A hydrometer measures the approximate alcohol levels of your fermented beverage, by comparing the amount of sugar in your mixture before fermenting and after fermenting. The level of alcohol is assumed by the decrease in the sugar levels. Using a hydrometer can help you determine when your beverage is ready for bottling. It’s also nice to know if your beer is 4% or 9%.
Original Gravity Reading: Before Fermentation
You need to know the sugar levels in your beverage before you add the yeast. This is the point before the yeast to converts the sugars to alcohol.
- Using your sterilized siphon, a wine thief or even a turkey baster, take a small sample of your beer/wine/cider right before you add the yeast.
- Transfer the liquid to your testing cylinder (one usually comes with your hydrometer). There should be enough liquid to fully suspend the hydrometer. If the hydrometer isn’t floating then you need more liquid.
- Make sure that your hydrometer isn’t touching the sidewalls of the cylinder and take the measurement from the bottom of the meniscus.
- It’s important to record the Original Gravity reading. I have a fermentation diary that I use for all my complex ferments –beer, wine, cheese -. Some people write it on the carboy.
Final Gravity Reading: After Fermentation
Take a Final Gravity reading once fermentation is nearing the end. Using a sterilized siphon, draw your sample and record your reading as described above. This second reading is your Final Gravity.
With beer this will be after about 4 weeks in the carboy. I recommend testing wine after the primary fermentation is over and again before you bottle, as it can take a while for all of the sugars to be consumed in a dry wine. Fermentation for cider is generally halted before all the sugars are used up (otherwise you would end up with wine!) So test before bottling for an approximate alcohol level.
Calculating Alcohol Levels
Calculating alcohol levels is done with by converting the specific gravity reading to a potential alcohol level using a handy table. You probably got a copy of this table with your hydrometer, so don’t throw your paperwork out!
Regardless of the scale used the calculation is pretty simple.
- Take your Original Gravity reading and look at the table to figure out the POTENTIAL alcohol level.
- Take your Final Gravity reading and look at the table to figure out the REMAINING POTENTIAL alcohol level.
- Calculate the difference between the original potential alcohol level and the remaining potential alcohol level to find out the actual alcohol level of your beer/wine/cider.
- My Original Gravity reading is 1.060, which has a potential alcohol level of 7.8% before fermentation.
- My Final Gravity reading is 1.012, which has a potential alcohol level of 1.3%.
- The actual alcohol level is 6.5% (7.8 – 1.3 = 6.5).
If math really isn’t your thing, then you can just put your Original Gravity and Final Gravity readings into this handy calculator to figure out the alcohol level.