The question on almost everyone’s lips when they first try a home-brewed beer is: How strong is it? When I first started to brew beer at home, this seemed like an irrelevant question, I was just happy to have made something drinkable. However, I now see that knowing the alcohol content of a beer that you have brewed is actually a really useful tool. Afterall, each recipe is designed with an alcohol content in mind. If the ABV ends up go above or below this, the beer can become unbalanced or, at the very least, not quite the beer it was supposed to be. So, how do we measure this magic number?
What is a hydrometer?
The alcohol content of beer can be calculated using a hydrometer. A hydrometer is a very simple, retro-looking piece of scientific equipment that almost every homebrewer needs. Whether you’re interested in knowing the alcohol content of your brews or not, a hydrometer can still let you know how the brew day went and how the fermentation is going as it progresses.
What do they measure?
Hydrometers do not actually measure alcohol content. This means that you cannot simply drop a hydrometer into finished beer and get a reading. Hydrometers actually measure the density or specific gravity (aka ‘gravity’) of a liquid. In beer, the density is affected by the amount of sugar contained in the solution, the more sugar you extract from your all-grain mash, the higher the density (aka original gravity – gravity at the start of fermentation). This is a useful, as every recipe is formulated to give an exact original gravity. Knowing the original gravity therefore lets you know if you brewed the beer correctly.
During fermentation, the sugar concentration drops as it is converted to alcohol. Therefore, knowing the original gravity and the current or final gravity of the beer lets you know how much sugar has been converted to alcohol, and therefore, allows you to calculate how much alcohol you have in your beer! Monitoring the gravity during fermentation is a great way to monitor the progress. If the gravity continues to drop, then the beer is still fermenting. If the scale stays the same for several days then it is pretty safe to assume that your beer is ready for the bottle.
How do I use a hydrometer?
Each hydrometer is calibrated against pure water and sugar solutions. They each have a scale of specific gravity units on them that will typically (for a brewing hydrometer) range from 0.980 right up to 1.250. To measure the gravity of the liquid simply:
- Ensure the liquid has cooled to room temperature.
- Draw some of the liquid into a tall sample jar.
- Drop the hydrometer into the liquid.
- Gently spin the hydrometer.
- Move your eye-line down to the level of the hydrometer (don’t lift the hydrometer to your eye-line).
- The liquid at the top of the trial jar will have a curve to the surface known as a meniscus. The reading of the hydrometer is taken from the bottom of the meniscus, not the top
- Whatever the number at the bottom of the meniscus is, that is the density of the liquid.
To measure the alcohol content of your beer simply do this process immediately after brewing, prior to pitching yeast. This is your OG (original gravity). Then, continue to repeat this process during fermentation. When the gravity remains constant, the beer is finished. The gravity at this point is known as the FG (final gravity). To convert these numbers into an ABV simply use the following equation:
ABV = (OG – FG) X 131.25
EXAMPLE, OG = 1.050, FG = 1.008
ABV = (1.050 – 1.008) X 131.25 = 5.5 %
Now you know! Whether you want to measure the alcohol of your beer of simply know when it’s done, a hydrometer can help you to achieve this and now you have all the tools you need to measure up!
Happy Brewing,
Jim