A hydrometer works by measuring the approximate alcohol level of your fermented beverage by comparing the concentration of sugar in the liquid before fermenting and after fermenting. The alcohol level is assumed by the decrease in sugar concentration.
Why Should you use a hydrometer?
Here are a few reasons why homebrewers use hydrometers.
- It measures the sugar concentration prior to fermenting. This allows you to add more sugar, if needed, to hit the level required for the recipe. This is particularly important when brewing cider and wine with whole fruit or homemade juice. Sugar levels of fruit can vary widely based on variety and ripeness.
- A hydrometer helps determine when the beverage is ready for bottling. Particularly important for hard cider, which is often bottled before all the sugar is consumed.
- It can help you determine if a ferment has stalled early. If fermentation has stopped, and there’s still plenty of sugar in the beverage, you likely need to add some yeast nutrient to kick-start it.
- It’s also nice to know the final ABV. There’s a big difference between beer that is 4% or 9% ABV!
When to use a hydrometer
The hydrometer is used to determine the final alcohol level, as well as to figure out when something is ready for bottling. Here is when to use the hydrometer:
- Beer: Beer is usually ready for a Final Gravity reading after 4 weeks in the carboy.
- Wine: I recommend testing wine when racking to a clean jug, and again when you bottle it. It can take a while for all of the sugars to be consumed in a dry wine.
- Cider: 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.
Step-by-step Guide to Using a hydrometer
Original Gravity Reading: Before Fermentation
The original gravity is the sugar concentration in your beer/wine/cider before the yeast is added. Here’s how to take an original gravity reading:
- Using a sanitized siphon, wine thief, or 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). You need to use enough liquid to fully suspend the hydrometer. If the hydrometer isn’t floating then add 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 calculate the final ABV. I have a fermentation diary that I use for all my complex ferments (beer, wine, cheese). Some people write directly on the carboy.
Final Gravity Reading: After Fermentation
Take a Final Gravity reading when bottling your beverage. (I often take an extra reading, when racking to a clean carboy just to test whether everything is fermenting nicely).
Here’s how to take the final gravity reading:
- Using a sanitized siphon, draw a sample.
- Pour the liquid into the testing cylinder and record the reading from the bottom of the meniscus. This second reading is the Final Gravity.
Calculating Alcohol Level
Calculate the alcohol level 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. (It should be a smaller number, and it probably won’t be zero).
- Calculate the difference between the original potential alcohol level and the remaining potential alcohol level to find out the actual alcohol.
Using the photos above, here’s how to calculate the alcohol in my perry:
- My Original Gravity reading was 1.080, which has a potential alcohol level of 10.2% before fermentation.
- My Final Gravity reading was 1.012, which has a potential alcohol level of 1.3%.
- The actual alcohol level was 8.9% (10.2 – 1.3 = 8.9).
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.