A hydrometer is a tool that measures the specific gravity of a liquid, which makes it a useful tool for home brewing to estimate the alcohol content of a brew. Some hydrometers may have different scales on them, making it harder for new brewers to know which scale or read.

How do you read a hydrometer? Fill the graduated cylinder with your target liquid, lower the hydrometer into the liquid, then read the numbers off the scale at the bottom of the meniscus of the liquid.

Since different factors can impact the accuracy of the ready, I’ll explain in more detail how to get the most accurate reading, some recommendations if you need to factor in a correction, and other alternatives to using a hydrometer.

Reading the Hydrometer

When you buy your hydrometer, make sure that the product will have a scale for specific gravity. Specific gravity is used to help determine the alcohol content of the brew.

I like using a hydrometer with a plastic graduated cylinder, after having broken two glass ones by being clumsy, like this one from Amazon:

Measure the specific gravity of your brew before fermentation, after it’s cooled and you’ve pitched your yeast, but before you put in the airlock. You can read more about getting started home brewing in my post here. Check the specific gravity periodically while the brew is fermenting. If the specific gravity stays consistent for a week or so, then the fermentation process is likely done.

Subtract your final specific gravity (final gravity) reading from your original reading, then multiply by 131.25 to get your alcohol percentage, or ABV %.

However, reading the tiny numbers on your hydrometer can be difficult, especially when your brew is dark, cloudy, or bubbly. Most hydrometers have tick marks on their scale about every 0.002. Even if you misread your number by 0.002, your ABV is likely only off by 0.2-0.3%.

To help read the number easier if there are bubbles on the surface, pinch the top of the hydrometer and spin with your fingertips very lightly. Once the hydrometer stops spinning and bobbing, the bubbles have hopefully been pushed away.

If the brew is dark, try using a flashlight to help shine light through the brew. This may also help if your brew if fairly cloudy.

Correction Factors for Specific Gravity

Most hydrometers are calibrated to measure specific gravity at a certain temperature – usually around 68-70F. If your brew is especially colder or hotter, then you may need to recalculate the specific gravity reading to correct for temperature.

Temperature effects the density of liquids. It’s easy to notice that water will expand in volume when frozen into ice cubes. However, even as a liquid, the volume of water will expand slightly at lower temperatures, making it less dense. The same is true for other liquids, such as alcohol, as well.

Brewer’s friend has a very easy online calculator to help you correct your specific gravity for temperature. You can find their calculator here.

Other factors may impact the reading, but you may not be able to correct for it. Dissolved gases, such as carbon dioxide, can make the liquid seem much lighter. If you take a reading that indicates a lot higher alcohol content than you would have expended, it may be due to an inaccuracy with the reading. Additives help remove undesirable dissolved gasses.

Alternative to Using Hydrometers

A cool alternative to using a hydrometer while home brewing may be using a brix refractometer. These devices use a drop of the liquid on the viewing plate, which uses far less of your brew than the hydrometer. You can read the specific gravity by looking through the lens.

The liquid will refract a line on the scale like this:

The trouble with these devices is that the get less accurate with the presence of more alcohol. The measurement will be highly accurate before the fermentation process begins, and get less accurate towards the finish. However, you will see when fermentation is done when the reading does not change over a couple of days.

Final Recommendations

To draw out wort to measure the specific gravity, feel free to use something like a turkey baster. I also like using a device called a wine thief, which resulted in far less mess. You can insert the collection tube into narrow openings, place your thumb over the top, then release your thumb, and the liquid, into your graduated cylinder. For a refractometer, disposable, sterile pipettes are quick and easy to use, unless you don’t like wasting plastic.

While conducting your readings, always, always remember to sanitize anything and everything that comes into contact with your brew!

Sugar Measurements via Hydrometer by James Fischer is licensed under CC by 2.0