Knowing how much alcohol is in your home-brew beer is important for many reasons. Luckily, if you have a hydrometer handy, it’s really easy to measure exactly how much alcohol is present in your home-brew.
So how exactly do you measure the alcohol content of your home-brew? In order to accurately calculate the alcohol content of your beer you need to subtract the Final Gravity (density after fermentation) from the Original Gravity (density before fermentation) then multiply the result by 131.25. Formula: (OG -FG) x 131.25. Example: (1.050 – 1.010) x 131.25 =5.25 ABV%
It’s one of the questions that many of us ask when drinking a beer, how strong is it? Of course, we mean how much alcohol it has in it.
Whether you are looking for something to really knock your socks off like a Belgian Trippel or something like a light beer so that you can get up for work in the morning, ABV matters!
In this article I have gone not only answer how to calculate the alcohol content of your home-brew beer, I have also tried to think of some other question you might have and have researched the answers to them too. Read on for more details on measuring ABV in beer.
What is alcohol by volume exactly and how is it determined?
ABV stands for alcohol by volume and is an international measurement of the alcohol content in drinks.
It actually measures the total pure ethanol content in milliliters per 100 ml (around 3.4 fl oz) at a temperature of 68°F (20°C).
As alcohol is less dense than water, the closer the final gravity to 1.099 the higher the ABV is going to be. As a brewer, we can determine the ABV of our home-brew beer by measuring its specific gravity at certain times of the brewing process.
How to measure your Original Gravity and Final Gravity
In order to find out your ABV, you need to know your OG and FG (original gravity and final gravity). By this, we mean just the density of your wort or beer at two particular times during the brew.
Most people will say that you want to take your original gravity reading just before you pitch your beer. However, it is really advisable that you take several readings during different stages of the process such as post-mashing, during the boil and just before the end of the boil. Although hydrometers are a good instrument, to save wasting wort and time, consider investing in a refractometer (see my recommended gear).
As a general rule for a pre-pitching sample, this measurement should be taken when the beer is between 80°F-68°F (26°C-20°C) and when the wort is in the fermenter. Take a small sample of your wort and use a hydrometer or refractometer to gain a density measurement.
Make sure that you are reading the hydrometer correctly and write the information down somewhere. See my article about hydrometers for more tips and advice.
Final gravity should be taken only after the fermentation process has completely finished. Again, you want to wait until the newly brewed beer is cooled enough to take an accurate reading.
You should notice that the FG is a lower number than the OG. This will show that the yeast in your wort has converted the sugars present at the beginning of the process into lovely ethanol and CO2. You should also have a target FG which is given for the recipe that you are following. If your FG seems higher than the expected gravity given, this probably means that fermentation hasn’t ended yet.
How to tell when fermentation is over.
Your FG is a good indicator of the progression of the fermentation stage of the brew. Fermentation requires the participation of yeast, tiny little fungi, which transform your beer wort soup into something useful, tasty beer. However, fermentation requires a lot of factors to ensure that it takes place successfully. If any of these variables are not perfect, your fermentation may take longer than expected, or not at all.
Usually, we can see when fermentation is taking place from the bubbling of CO2 on the surface of your wort, or through your airlock and also the presence of krausen (see my definition).
But, sometimes we may think everything is going well but then your fermentation slows or stops. If you are in doubt, let your brew settle for another day or so and take further density readings with a hydrometer. If the FG doesn’t change but is still high, then you may have made an error earlier in the brewing process. If you want more details on issues with fermentation, check out my article on it.
How to measure the Original Gravity and Final Gravity to get ABV without a hydrometer
Use a refractometer
If you don’t have a hydrometer, you can use a device called a refractometer which measures the amount that light is refracted through your wort or beer. It will give you a reading directly on the screen or viewing plate.
The refractometer will give you a result in Brix, and you will have to convert this in order to get a specific gravity reading.
The formula is: 1 + (0.004 * Brix). E.g 11 Brix becomes 1 +(0.004 * 13) = 1.052
Another quicker method is the ‘by four’ rule, just multiply the Brix by four and you will get the decimal amount after 1.0. For example, 5 would give you (5 * 4) 20 or 1.020.
Don’t have a hydrometer or a refractometer?
You can work out the specific gravity of your wort or beer by weighing it.
Density can be determined by dividing mass by volume, (D = M/V) so if you weigh your wort or beer (account for the container it is in) and you know exactly how much liquid is there in gallons (or liters), you take the weight in pounds or Kilograms and divide it by the gallons or liters.
As a rough guide, 0.001 SG is equal to 3/4 of a pound.
How to measure the alcohol content of beer without knowing the Original Gravity
Sometimes as brewers we can forget important steps. Often for newer brewers, these vital acts aren’t fully appreciated until the end of the first brew. Not taking an accurate Original Gravity reading before pitching is a common mistake, but doesn’t it mean your ABV will always remain a mystery to you? Not necessarily.
It is possible to calculate your ABV for your beer by using both a hydrometer and a refractometer.
If you get an accurate density reading with your hydrometer and take a reading of the same beer with a refractometer you can calculate a fairly close ABV with the FG and Brix reading. There are plenty of apps and calculators online to do this, so don’t worry about the Math.
If you don’t have either of these fairly inexpensive tools, then the only other way I can think of to get an accurate ABV involved breaking the law, spending a night in the police station and then calculating your blood alcohol content back into ABV. Not worth it really just to solve this one, eh Watson?
How much alcohol has a standard drink got in it?
Many countries have different standard drink measurements and so the amount of alcohol in them changes from nation to nation.
If we take the example of a beer with an ABV of 4.5%, let’s look at the number of standard drinks a 16 fl oz brewski (500ml) would represent.
In the United States, a 16 fl oz beer with an ABV of 4.5% would be 1.27 standard drinks, in Australia the same drink would be equal to 2 standard drinks. It’s 1.4 standard drinks in Canada and makes up 2 standard drinks in both New Zealand and Ireland. What about the UK? Well not only do we drive on the correct side of the road (jokes!), and spell things with “U” and “S”, we also use a different system to talk about alcohol consumption. It’s simply called units of alcohol.
How to convert alcohol by volume (ABV) into units of alcohol (UK).
In the United Kingdom, we don’t use the idea of standard drinks but talk about units of alcohol consumed.
If you are a fellow Brit and you need to know how many units of alcohol your home-brew has in it so that you can keep your GP happy, there is a fairly easy formula you can use.
To get the unit of alcohol you just need to multiply the volume of alcohol (in ml) by the ABV then divide this number by 1000.
For example, an imperial pint is the same as 568 ml. Let’s say it has an ABV of 6%
568 x 6 = 3408 / 1000 = 3.408 so 3.4 units of alcohol.
If we were talking about a standard brew of 5 gallons (18927.1 ml) with the same ABV you would have 113.5 units of alcohol on your hands.
18927.1 x 6 =113,562.6 / 1000 = 113.5 units
How to adjust your beer’s specific gravity to get your target ABV
The ABV, which is the volume of alcohol in your finished beer is wholly dependent on how much ethanol yeast can produce from the wort it is given to process. If the yeast isn’t given the right amount of sugars to convert, it will not produce enough alcohol for you to meet your expected FG. So, what can you do to adjust your specific gravity, in this case, OG, in order to hit your target ABV or thereabouts?
This is where following the recipe and supplier’s instructions come into their own. If when you take your original gravity reading just before priming your wort and discover that it is way off the expected OG you were given, it’s time to act. In fact, this type of problem is going to be easier to spot if you take several readings throughout the process, especially before the boil.
If your OG is too low
If you find that your original Gravity is lower than you had anticipated, there is a fairly easy fix. By adding some Dry Malt Extract (DME) you are going to be able to raise your wort’s density.
What you need to do is to measure the difference between your estimated OG and your actual OG. You then multiply that difference by 1000 and this will give you the ‘points’ you need to raise your wort’s specific gravity by.
For example, if your target OG is 1.065 and your actual OG is 1.042 you need to change the density by 23 points ( 1.065 – 1.042 x 1000 = 23).
This means that you need to add a quantity of DME which is equivalent to 23 points per gallon of DME. Right, are you confused because I was?
What this means is that we need to know more about DME and how it relates to Specific Gravity.
So, DME has a potential of 1.046 and will add 46 points for every pound of DME added to your wort. So, in our case we only need 23 points, so divide 23 by 46 and you get 0.5 or half a pound of DME.
Of course, if you are brewing more than a gallon of future beer, you need to adjust those sums accordingly.
When should you add the DME?
This obviously depends on when you discovered the difference in your OG and the one you were aiming to get. If your wort is cool and you are about to pitch it, you may have to just proceed with what you have.
However, if you have discovered the mistake during the boil, you can simply extend the process by an extra 10 to 15 minutes or so to really sanitize the DME and remove the risk of a later bacteria infection in your beer.
The extra boil time might affect the bitterness of the beer, but this might be a good compromise if you want to hit your expected FG. To avoid altering the IBUs of your finished product, you can get into a habit of taking a measurement of the OG 20 minutes before the end of the boil and that’ll give you time to act without extending the boil time.
If you have managed to spot the difference before the boil, your task is much easier and you can add the DME there and then to bring your OG up to the expected OG.
What if your OG is too high?
Now, this may seem a more alarming prospect because it has to be easier to increase something in a recipe than decrease it, right? Well not really.
If your OG is much higher than anticipated, then you can just use water to dilute it.
If your target OG is 1.055 and your actual OG is 1.065. If you divide 65 by 55 you get 1.181. Assuming that we have a yield of 1 gallon in order to restore the OG to its intended level you need to add 0.181 gallons of water.
What is the alcohol content of the most popular home-brew beers?
This is a list of the top voted home-brew recipes from homebrewtalk.com and their potential ABV%. On average, these home-brew recipes will produce a beer with an average ABV of 6.17%