A hydrometer is one of the most useful tools in home brewing and winemaking. It measures the specific gravity of beer, wine, cider or mead, showing how much dissolved sugar is present in the liquid.
Taking a reading before fermentation gives you the original gravity. Taking another once fermentation has finished gives you the final gravity. These two readings can be used to estimate the alcohol content of your finished drink.
A hydrometer can also help you monitor fermentation, identify a possible stalled fermentation and confirm when it is safe to move on to clearing, back-sweetening or bottling.
Use Your Readings
What Does a Hydrometer Measure?
A hydrometer measures the density of a liquid compared with water. This is shown as specific gravity, usually written as a number such as 1.080, 1.010 or 0.996.
Water has a specific gravity of approximately 1.000 at the hydrometer’s calibration temperature. A sugary liquid is denser, causing the hydrometer to float higher and display a larger number.
As yeast converts sugar into alcohol and carbon dioxide, the liquid becomes less dense and the gravity reading falls.
Original Gravity
Original gravity, usually shortened to OG, is the reading taken before fermentation begins. It provides a starting point for calculating alcohol content.
Current Gravity
A current gravity reading is taken during fermentation. Comparing current readings allows you to see whether fermentation is still progressing.
Final Gravity
Final gravity, usually shortened to FG, is the stable reading recorded after fermentation has finished. It is used with the original gravity to calculate the approximate alcohol by volume.
What You Need
- A suitable wine and beer hydrometer
- A clean and sanitised hydrometer trial jar
- A sanitised wine thief, syringe, turkey baster or sampling tap
- A thermometer
- A notebook or brewing record
A trial jar allows the hydrometer to float freely while using a much smaller sample than would be needed to take a reading directly inside a fermenting bucket.
Check Your Hydrometer Before Using It
Look carefully at the hydrometer and make sure there are no cracks, chips or breaks in the glass. A damaged hydrometer should not be placed in your drink.
Check the calibration temperature printed on the hydrometer or its packaging. Many homebrew hydrometers are calibrated at 20°C, but this is not universal.
You can perform a simple check by placing the hydrometer in clean water at its stated calibration temperature. It should read approximately 1.000. A small difference can be recorded and allowed for when taking future readings.
How to Take a Hydrometer Reading
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Sanitise the Equipment
Clean and sanitise the trial jar, hydrometer and anything being used to remove the sample. This is particularly important when sampling during or after fermentation.
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Take a Representative Sample
Draw enough liquid from the fermenter to fill the trial jar to a level at which the hydrometer can float without touching the bottom.
Avoid collecting thick sediment from the bottom of the fermenter. Sediment, fruit pulp and other solids can interfere with the reading.
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Remove Excess Foam and Bubbles
Foam and carbon dioxide bubbles can cling to the hydrometer and make it float higher than it should.
Gently swirl or stir the sample to release excess gas. Avoid shaking it so vigorously that more foam is created.
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Lower the Hydrometer Carefully
Place the hydrometer gently into the trial jar. Do not drop it, as the glass can strike the bottom and break.
Give the hydrometer a light spin as you release it. This helps dislodge bubbles that may be attached to the glass.
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Allow It to Settle
Wait until the hydrometer has stopped moving. It must float freely without touching the side or bottom of the trial jar.
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Read at Eye Level
Bring your eyes level with the surface of the liquid. Most brewing hydrometers should be read where the flat surface of the liquid crosses the scale, rather than at the raised edge of the meniscus.
Check the instructions supplied with your hydrometer because some specialist instruments may be calibrated differently.
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Record the Temperature
Measure and record the temperature of the sample. Readings taken above or below the hydrometer’s calibration temperature may require correction.
For hydrometers calibrated at 20°C, enter the measured gravity and sample temperature into our Hydrometer Temperature Correction Calculator.
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Write Down the Reading
Record the gravity, temperature, date and stage of fermentation. Keeping proper records makes it much easier to monitor progress and diagnose problems.
Do Not Return the Sample
Once a sample has been removed, it is normally best not to pour it back into the fermenter. Returning the sample creates an unnecessary contamination and oxidation risk.
You can taste the sample after taking the reading, provided all equipment has been properly sanitised.
How to Read the Scale
A standard beer and wine hydrometer commonly includes a specific gravity scale. Some models may also show potential alcohol, sugar concentration, Brix or colour-coded fermentation bands.
Use the specific gravity scale when recording your OG and FG. Read all three decimal places where possible.
| Displayed reading | Record it as |
|---|---|
| 1.090 | 1.090 OG |
| 1.014 | 1.014 current gravity |
| 0.996 | 0.996 FG |
Do not record 1.090 as 1090 unless the form or calculator specifically asks for gravity points.
How to Tell When Fermentation Has Finished
Airlock activity alone cannot confirm whether fermentation has finished. Gas can escape from an imperfect seal, and changes in temperature or air pressure can cause an airlock to move even when fermentation is complete.
The most reliable homebrew check is to take hydrometer readings over several days. Fermentation is likely to have finished when:
- The gravity is within the expected range for the recipe or kit
- The same corrected gravity is recorded on consecutive readings taken several days apart
- There are no other signs of active fermentation
Do not bottle solely because a fixed number of days has passed. A fermentation that has not finished can restart inside the bottle and create dangerous pressure.
How to Calculate Alcohol by Volume
To estimate alcohol by volume, you need the original gravity recorded before fermentation and the final gravity recorded once fermentation is complete.
Enter both figures into our ABV Calculator. The calculator will estimate the alcohol by volume and apparent attenuation.
Example
- Original gravity: 1.090
- Final gravity: 0.996
- Approximate ABV: 12.34%
This calculation is an estimate. Hydrometer accuracy, temperature, sugar additions made during fermentation and inaccurate volume measurements can all affect the result.
Adding Sugar in Stages
Some country wine and high-alcohol recipes add sugar in more than one stage. A basic OG-to-FG calculation will not automatically account for sugar added after the original reading.
Record the amount and timing of every sugar addition. For the most useful result, follow the calculation method supplied with the recipe or use a brewing calculator designed for staggered additions.
Correcting a Reading for Temperature
A hydrometer is most accurate when the sample is close to its stated calibration temperature. Warmer and cooler samples have different densities and can produce slightly different readings.
Allowing a hot sample to cool is normally more reliable than applying a very large correction. When the temperature difference is modest, use our Hydrometer Calculator to correct a reading from a hydrometer calibrated at 20°C.
Do not assume every hydrometer is calibrated at 20°C. Check the instrument before applying a correction.
Common Hydrometer Mistakes
Reading the Top of the Meniscus
The liquid rises slightly around the hydrometer stem. Most brewing hydrometers are read at the lower, flat liquid level rather than the raised edge.
Allowing the Hydrometer to Touch the Jar
If the hydrometer rests against the side or bottom, it cannot float correctly and the reading will be unreliable.
Leaving Bubbles Attached
Carbon dioxide bubbles can lift the hydrometer and produce an artificially high reading. Gently spin the instrument to remove them.
Taking a Reading Through Foam
Foam makes the liquid level difficult to see. Remove the foam or wait for it to settle before reading the scale.
Ignoring Sample Temperature
A warm sample may need correcting before it can be compared with a previous reading.
Using the Wrong Scale
Triple-scale hydrometers contain several sets of numbers. Make sure you are reading the specific gravity scale when recording OG and FG.
Relying on a Single Final Reading
One low reading does not prove that fermentation has completely stopped. Confirm that the result remains stable over several days.
Frequently Asked Questions
Can I put the hydrometer directly into the fermenter?
You can, provided the hydrometer can float freely and the vessel is deep enough. However, a trial jar gives a clearer reading, is easier to bring to eye level and avoids placing a fragile glass instrument inside the full batch.
Why has my wine finished below 1.000?
This can be normal. Alcohol is less dense than water, so a dry fermented wine can finish slightly below 1.000.
Can I calculate ABV without an original gravity?
Not accurately using the standard OG and FG method. A potential alcohol scale or recipe estimate can provide a rough indication, but it cannot replace a recorded original gravity.
Does a low gravity mean fermentation has finished?
Not by itself. The reading must also remain stable. A fermenting drink can reach a low gravity and continue dropping.
Can I use a refractometer instead?
A refractometer is very useful before fermentation and requires only a small sample. Once alcohol is present, the reading requires additional correction. Use our Refractometer Calculator when measuring during or after fermentation.
Should I take a reading before back-sweetening wine?
Yes. Record the true final gravity before adding sweetener or sugar. Added sweetness changes the density and means the later reading is no longer the original fermentation FG.
Read our guide to back-sweetening wine before adding sugar or sweetener to a finished wine.