Reading a hydrometer, and what the numbers mean

A hydrometer is the cheapest tool in the brewery and the one that answers the most questions. It tells you how strong the beer will be, whether fermentation has finished, and whether something has gone wrong — none of which you can determine by looking at an airlock.

What it measures

A hydrometer measures the density of a liquid relative to water. Water is 1.000. Dissolved sugar makes wort denser, so unfermented wort might read 1.048. As yeast converts sugar to alcohol — which is less dense than water — the reading falls.

Two readings give you the alcohol content:

ABV ≈ (OG − FG) × 131.25

So a beer that started at 1.048 and finished at 1.010 is (0.038 × 131.25) = about 5.0%.

Reading it correctly

Fill a trial jar rather than floating the hydrometer in the fermenter — you want it away from the sides and free of CO2 bubbles, which cling to the stem and lift it, giving a false high reading. Spin it gently to shake them off.

The liquid climbs slightly where it meets the glass, forming a meniscus. Read at the bottom of the curve, at eye level. Reading the top adds a point or two consistently.

Temperature correction

Hydrometers are calibrated at a specific temperature — usually 20 °C, sometimes 15 °C. Check yours; it is printed on the paper scale inside.

Warmer liquid is less dense, so a hot sample reads low. Roughly:

• At 30 °C, add about 0.002.
• At 40 °C, add about 0.005.

Taking a reading straight from a 60 °C sample and not correcting it is a common way to convince yourself the mash failed. Cool the sample, or correct the number.

Finished, or stuck?

This is what the hydrometer is really for. An airlock tells you nothing — it stops bubbling when CO2 production falls below the seal's leak rate, which can happen with plenty of sugar left.

The test is simple: two readings, two to three days apart, at the same temperature. If they are identical, fermentation is finished. If the second is lower, it is still going.

One reading, however low, proves nothing.

If the readings are stable but well above where you expected — say 1.020 on a beer that should finish at 1.010 — you have a stuck ferment. Common causes:

• Fermentation temperature dropped and the yeast went dormant.
• Underpitching, so too few cells for the sugar available.
• A mash run too hot, producing more dextrins than the yeast can ferment. This is not really stuck — the beer is finished, it just finished sweet.

Rousing the yeast by gently swirling the fermenter and raising the temperature two or three degrees restarts most genuinely stuck ferments.

Why final gravity reads lower than the sugar suggests

Worth knowing, because it confuses people: a hydrometer measures total density, and finished beer contains alcohol, which is less dense than water. So a beer at 1.010 has more residual sugar than a 1.010 sugar solution would.

This means the ABV formula is an approximation — good to a couple of tenths of a percent, which is close enough for anything you are doing.

Hydrometer or refractometer?

A refractometer needs two drops and reads instantly, which makes it excellent for checking gravity during the mash and boil, where a hydrometer sample means cooling 200 ml of hot wort.

Its weakness is that alcohol distorts refraction, so readings taken during or after fermentation need a correction formula and are less reliable.

The sensible combination: refractometer for pre-fermentation, hydrometer for final gravity. If you are only buying one, buy the hydrometer — it is the one that answers "is it finished?" without arithmetic.

Buy two

Hydrometers are glass, thin, and get dropped into hard sinks. They are inexpensive. Having a spare means a broken one costs you an afternoon rather than a batch.

Check yours occasionally against plain water at its calibration temperature. It should read 1.000. If it does not, note the offset and apply it — a hydrometer that is consistently two points out is perfectly usable once you know.