Do you really need to own a hydrometer? If you want to know the alcohol content of your beer, wine, mead, or cider – then yes! Or, if you want to know how your fermentation is coming along – then yes! Here’s instructions on how to use a hydrometer for brewing beer and how to determine the alcohol of your beer.
What is a hydrometer and how does it work?
A hydrometer is a simple device that helps home brewers and winemakers determine the alcohol content of their beer and wine. It’s a glass tube with a weight on the bottom and it floats in a sample of wort, must, beer, or wine. Based on how high or low the hydrometer floats, we can determine the amount of sugar suspended in the liquid. By taking one reading before fermentation and one reading after, we can easily measure how much sugar was consumed by the yeast and converted into alcohol.
How to take a proper hydrometer reading
To take a hydrometer reading, collect a sample of wort or beer into a graduated cylinder or hydrometer testing jar. Some homebrewers use the tube that the hydrometer comes in, but they’re really not made for this purpose, and that’s not a recommended way to use a hydrometer.
Carefully suspend the hydrometer in the sample and give it a spin to shake off any bubbles. When it slows, read the measurement (in the form of 1.0xx). Remember, if you want to return the wort or beer to the fermenter, the testing cylinder and the hydrometer should both be sanitized.
There are two keys to taking an accurate hydrometer reading:
- You must take the reading at the level of the top of the liquid sample. Liquids will “climb” up the side of the hydrometer due to the surface tension of the liquid, so measure from the level of the liquid, not where the liquid sticks to the hydrometer (see diagram).
- You must either take the reading at the appropriate temperature, or correct for the difference in temperature.
To expand on the second point, every hydrometer is calibrated for a certain temperature, usually 60 or 68 degrees Fahrenheit. I suggest chilling your wort or beer sample by running it under cold water or sticking it in the freezer – just don’t forget about it! Once you’re relatively close to the calibration temperature, just plug the values into a calculator such as this one to adjust the reading.
How to use a hydrometer to calculate alcohol content
To calculate alcohol content, take one hydrometer reading before fermentation (prior to pitching yeast) and one reading after fermentation. Multiply the difference between the two readings by 131.25.
Starting gravity (SG) = 1.060
Final gravity (FG) = 1.010
1.060 – 1.010 = 0.050
0.050 * 131.25 = 6.56% ABV (alcohol by volume)
Alternatively, you can do the same calculation with gravity points.
60 – 10 = 50
50 * .13125 = 6.56% ABV
While using a hydrometer to measure alcohol makes it important enough to have, by itself, the hydrometer is also valuable for tracking fermentations and for verifying that a fermentation has completed once that activity has topped. There such a thing as stuck fermentations that need to be looked out for.
So, that’s all there is to it! That’s how to use a hydrometer. The explanation gets a little technical, but after a couple tries you’ll have a handle on reading hydrometers in no time!
David Ackley is a beer writer, brewer, and self-described “craft beer crusader.” He holds a General Certificate in Brewing from the Institute of Brewing and Distilling and is founder of the Local Beer Blog.