A hydrometer is one of the simplest and most useful tools in home brewing. It measures the density of wort or fermented beer compared with water, helping you track fermentation, estimate alcohol content, and decide when a batch is ready to package.
What a Hydrometer Measures
A brewing hydrometer measures specific gravity, usually abbreviated as SG. Pure water has a specific gravity of approximately 1.000 at the instrument’s calibration temperature. Wort containing dissolved sugars is denser, so an unfermented beer might have a reading such as 1.050. As yeast converts sugar into alcohol and carbon dioxide, the liquid becomes less dense and the reading falls.
Three readings are especially useful:
- Original gravity (OG): The gravity before yeast is added.
- Final gravity (FG): The gravity after fermentation is complete.
- Intermediate gravity: Any reading taken during fermentation to monitor progress.
The difference between OG and FG can be used to estimate alcohol by volume. A commonly used approximation is:
ABV = (OG - FG) × 131.25
For example, a beer that starts at 1.050 and finishes at 1.010 has an estimated alcohol content of about 5.25% ABV. This is an estimate rather than a laboratory measurement, especially for beers with unusual ingredients, very high gravity, or significant fermentation byproducts.
Equipment You Need
Gather the following before taking a reading:
- A clean, sanitized hydrometer
- A hydrometer test jar or tall, narrow sample cylinder
- A sanitized wine thief, turkey baster, pipette, or sampling valve
- A thermometer
- A small flashlight, if reading the scale is difficult
- Brewing notes or a digital tracker
- Sanitizer suitable for brewing equipment
A hydrometer must float freely. It should not touch the sides or bottom of the test jar, because contact can make the reading inaccurate. The sample jar should be tall enough for the hydrometer to float with several centimeters of liquid above its bottom.
Sanitation matters even though the sample is not going back into the fermenter. Dirty sampling equipment can contaminate the batch, and a sample taken from an actively fermenting beer can also expose the liquid to unnecessary oxygen. Clean and sanitize everything that will touch the wort or beer.
Check the Hydrometer Before Brewing
Most brewing hydrometers are calibrated at either 60°F (15.6°C) or 68°F (20°C). The calibration temperature is usually printed on the hydrometer or its packaging. The scale is most accurate when the sample is near that temperature.
Before using a new hydrometer, inspect it for cracks, chips, or liquid inside the glass. Gently place it in a container of clean water at the calibration temperature. It should read close to 1.000. A small difference may be caused by temperature, manufacturing tolerance, or the way the scale is read. Record any consistent offset so you can account for it later.
Do not test the hydrometer in hot wort. Sudden temperature changes can damage the glass, and hot samples produce misleading readings. If your sample is too warm, cool it in a sealed or covered container before measuring.
How to Take an Accurate Hydrometer Reading
Follow these steps for a reliable reading:
- Sanitize the equipment. Sanitize the hydrometer, test jar, sampling tool, and thermometer. Avoid wiping sanitized equipment with an unclean towel.
- Mix or swirl only when appropriate. Before pitching yeast, stir the wort thoroughly so sugars are evenly distributed. After fermentation begins, avoid vigorous stirring because it can introduce oxygen and disturb the yeast or sediment.
- Collect a representative sample. Draw enough liquid to fill the test jar so the hydrometer can float freely. Take the sample from the liquid, not from a thick layer of trub or settled yeast.
- Transfer the sample carefully. Pour slowly down the side of the test jar to reduce foam. If foam forms, wait for it to settle before reading.
- Remove bubbles. Small bubbles can cling to the hydrometer and make it float higher than it should. Gently spin the hydrometer once or twice to release them. Do not flick or shake it aggressively.
- Let the hydrometer settle. Wait until it stops moving and is not touching the container. Read at eye level.
- Read the bottom of the meniscus. Most brewing hydrometers create a curved surface where the liquid meets the glass. For a clear sample, read the lowest point of that curve. Use the same method every time.
- Measure the temperature. Record the sample temperature along with the displayed gravity.
- Write down the result. Include the date, gravity, temperature, batch name, and any relevant observations.
Take the sample out of the test jar before adjusting the hydrometer or moving the jar. Glass hydrometers break easily, and a broken instrument can leave sharp fragments in the sample.
Taking the Original Gravity Reading
Original gravity should be measured after the wort is completely mixed and before yeast is pitched. This is important for extract brewing, where dense syrup or dissolved malt extract can settle before the wort is diluted to its final volume.
For an extract batch, stir thoroughly after topping up with water. Do not rely on a sample from the top of the kettle or fermenter, because the sugar concentration may not be uniform. Once the wort is mixed, cool a sanitized sample to the hydrometer’s working temperature and take the reading.
For an all-grain batch, you can measure gravity before the boil, after the boil, or at both points. The post-boil reading is usually the OG used for calculating alcohol. If the gravity is higher than expected, dilution with sanitized water may be possible before fermentation, provided the water is suitable for brewing and the batch remains properly mixed. If the gravity is lower than expected, boiling longer can concentrate the wort before yeast is added.
Record the measured OG rather than substituting a recipe’s predicted value. The actual result reflects your efficiency, volume, evaporation, ingredient differences, and measurement technique.
Measuring Fermentation Progress
Once yeast has been pitched, gravity readings show whether fermentation is progressing. Avoid opening the fermenter repeatedly just to collect samples. Wait until visible fermentation is well underway or until the expected fermentation time has passed, depending on the style and yeast.
To monitor fermentation:
- Take a clean sample using a sanitized thief or valve.
- Compare the reading with the OG.
- Look for a downward trend over time.
- Replace the fermenter lid or airlock promptly.
- Never return the sample to the fermenter.
An airlock bubbling does not prove that fermentation is active, and a quiet airlock does not prove that fermentation is finished. Leaks, temperature changes, dissolved carbon dioxide, or pressure differences can all affect airlock activity. Gravity readings provide much stronger evidence.
Fermentation is generally considered complete when two readings taken several days apart are the same and the beer is within a reasonable range for the recipe and yeast. A stable reading alone does not guarantee that the beer is ready; the beer should also taste and smell appropriate, and obvious signs of infection should be considered.
Measuring Final Gravity and Calculating ABV
Take the final gravity after fermentation has stabilized. Degas the sample if it contains a lot of dissolved carbon dioxide. Carbon dioxide bubbles can cling to the hydrometer and cause an artificially high reading. Pour the sample gently between sanitized containers or stir it carefully with a sanitized utensil until bubbling subsides.
Use the following formula:
ABV = (OG - FG) × 131.25
For example:
- OG: 1.062
- FG: 1.012
- Gravity difference: 0.050
- Estimated ABV: 0.050 × 131.25 = 6.56%
The result may differ from the alcohol content shown on a commercial laboratory analysis. Hydrometer-based ABV estimates become less dependable when the starting gravity is extremely high, when sugar additions are unknown, or when the beer contains substantial non-fermentable extracts.
| Reading | When to take it | Main purpose |
|---|---|---|
| OG | Before pitching yeast | Record starting sugar density |
| Intermediate gravity | During fermentation | Confirm fermentation progress |
| FG | After readings stabilize | Estimate attenuation and ABV |
Temperature Corrections
Hydrometers are calibrated for a specific temperature. Warmer liquids are less dense than cooler liquids, so a warm sample may appear to have a lower gravity than it actually has. Many brewing calculators include temperature correction automatically, but you need to enter the hydrometer’s calibration temperature and the sample temperature correctly.
The correction is usually modest near room temperature but can become significant with hot samples. As a practical rule, cool samples before measuring rather than trying to correct a very hot reading. Do not place a hot glass hydrometer into ice water, because thermal shock may crack it.
If you use a refractometer during brewing, remember that refractometer readings require different calculations after fermentation because alcohol changes how light bends. A hydrometer reading is usually easier to interpret for final gravity and ABV, while a refractometer can save sample volume during brewing.
Common Problems and Troubleshooting
The reading is unexpectedly high. The sample may be too warm, poorly mixed, or concentrated by evaporation. In extract brewing, undissolved extract or stratified wort is a common cause. Mix the wort thoroughly before taking a new pre-fermentation sample.
The reading is unexpectedly low. The batch may contain more water than planned, or the sample may have been taken from a less concentrated layer. Check the actual volume and remix the wort before measuring. If the reading is low but the volume is correct, brewing efficiency or ingredient measurement may explain the result.
The hydrometer will not float upright. The test jar may be too shallow, the sample may contain too much foam, or the hydrometer may be damaged. Use a taller cylinder, remove foam, and inspect the instrument.
Bubbles cling to the hydrometer. Spin it gently, wait for the bubbles to release, and read again. Degas fermented samples before measuring.
The sample is cloudy. Cloudiness does not automatically make the reading unusable, but sediment attached to the hydrometer can interfere. Let large particles settle, then take a clean sample from above the sediment.
The gravity has stopped falling too early. Check fermentation temperature, yeast health, pitching rate, and whether the wort contains a high proportion of unfermentable sugars. Gently rouse the yeast only if appropriate for the beer and without introducing oxygen. Give the batch additional time, then take another reading.
The final gravity is still falling. Do not package yet, especially in bottles. Continued fermentation can create excess pressure and dangerous bottle bombs. Wait for stable readings on separate days.
Limitations and Safer Alternatives
A hydrometer uses a sample, so repeated measurements reduce the batch volume. This is one reason to keep sampling to the minimum needed for a decision. A refractometer requires only a few drops, making it useful for monitoring wort or small batches, but post-fermentation readings need alcohol correction and are easier to miscalculate.
Hydrometer readings also cannot identify every fermentation problem. They do not tell you whether a beer has an infection, whether flavors are mature, or whether carbonation is complete. Taste and aroma evaluation, visual inspection, and stable gravity readings all matter.
Never taste a sample that smells rotten, solvent-like, strongly sour when it should not be sour, or otherwise suspicious. Do not carbonate or bottle beer while gravity is still changing. Store the hydrometer in a protective case, rinse it after use, and keep it away from children and busy brewing areas.
With a sanitized sample, a temperature-aware reading, and consistent note-taking, a hydrometer turns fermentation from guesswork into a measurable process. Use it at the start, check progress when needed, and wait for stable final readings before packaging.