TL;DR
A hydrometer measures the density of your beer or wine relative to water. Take a reading before fermentation (Original Gravity) and after fermentation (Final Gravity), then plug both numbers into an ABV formula. The standard equation is ABV = (OG − FG) × 131.25. Always correct for temperature — most hydrometers are calibrated at 20 °C (68 °F) — and read at the bottom of the meniscus for an accurate result. A quality triple-scale hydrometer, a test jar, and a thermometer are all you need.
What Is a Hydrometer and Why Does It Matter?
A hydrometer is a sealed glass tube with a weighted bulb at the bottom and a graduated scale inside. When you lower it into a liquid sample, it floats at a level that corresponds to the liquid’s specific gravity (SG) — how dense the liquid is compared to pure water (SG 1.000 at 20 °C).
Before fermentation, your wort or must is loaded with dissolved sugars. That sugar makes the liquid denser than water, so the hydrometer sinks less and shows a higher number — your Original Gravity (OG). As yeast converts sugar into alcohol and CO₂, the liquid becomes thinner. After fermentation finishes, the hydrometer sinks deeper, giving you a lower number — your Final Gravity (FG).
The difference between OG and FG tells you exactly how much sugar was consumed, which translates directly into alcohol content. If you want a deeper dive into what those two numbers really mean, check out our article on Abv Calculator Og Fg Explained.
Equipment You Need
| Item | Purpose |
|---|---|
| Triple-scale hydrometer | Reads SG, Brix, and potential alcohol |
| Hydrometer test jar | Tall narrow cylinder to float the hydrometer in |
| Thermometer | For temperature correction |
| Wine thief or turkey baster | To pull a sample from the fermenter |
| Sanitiser (Star San or similar) | Everything touching your beer must be clean |
A good all-in-one option for beginners is the
Step-by-Step: Taking a Hydrometer Reading
Step 1 — Sanitise Everything
Spray or dip your hydrometer, test jar, wine thief, and thermometer in sanitiser solution. Any contamination you introduce could ruin an entire batch.
Step 2 — Pull a Sample
Use a wine thief or sanitised turkey baster to draw enough liquid to fill the test jar about 80 % full. Never float the hydrometer directly inside the fermenter — you risk contamination, and the vessel is usually too wide for an accurate reading.
Step 3 — Degas the Sample (Fermented Samples Only)
If fermentation is active or recently finished, dissolved CO₂ will cling to the hydrometer as tiny bubbles. These bubbles push the instrument upward and give a falsely low reading. To degas:
- Pour the sample back and forth between two sanitised containers 5–6 times.
- Or gently stir the sample with a sanitised spoon for 30 seconds.
- Or microwave the sample for 10 seconds (only if you plan to discard it afterward).
Step 4 — Float the Hydrometer
Lower the hydrometer gently into the test jar and give it a light spin with your fingers. The spin breaks any surface tension bubbles that cling to the stem. Wait for it to stop bobbing.
Step 5 — Read at the Meniscus
This is where most beginners go wrong. The liquid climbs slightly up the hydrometer’s stem, forming a curved surface called the meniscus. Always read the scale at the bottom of the meniscus — the flat part of the liquid surface — with your eye at the same level as the liquid.
| Mistake | Result |
|---|---|
| Reading the top of the meniscus | SG reads ~0.001–0.002 too high |
| Looking down at the sample | Parallax error makes SG read high |
| Looking up at the sample | Parallax error makes SG read low |
| Bubbles clinging to the stem | SG reads 0.001–0.003 too low |
Step 6 — Record the Temperature
Note the temperature of the sample right after reading. This is critical for the correction step below.
Temperature Correction: Why and How
Hydrometers are calibrated at a specific temperature — usually 20 °C (68 °F), though some older models use 15 °C (59 °F). Check the small print on your hydrometer or its packaging. If your sample is warmer or cooler than the calibration temperature, the density changes, and your reading will be off.
Temperature Correction Table (Calibrated at 20 °C)
| Sample Temp °C (°F) | Correction to Add |
|---|---|
| 10 (50) | −0.0006 |
| 15 (59) | −0.0003 |
| 20 (68) | 0.0000 |
| 25 (77) | +0.0007 |
| 30 (86) | +0.0016 |
| 35 (95) | +0.0027 |
| 40 (104) | +0.0040 |
Example: Your hydrometer reads 1.050 and the sample is 30 °C. Corrected SG = 1.050 + 0.0016 = 1.0516.
At fermentation temperatures (18–22 °C / 64–72 °F), the correction is small — often less than 0.001. But if you’re reading hot wort straight from the kettle at 60 °C+, the correction becomes massive and the hydrometer can be damaged by thermal shock. Always cool your sample to below 40 °C before measuring.
For an in-depth explanation of how temperature and reading technique interact, see Reading Hydrometer Step By Step.
Calculating ABV from OG and FG
Once you have both readings, the math is simple. The most commonly used formula is:
ABV (%) = (OG − FG) × 131.25
| Beer Style | Typical OG | Typical FG | ABV (%) |
|---|---|---|---|
| Light Lager | 1.035 | 1.006 | 3.8 |
| American Pale Ale | 1.050 | 1.012 | 5.0 |
| IPA | 1.065 | 1.012 | 7.0 |
| Imperial Stout | 1.090 | 1.018 | 9.5 |
| Barleywine | 1.110 | 1.025 | 11.2 |
Rather than doing the arithmetic by hand every time, just enter your OG and FG into our tool:
ABV CalculatorCalculate your alcohol by volume from gravity readings
It handles temperature correction and gives you ABV instantly.
Common Mistakes (and How to Avoid Them)
1. Not Taking an OG Reading at All
Without OG, you cannot calculate ABV. Period. Always pull a sample on brew day before pitching yeast.
2. Sampling Too Frequently During Fermentation
Every time you open the fermenter, you risk contamination and oxidation. Two readings are enough: OG on brew day, FG when fermentation appears complete (stable gravity for 2–3 consecutive days).
3. Confusing Specific Gravity with Brix
If your hydrometer has a Brix scale, remember that Brix readings are only accurate for unfermented wort. Once alcohol is present, Brix readings require a correction factor. This is a common source of confusion, and we cover it in detail in Hydrometer Vs Refractometer Which Better.
4. Not Degassing the Sample
Dissolved CO₂ is invisible but powerful. It can push your reading 2–4 gravity points lower than the true value. Always degas post-fermentation samples.
5. Dropping the Hydrometer
Hydrometers are fragile. A hairline crack in the weighted bulb lets liquid seep in, making all future readings inaccurate. If your hydrometer starts reading water at 0.998 or 1.003 instead of 1.000, it’s compromised — replace it.
How Accurate Is a Hydrometer?
A standard homebrewing hydrometer has graduations every 0.002 SG. With proper technique — degassed sample, correct temperature, reading at the meniscus — you can expect accuracy of about ±0.001 SG. That translates to roughly ±0.13 % ABV, which is more than adequate for homebrewing.
For higher precision, consider a precision laboratory hydrometer (graduated every 0.001 or 0.0005) or a digital density meter, though the latter costs several hundred euros.
Hydrometer Maintenance Tips
- Store vertically in the original tube or a padded case.
- Rinse immediately after every use — dried wort is hard to remove and can obscure the scale.
- Calibrate periodically by checking against distilled water at the calibration temperature. It should read 1.000.
- Avoid thermal shock — don’t plunge a room-temperature hydrometer into near-boiling wort.
When a Hydrometer Isn’t Enough
Hydrometers are the gold standard for homebrew gravity measurement, but they have limits:
- They require a relatively large sample (100–200 mL).
- They’re fragile.
- They can’t easily be used for in-fermenter monitoring.
If these limitations bother you, a refractometer requires only a few drops and is nearly indestructible. Digital hydrometers like the Tilt or iSpindel sit inside the fermenter and send readings to your phone. Each tool has trade-offs — more on that in our comparison article.
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Methodology
This guide is based on the ASBC (American Society of Brewing Chemists) Methods of Analysis for specific gravity measurement, the standard ABV formula (OG − FG) × 131.25 as documented by John Palmer in How to Brew (4th edition, 2017), and temperature correction tables derived from the OIML R 44 international recommendation for hydrometers. Equipment recommendations reflect hands-on testing and community feedback from forums including HomeBrewTalk and r/homebrewing. All typical gravity ranges reference the 2021 BJCP Style Guidelines.