine thief.

Using The Hydrometer To Take A Reading

These are the steps you need to follow to take a hydrometer reading, whether it’s for the original gravity, final gravity, or any other specific gravity reading you might want to take.

Step One

Ok, so first you need to get a sample of your wort from the fermentation vessel and into the test jar, as described earlier.

There needs to be enough liquid in the test jar for the hydrometer to be able to float.

However, you also have to be careful that you don’t add so much that the liquid overflows. Usually about 75% of the way should do just fine.

Step Two

Get the wort to the calibration temperature of the hydrometer.

For most hydrometers, the calibration temperature comes in at 20 degrees centigrade or 68 degrees Fahrenheit.

If the wort is not at the right temperature, this could really throw off the calculation and give you the wrong result for your alcohol content.

Step Three

Gradually lower the hydrometer into the wort. At this stage, you should also get rid of any bubbles that form by giving the hydrometer a gentle spin.

Step Four

Making sure that the hydrometer is not touching the bottom of the test tubes or the sides, you can now take a reading of the gravity.

When liquids are in glass containers, the line at which they stop tends to curve upwards slightly because the molecules are adhering to the glass. This upwards curve is called a meniscus.

Usually, when you take your reading, you should go by the bottom of the meniscus, but you should check this with the instruction manual of your hydrometer in case it says anything different.

Step Five

Once you’ve taken your reading, please remember to give the hydrometer a good clean before packing it away for storage, and prior to taking any other readings.

Frequently Asked Questions

So, by this point, you should understand all the terminology, and understand how to take a sample, and take a sample reading.

You should also be able to follow the quick and dirty alcohol content equation, and use all this knowledge to calculate how much alcohol is in each batch of beer you brew.

However, you may still have questions for me at this stage, so I’m going to do my best to answer the questions that tend to come up the most.

Here goes.

Do I Really Need To Measure Alcohol Content?

In my view, there’s no real need to start measuring your alcohol content from the get-go…

Brewing beer can be a tricky and time-consuming process, so if you’re only just learning how to brew your own beer, I would recommend getting the basics firmly under your belt first before you start doing other things like adding additional flavors or measuring your alcohol content.

That said, if you intend to sell your home brews, any customers you attract may like to know just how much alcohol they’ll be drinking so that they know what to expect before they drink.

If you are following a specific recipe and method to the letter, however, then the alcohol content of each batch should be fairly predictable, and there’ll be little to gain by investing in a hydrometer and taking gravity readings to work out the alcohol content.

When Should I Take a Gravity Reading?

To recap, the time to take the OG reading is at the first stage of brewing, before hops are added, when the sugars are steeped out of the malted barley through mashing.

And the FG reading is to be done when the fermentation process is complete and your beer is made.

However, that said, there may be other times during the process where you may want to take readings…

You can judge the brewing efficiency of your set-up by measuring the gravity before boiling the wort. That way, if you decide the sugar content is too low or too high, then you can either add more malt extract or add more water respectively.

You can also take gravity readings during the fermentation process to judge how well things are progressing.

Sometimes fermentation can get stuck before all the fermentable sugars are used up.

So you might want to check on specific gravity readings after the first 7 days, and then every other day until you get two matching readings, so that you can confirm that the fermentation process is complete.

Can I Pour The Sample Back Into The Fermenter Vessel?

I guess it would be ok to pour the sample back into the fermentation vessel on the proviso that everything that the sample has touched has been completely sanitized.

But, personally, I won’t risk it.

Final Thoughts

So, working out the alcohol content can be pretty straight forward if you go with the quick and dirty method.

But if you know your math and want a more precise method, you can use the more precise, slightly more complicated alcohol content equation that I laid out for you earlier.

Happy brewing!

Andrew Carr
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