A hydrometer (affiliate link) measures the approximate alcohol levels of your fermented beverage by comparing the amount of sugars in the liquid 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 will measure how much sugar there is before the yeast to converts the sugars to alcohol.
- Using your sterilized siphon, a wine thief or a turkey baster, take a small sample of your beer/wine/cider right before you add the yeast.
- Transfer the liquid to a testing cylinder (one usually comes with the 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 the 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 to keep track. I have a fermentation diary that I use for all my complex ferments -beer, wine, cheese -. Some people write it directly on the carboy.
Final Gravity Reading: After Fermentation
Take a Final Gravity reading once fermentation is nearing the end. Using a sterilized siphon, draw a sample and record the reading as described above. This second reading is the Final Gravity.
With beer take a reading after 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. 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 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 the paperwork out!
Regardless of the scale used the calculation is pretty simple.
- Take the Original Gravity reading and look at the table to figure out the POTENTIAL alcohol level.
- Take the 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 in 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.