A hydrometer is a tool that allows a brewer to calculate the alcohol by volume of a finished beer. The use of a hydrometer to calculate gravity throughout the brewing process is an important step in brewing consistent beer, especially if brewing the same recipe multiple times. Adding a hydrometer to your brewing gear and checking gravity diligently is one of the easiest and cheapest ways of improving your homebrew. Hydrometers allow you to:
- Accurately determine the amount of sugar present in the wort pre-fermentation.
- Identify whether your beer has finished fermenting or if it has become stuck part-way through the fermentation process.
- Calculate the alcohol by volume (ABV) of your homebrew (a good thing to know!)
A hydrometer is a long, thin glass tube with a weighted end at the bottom and 2-3 graduation scales printed running up and down the length of the tube. A hydrometer works by measuring the density (gravity) of the wort in comparison to water.
There are usually 2-3 scales present on a homebrewing-level hydrometer, but only two of them are relevant to most homebrewers: the Brix scale and the Potential Alcohol scale. For the purpose of small-batch homebrewing, I recommend focusing on the potential alcohol scale, which is the easiest to use. The following instructions will utilize the potential alcohol scale for reading gravity and calculating ABV.
If a hydrometer is placed in distilled water, it should read 1.000. As more sugar is included in the wort, the hydrometer will float higher in the liquid, but as the sugar ferments into alcohol (which is lighter, or less dense than unfermented wort), it will float lower in the liquid.
A hydrometer cannot provide ABV based on one measurement alone. To correctly calculate the alcohol content of your beer, you must take a gravity reading before you ferment the wort. This first reading is called the “Original Gravity” and the final reading is known as the “Final Gravity”. Most recipes or kits that your find at your local homebrew shop (LHBS) or online shops like Northern Brewer will have the target Original Gravity and target Final Gravity listed for you, so it is good to compare your readings to theirs to make sure you’re on target.
Make sure you record your Original Gravity after you have cooled the wort but before pitching your yeast. The best way to do this is to grab a sample into the cylinder about halfway through draining the wort from the brew kettle to the fermenter. To take the reading, fill the cylinder about ¾ full of wort (don’t overfill it, the hydrometer will displace some of the worth when inserted) and place the hydrometer inside, allowing it to settle in the center of the wort not touching the sides of the cylinder.
The graduation lines on the Potential Alcohol scale represent specific gravity points. Record the number on the hydrometer where the wort crosses it at the bottom of the meniscus (the lowest point). From the picture, you can see that the original gravity reading of this beer was 1.068.
Once you have recorded the number, you will need to adjust for temperature. The wort’s gravity will be different at different temperatures, so you will need to adjust your OG reading slightly based on the temperature. Most hydrometers are made to assume a temperature of 59 degrees Fahrenheit/15 degrees Celsius. You should know the temperature of your wort from the cooling process after the boil, so using the temperature of the wort and the OG you measured with the hydrometer, use the calculator below to adjust your Original Gravity. For example, the hydrometer in the example above read 1.068, but because it was 72 degrees, we must add 0.0016 to the OG, giving us a corrected OG of 1.0694 (we can round that to 1.070, which is a pretty big beer!). Make sure you record this as your Original Gravity. I do not recommend putting the wort sample into the fermenter after taking the reading! It would add a high risk of contamination, so I generally take the opportunity to taste the wort from the sample at this point and make some notes about pre-fermentation flavor and aroma.
If you are planning to do a secondary fermentation once fermentation has slowed or stopped in your primary, I recommend taking another sample while transferring from primary to secondary following the same steps as above and adjusting for temperature. This reading should give you an idea of whether fermentation is almost done or if it got stuck earlier in the process, leaving excess sugar. If you are not doing a secondary fermentation, I do not recommend opening the fermenter to take a reading halfway through the fermentation process as it could result in contamination. Like the first time, I do not recommend putting the sample back into the fermenter, and instead recommend taking a taste and noting the changes in flavor/aroma. Make sure you record the gravity reading.
Finally, when your beer has finished fermenting and it is time to keg or bottle, you’ll need to take a Final Gravity reading. Follow the same steps as above to use the hydrometer to take the reading and adjust for temperature. For the example I have provided so far, I ended up with a Final Gravity of 1.012
Now you should have your reading for your Original Gravity and your reading for your Final Gravity recorded in your notes (or on your Brew Day Worksheet). To calculate ABV, subtract the Final Gravity from the Original Gravity and multiply by 131.25. More simply put, apply the following formula to the two gravity readings:
(OG-FG) x 131.25 = ABV
So, using the formula for the example I have provided, I would use the formula to calculate my ABV:
(1.069-1.012)x 131.25 = 7.48%
Record your ABV and then bottle or keg your beer. Knowing the ABV of your homebrew helps avoid over-consumption and provides you with a more consistent homebrew, especially as you dial in your favorite recipes.
Don’t want to bother with the math? Check out our ABV Calculator and our ABV Chart to find your ABV based on your gravity readings!
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