How to use a hydrometer

A hydrometer is a weighted glass float with a paper scale inside it. Drop it in a liquid and it sinks until it has displaced its own weight; sugary wort is denser than water, so it floats higher. That is the entire instrument — no battery, no calibration drift, nothing to go wrong except the way you read it. Which is why it is slightly absurd that the pages telling you how to read it do not agree.

What does a hydrometer actually measure?

Density, relative to water. Water reads 1.000. A wort with enough dissolved sugar to make a five-percent beer reads about 1.050 — it is 5% denser than water. As yeast turns that sugar into alcohol, which is less dense than water, the number falls. The gap between the reading before fermentation (original gravity, OG) and after (final gravity, FG) is what the ABV calculator turns into a percentage.

It measures density, not sugar — and that matters once alcohol is in the beer, because alcohol drags the reading down independently of the sugar left. A hydrometer handles that correctly and a refractometer does not, which is the whole reason refractometer readings need a second correction after fermentation starts.

How do you take a hydrometer reading?

1. Sanitize whatever goes in the beer. The thief or baster, the trial jar, and the hydrometer if it is going into the fermenter rather than into a jar. Drawing into a jar is the safer habit: less time with the lid off, and one less thing to sanitize on brew day.
2. Fill the jar about three-quarters full. The hydrometer displaces its own volume, so a jar filled to the brim overflows when you lower it in — and a jar too shallow lets it touch bottom, which reads whatever you like.
3. Lower it in and spin it. A gentle twist sheds the bubbles that cling to the bulb, and each one is buoyancy you did not intend. Let it stop drifting before you read.
4. Get your eye level with the surface. Reading down at the scale from above adds a point or two of parallax on its own.
5. Read it near the temperature on the paper — most are 60 °F (15.6 °C), many metric ones 20 °C. If your sample is hotter, the correction calculator has the chart and the arithmetic.

Top or bottom of the meniscus — which is right?

Wort climbs the glass where it touches, so the surface in the jar is a shallow dish: a low flat middle and a raised rim. That curve is the meniscus, and the ranking pages send you to opposite edges of it. The American Homebrewers Association says to "carefully look where the bottom point of meniscus is and take a reading from that level". Home Brew Answers says to "note the level of the beer, not where the meniscus curves up to meet the hydrometer". DeFalco's instruction sheet says to "read the scale inside the instrument at the level where the liquid contacts the glass" — which is the top. Australian Homebrewing splits the difference with "where the surface of the sample intersects with the scale".

Read at the meniscus position specified by the maker. Many brewing hydrometers are calibrated at the bottom of the meniscus, but check the instructions for yours. Reading another part of the curve introduces a bias whose size depends on the instrument and sample.

Does reading it wrong actually change your beer?

Here is a worked example of a consistent reading bias. Suppose you read the rim every time — consistently high, on the OG and on the FG. This is what that does to the numbers you draw off a 21 L batch of 5 kg pale malt that really went 1.050 to 1.010:

Apparent attenuation and brewhouse efficiency, computed from this site's engine for 5 kg of 37 PPG pale malt into 21 L. The misread is applied to both readings, as a consistent habit would.

The ABV does not move at all. Not "barely" — it is 5.25% on every row, because alcohol is calculated from the difference between two readings and a constant error subtracts itself out. The number on the bottle, the number your friends ask about, is completely immune to the argument the internet is having.

Everything measured against a single reading moves. Apparent attenuation falls from 80% to 75%, which is the difference between a yeast performing as the lab said and one you would write off as lazy. And brewhouse efficiency climbs from 68% to 72% — four points, for free, from a habit. Four points is the gap people buy a new mill over.

What are the three scales on a triple-scale hydrometer?

Specific gravity, Brix (or Balling — near enough the same thing at brewing strengths), and potential alcohol. The first two are two ways of saying how much sugar is dissolved, and the gravity converter moves between them properly. The third is a trap.

Brix from the ASBC polynomial. Potential alcohol from the winemaking divisor that reproduces DeFalco's published table to within a quarter point of ABV above 1.040; below that it runs high, and the guide says so rather than printing it as fact.

The potential-alcohol scale estimates what an unfermented liquid might produce under an assumed dry finish. It cannot account for the actual fermentability of beer wort or a particular yeast. Many beers finish above 1.000 because dissolved extract remains, while highly attenuated beers can finish below it. Use original and final readings in the ABV calculator for a finished-batch estimate.

Should you pour the sample back in?

The AHA is blunt: "do not return your sample to the fermenter, as it could cause contamination." That is the safe answer and it is the right default, especially for the OG reading, where the wort is a sterile, unprotected sugar solution and anything you drop in has a clear run at it.

Discarded samples reduce the packaged yield. A 150 mL trial jar, 4 readings across a brew — OG, one mid-fermentation, and two days apart at the end to confirm — is 600 mL, or 2.9% of a 21 L batch. Run it through the packaging ledger and it is the difference between 36 and 34 half-liter bottles. Two bottles is the price of knowing when your beer is finished, which is a bargain — but pour the samples into a glass and taste them rather than down the sink. Flat, warm, young beer is the best early warning of an infection you will get.

My hydrometer reads 1.002 in water — is it broken?

No, and the fix is free. The correction calculator covers running the check in distilled water at the paper's temperature; what matters is what you do with the result. As the BYO piece on MoreBeer puts it, "if it reads 1.002, for example, 0.002 becomes your correction factor for use with all future readings." Apply the offset consistently and recheck it periodically. Write it on the tube in permanent marker so the next person to pick it up knows too.

Follow the instrument maker's meniscus instructions consistently in water and wort. A water check reveals a useful zero offset, but it cannot prove accuracy across the whole scale or remove every reading error. Recheck calibration if the instrument is damaged or the readings become suspect.

It reads 1.020 after two weeks — is it stuck or done?

Take readings several days apart under comparable conditions. If they match, the beer was stable over that interval; it may be finished or stalled. An unexpectedly high result needs an explanation before bottling. Compare the recipe and strain with the attenuation calculator, then follow the stuck-fermentation checks. A cold or stressed culture can restart after stable readings.

For temperature adjustment, Brewer's Friend's reference distinguishes the sample temperature from the instrument calibration. Follow your own instrument's instructions for the meniscus and operating range.

Apply temperature and calibration corrections before comparing results. Those corrections improve the measurement; they do not by themselves tell you whether fermentation is complete. Record the readings and conditions in Brew Day so the trend is easy to check.

Corrected October 10, 2026: Clarified fermentation stability and the limits of calibration checks.