How to Use a Hydrometer

One question we get from a lot of new homebrewers is, “How do I know how strong my beer will be?”  Luckily, the answer is easy: use a hydrometer!

In brewing, hydrometers measure the density of beer and wort. Alcohol content can be calculated based on these densities. Using a hydrometer is easy if you follow a few simple steps to ensure consistent and accurate results.

What Is A Hydrometer?

A hydrometer is a basic instrument used by brewers to figure out the alcohol content of a beer. They are one of the first pieces of equipment you learn how to use as a homebrewer. Very easy to use, hydrometers are a handy tool for any brewer from beginner to professional.

Hydrometers float in liquid -- for brewers that means wort or beer -- to measure its density. 

Usually made from glass, hydrometers have an unmistakable shape. A long skinny tube connects to a thicker, heavy base. Along the tube is a graduated marking with a density scale useful in the range for brewing.

Across the brewing, wine-making, and distilling worlds, there are several common units of measure:

  • Specific Gravity (SG): The most common unit for U.S. homebrewers. SG is the ratio of the density of a test liquid relative to the density of water. Water has a specific gravity of 1.000 and fluids with sugars, like wort, are denser and have an SG higher than 1.000. 
  • Plato:  Used mostly by professional brewers. Plato is a measure of the concentration of dissolved solids (mainly sugars) in a wort. 1-degree Plato equals roughly 4 points of specific gravity
  • Brix: Mainly used in wine-making, one degree Brix is 1 gram of sucrose in 100 grams of solution.

Since SG is most common in homebrewing, we’ll be focusing on this unit.

When To Use A Hydrometer

Measuring the specific gravity of your beer provides useful information about the brewing process. Brewhouse and fermentation efficiency, gravity stability, and alcohol percentage are all important to know for brewers.

There are a few crucial times to check the gravity of your beer:

OG

Once you’ve boiled and chilled your brew, you’re ready to pitch the yeast and start the fermentation process. It’s important to measure the gravity at this point. This is called the original gravity (OG). The OG of your wort tells you the potential for alcohol production. For beer, OGs can range from about 1.030 to 1.100. For a 5% beer, expect an OG of about 1.050.

FG

The final gravity (FG) of your beer will let you calculate the alcohol content. When the gravity is constant over a few days, your beer is done fermenting. The FG of beer ranges between 1.000 and 1.020. The lower the FG, the drier the beer. A dry beer will generally have a thinner mouthfeel where a higher FG beer will be sweeter and fuller.

During Fermentation

Aside from the OG and FG, it is useful to take readings during fermentation to track the beer’s progress. Some brewers like to dry hop at specific gravities during fermentation. For brewers who naturally carbonate in the fermenter, they need to know how much sugar is left to ferment.

How Does A Hydrometer Work?

Hydrometers use the principle of buoyancy to produce readings of liquid density. Brewers use specifically calibrated hydrometers that measure the density of wort and beer. 

How do you read a hydrometer?

To get the best reading out of your hydrometer, use an adequately sized hydrometer jar. These should be about 10-14” tall. Some larger hydrometers might need larger test jars. It should be made of clear glass or plastic.

How to Measure Alcohol with a Hydrometer - Home Winemaking

When you float your hydrometer in your beer or wort, the top of the liquid will line up with a marking on the hydrometer’s scale. Looking from eye level, observe where the liquid crosses the graduated tube. Due to surface tension, the liquid will curve upward. Take the reading from the low point of the curve -- called the meniscus.

How do you calibrate a hydrometer?

All hydrometers should come calibrated to measure water at the temperature written on the side of the tool. At sea level, water should read an SG of 1.000. To calibrate your hydrometer, check the gravity of distilled water at the rated temperature. Most hydrometers are rated at 60F or 68F. If it reads 1.000, your hydrometer is properly calibrated. If it reads low, like 0.998 -- then you know to add 0.002 to all hydrometer readings. Similarly, if it reads 1.002, subtract 0.002 from future readings.

There are ways to correct the weight of the hydrometer to fix the reading. You can add mass or gently file away material. This can be tricky and dangerous.  Be careful when working with hydrometers as they are fragile.

Types Of Hydrometers

Hydrometers are used to measure buoyancy in fluids in various industries. For brewing, you want to seek out specifically designed models for beer or wine. There are a few different types of hydrometers so make sure you buy the right one.

Triple Scale Hydrometer

Triple scale hydrometers are the most common type that you’ll see. Most starter homebrew kits come with one of these. A triple scale hydrometer is usually about 9-12 inches long. They usually measure specific gravity, potential alcohol content, and Brix. These can range in price and the more you pay, the higher the precision. These are great tools and suitable for most brewers who want to have a close estimate of alcohol content.

Narrow Range Hydrometer

Narrow range hydrometers are specialized tools to measure specific gravity with a high degree of accuracy. When monitoring fermentation, it’s important to know when it has stopped so you can package safely. A hydrometer with a narrow range of specific gravity between  .990-1.020 lets you see exactly when your beer has finished fermenting. These are more common in professional breweries.

Hydrometer With Correction Scale

You don’t have to worry about making any temperature corrections with this type of hydrometer -- it does it for you! These nifty tools have a built in thermometer. You can instantly correct temperature differences using the conversion chart on the hydrometer.

How To Use A Hydrometer

Hydrometers are popular because they’re inexpensive, accurate, and easy to use. Follow these basic steps and you’ll be sure to gather consistent readings.

Pull a sample

Make sure your test jar and hydrometer are both clean. Any debris or dried beer on your hydrometer can throw off the reading.

The best way to pull a sample is through a valve. If your kettle or fermenter has a valve, use that to fill the test jar. 

If you don’t have a valve, you can grab a sample using a device called a “wine thief”. These are useful when fermenting in carboys as you can slide a thief through the narrow neck to pull a sample. If you don’t have a wine thief, a turkey baster works well too. Always be sure that anything that comes into contact with the beer is sanitized.

We don’t recommend floating the hydrometer directly in the wort or beer. If it happens to break, the shattered glass will ruin your batch.

Fill the test jar almost to the top to make reading the hydrometer easier.

Take a reading

Once you have filled your test jar with beer/wort, carefully drop in the hydrometer. A bit of liquid might overflow due to volume displacement, so watch out for spills. 

Wait until the hydrometer stops bobbing or spinning and take the reading.

Record the reading

Keeping detailed records of each reading helps track your beer’s progress. Record the date and gravity reading, including temperature. You can use a brewing journal or homebrew software/app to keep track of your gravities.

Make any temperature correction, as needed, using a chart or an online calculator

Once the FG is stable over a few days, you know your beer has finished fermenting and it’s safe to bottle or keg.

Calculating ABV

To calculate the alcohol content of your beer, you need your OG and your FG. Find the ABV by applying the following simple formula (OG and FG in units of specific gravity): 

(FG − OG) × 131.25 = ABV 

If you forget this formula, there are plenty of online calculators to help.

Conclusion

Hydrometers are an elegant and useful tool for measuring the gravity of your beer. Their ease of use and precision have made them a mainstay in the brewing world, both professionally and at home.

By properly reading and handling your hydrometer, you’ll hone in on your brewing process. Tracking the gravity of your brew will help make consistent and great quality beer.

FAQ

What does a hydrometer measure?

Hydrometers measure the density of fluids. In brewing, that means sugar concentration in wort or beer. The higher the density, the higher the sugar content. As beer ferments, sugar is converted into alcohol and carbon dioxide. The density of the beer decreases as this conversion takes place.

Can I measure the alcohol of a beer without its original gravity (OG)?

If you forgot to take your OG, there is no way to know your ABV. What you can do is estimate the OG based on the recipe. From that, you can calculate an estimated ABV using the FG of the beer.

My reading seems High / Low. Why?

We often hear about homebrewers frustrated with inexplicably high or low gravity readings. There are a few troubleshooting tips to consider before buying a new hydrometer:

  • Calibrate hydrometer: check that your hydrometer reads 1.000 in distilled water. If it is off, perform the calibration steps that we described above.
  • Correct for temperature: make sure you take readings at your hydrometer’s rated temperature, otherwise you need to correct for temperature. As density changes with temperature, colder fluids will give higher SG readings and hotter fluids will read lower.
  • Clean hydrometer: ensure your hydrometer is clean. Any stuck on debris could give you a false reading.
  • Ensure well-mixed wort: Stratification can cause heavier sugars to sink to the bottom of your kettle or fermenter. When taking your OG, ensure that your wort is well mixed and homogenous. This is especially important in malt extract batches where you mix cold water with concentrated wort in the fermenter.
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