Introduction: Measuring Alcohol Content With a Hydrometer

The first couple times you brew your own beer or wine, you're more concerned about the end product - delicious, delicious alcohol. The exact amount is only a secondary concern.

As you start getting more and more into the hobby, knowing the percent alcohol can be useful. Both for knowing what to expect in your final product, but also for monitoring progress in the fermentation process.

In this Instructable, I will go over how to use a hydrometer to take density measurements of your brew, and how to transform these density measurements into a more familiar percent alcohol.

You will need:

  • A Hydrometer
  • Sterilizing Equipment
  • Something to Measure
  • Pipette or Baster to Take a Sample

Step 1: Your Hydrometer

A hydrometer is a tool used for measuring density. Hydrometers used for brewing will tell you the specific gravity of a liquid - a ratio of the liquid's density to that of water. Something with a SG of less than 1 is less dense than water (and will float), and something with a SG of above 1 is more dense and will sink.

The hydrometer is essentially a specially weighted bob - you place it in a liquid, and it will sink to a certain depth, which depends on the density of the liquid. My seeing how far it sinks, you can determine the liquid's density.

Since alcohol has a different density than the sugar/water mixture you started with, you can calculate the percent alcohol by measuring the initial SG and the final SG.

Step 2: Sanitizing

As with any homebrewing project, sanitation is critical. There's nothing worse than an infected brew.

Before beginning, mix up some sanitizer. I personally use Easy Clean, and mix up a one gallon batch following the instructions on the back (one tbsp per gallon warm water). Easy Clean is a no-rinse sanitizer, meaning that it isn't necessary to rinse your equipment after cleaning them (which could potentially reintroduce bacteria).

Sanitize all the equipment you'll be using - the hydrometer, the hydrometer tube, and whatever you'll be drawing your sample with.

And of course, thoroughly wash your hands before continuing.

Step 3: Measuring

First, place the freshly sanitized hydrometer in its tube. Using your sampler, draw off a small sample of your brew from the middle.

Begin filling the hydrometer tube until the hydrometer begins to float freely, and then stop filling. If your equipment was properly sanitized, it is safe to return the excess liquid.

Place the hydrometer on a flat, level surface, and gently spin the hydrometer back and forth to release any bubbles that may have formed on its surface. Ensure the hydrometer is floating freely, and is not touching the sides of the hydrometer tube.

From eye-level, read the hydrometer's measurment from the bottom of the meniscus. The meniscus is the curved surface caused by surface tension - and can make a significant difference in your results if you are not consistent.

Note down your reading, and the date the measurement was taken.

From here, you can return the sample to the larger brew, assuming all the equipment was properly handled and sanitized.

Step 4: Calculation

Now, it's time to turn the specific gravity readings into a useful number.

For an approximate estimation, the magic number is 131.25. Your percent alcohol can be given by the formula:

ABV(%) = (Initial Gravity - Final Gravity) * 131.25.

So if your initial gravity was 1.108, and your final gravity was 1.041, your beer is approximately 8.79% alcohol by volume.

Once again, this is only an approximation, and loses accuracy as the alcohol content goes up. For a more precise number, or if you're brewing something particularly strong, there exist a number of online calculators that can do the heavy lifting for you.

Thanks for reading, brew and enjoy responsibly!

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