You can’t measure the alcohol content of wine directly with a hydrometer. Instead, you take two readings: one before fermentation starts and one after it finishes. The difference between those readings tells you how much sugar was converted into alcohol, which you plug into a simple formula to get your alcohol by volume (ABV).
How a Hydrometer Actually Works
A hydrometer is a weighted glass tube that floats at different heights depending on how dense a liquid is. Sugar dissolved in grape juice adds mass to the liquid, making it denser than plain water. Density is mass divided by volume, so more sugar means higher density, and the hydrometer floats higher. As yeast converts that sugar into alcohol and carbon dioxide during fermentation, the liquid becomes less dense and the hydrometer sinks lower.
The scale on the hydrometer measures specific gravity (SG), which compares the density of your liquid to the density of pure water. Water has a specific gravity of 1.000. Unfermented grape juice typically reads somewhere between 1.070 and 1.100, depending on the grape variety and ripeness. A fully fermented dry wine usually lands right around 1.000 or just below it.
What You Need
Beyond the hydrometer itself, you’ll need a few inexpensive items:
- A test jar: A tall, narrow cylinder (usually glass or plastic) that holds enough liquid for the hydrometer to float freely without touching the sides or bottom.
- A wine thief or turkey baster: Used to pull a sample from your fermenter without contaminating the batch. A wine thief doubles as a test jar, so you can drop the hydrometer right into it.
- A thermometer: Hydrometers are calibrated to read accurately at a specific temperature, usually 60°F (15.5°C) or 68°F (20°C). Check the label on your hydrometer. If your sample is warmer or cooler than the calibration temperature, your reading will be slightly off.
- Sanitizer: Anything that touches your wine needs to be sanitized first to avoid introducing bacteria or wild yeast.
Taking Your First Reading
Your first reading, called the original gravity (OG), should be taken after you’ve crushed and prepared your juice but before you add yeast. This captures the full sugar content that yeast will later consume.
Sanitize your test jar, hydrometer, and wine thief. Draw a sample large enough to fill the test jar so the hydrometer can float without resting on the bottom. Gently lower the hydrometer into the sample and give it a spin, like a top, as you release it. Spinning dislodges any tiny air bubbles clinging to the glass, which would make it float higher than it should and throw off your number.
Once the hydrometer settles and stops bobbing, bring your eye down to the level of the liquid surface. You’ll notice the liquid curves slightly upward where it meets the glass of the hydrometer. This curve is called the meniscus. Take your reading at the bottom of that curve, not at the top where it climbs the glass. Write the number down. A typical starting gravity for wine might be something like 1.085.
Taking Your Final Reading
Wait until fermentation is complete before taking your second reading. A good rule of thumb: fermentation is done when you see one bubble per minute or less coming through the airlock. But the only way to confirm it is with the hydrometer itself. Take a reading, wait two days, and take another. If the number hasn’t changed, fermentation has stopped.
For a dry wine, your final gravity (FG) should be around 1.000 or slightly below. If you’re aiming for a sweeter style, the final gravity will be higher because residual sugar remains in the wine.
The sample-taking process is identical to your first reading: sanitize everything, pull a sample, spin the hydrometer, read at the meniscus. If your wine is still producing carbon dioxide, you need to de-gas the sample first. CO2 bubbles cling to the hydrometer and make it float higher, giving you a falsely elevated reading. Pour the sample back and forth between two clean containers a few times, or gently swirl it in the jar until the visible fizzing stops. Blow away any foam sitting on the surface before reading.
Calculating Alcohol by Volume
With both numbers in hand, the formula is straightforward:
ABV = (Original Gravity − Final Gravity) × 131.25
So if your original gravity was 1.085 and your final gravity is 0.998:
ABV = (1.085 − 0.998) × 131.25 = 0.087 × 131.25 = 11.4%
That’s your wine’s approximate alcohol content. This formula is the standard version used by most home winemakers and works well for typical wine-strength fermentations.
Some Hydrometers Have a Potential Alcohol Scale
Many hydrometers sold for winemaking include a “potential alcohol” scale printed alongside the specific gravity markings. This scale converts the sugar content directly into potential alcohol percentage. You read the potential alcohol before fermentation, read it again after, and subtract the second number from the first. The result is the same as the formula above, just without the math. If your hydrometer has this scale, it’s the easiest route.
You may also see a Brix scale on your hydrometer, which measures sugar content as a percentage by weight. Winemakers frequently use Brix because it translates intuitively: 24 Brix means 24% of the liquid’s weight is sugar. A finished dry wine typically reads around negative 1.5 to negative 2.0 Brix, since alcohol is less dense than water and pulls the reading slightly below zero.
Common Mistakes That Skew Your Numbers
The biggest source of error is temperature. If your sample is 80°F and your hydrometer is calibrated for 60°F, the liquid is less dense than it would be at the calibration temperature, and your reading will be artificially low. Correction charts are widely available online. Let your sample cool to the calibration temperature when possible, or note the temperature and adjust.
Bubbles are the second most common culprit. Even a few tiny CO2 bubbles clinging to the hydrometer can raise its float point noticeably. Always spin the hydrometer after placing it in the sample, and blow off any foam or particles sitting at the surface. Make sure the hydrometer isn’t touching the walls of the test jar, which can also prevent it from floating freely.
Finally, read at eye level. Looking down at the hydrometer from above creates parallax error, making the reading appear lower than it actually is. Get your eyes level with the surface of the liquid, find the bottom of the meniscus, and read there.
What If the Reading Stops Moving Too Early?
If your gravity reading stalls well above 1.000 and the airlock has gone quiet, you may have a stuck fermentation. The telltale sign is two readings, taken a couple of days apart, that are identical but still above your target. For a dry wine, you’re aiming for a final gravity near 1.000 or below. If it’s sitting at, say, 1.020, the yeast quit before finishing the job.
Stuck fermentations happen for several reasons: the temperature dropped too low for the yeast to stay active, the alcohol level climbed high enough to stress them, or they ran out of nutrients. Gently warming the fermenter, adding yeast nutrients, or pitching a fresh dose of a more alcohol-tolerant yeast strain can sometimes restart it. Taking hydrometer readings every few days during fermentation helps you catch a stall early, before it becomes harder to fix.