For my first few batches I treated the hydrometer like a decoration — something that came in the kit that I floated once, squinted at, and ignored. Then I bottled a batch I assumed was finished, and it wasn't, and I learned the hard way that this cheap glass tube is the one instrument that actually tells you what your yeast is doing. It answers two questions a bubbling airlock never can: how strong will this beer be, and is fermentation actually done? Both come down to reading a number off a floating scale, and once the habit sticks it takes about two minutes a batch.
What the floating number is telling you
A hydrometer measures specific gravity — the density of your liquid compared to plain water, which reads 1.000 at the calibration temperature. Dissolved sugar makes a liquid denser, so unfermented wort packed with malt sugar floats the hydrometer high, somewhere around 1.040 to 1.060 for a typical beer. As yeast eats that sugar and turns it into alcohol and carbon dioxide, the density drops, because alcohol is actually less dense than water. So the number falling over the days of fermentation is a direct readout of sugar being converted, and where it stops tells you how much got converted in total.
That is the whole idea behind the two readings that matter. The gravity before the yeast goes in is your original gravity (OG); the gravity once fermentation has finished is your final gravity (FG). The gap between them is the sugar the yeast ate, and that gap is what you turn into an alcohol figure.
Pulling an original gravity reading
Take the OG right after you've cooled the wort and topped up to your final volume, but before you pitch the yeast — once the yeast is in and working, the number is already moving and no longer represents your starting point. Draw a sample with a sanitized wine thief, siphon, or turkey baster into a tall test jar, enough that the hydrometer floats without bumping the bottom or clinging to the side. Give it a gentle spin to knock loose any bubbles riding on the glass, since a bubble-lifted hydrometer reads falsely low, then let it settle.
Read the number where the liquid surface actually crosses the scale, at eye level, looking at the bottom of the curved meniscus rather than the film that creeps up the stem. This is the single most common way readings go wrong — even brewers who have been at it for years quietly misjudge the meniscus and record a point or two off every time. Write the figure down the moment you have it; you will not remember it accurately three weeks later when you need it.
Pulling a final gravity reading, and confirming you're done
The FG reading does double duty. Beyond feeding the alcohol math, it is how you prove fermentation has genuinely stopped rather than merely gone quiet. An airlock that has stopped bubbling proves nothing — it can go still while the yeast is still slowly working, and it can bubble from a temperature swing after fermentation is over. The gravity number is the honest signal.
So take a reading when activity looks finished, then take another two or three days later without pitching or disturbing anything. If the two match, the yeast has done all it is going to do and the beer is safe to package. If the second number is still lower than the first, fermentation is ongoing and bottling now risks overcarbonated, potentially dangerous bottles down the line. I have never regretted waiting for two matching readings, and the one time I skipped that check is the batch that taught me to stop skipping it.
Turning two numbers into an alcohol figure
The standard homebrew formula is simple arithmetic: subtract the final gravity from the original gravity, then multiply by 131.25. So a beer that started at 1.050 and finished at 1.010 gives you (1.050 − 1.010) × 131.25, which is 0.040 × 131.25, or about 5.25% alcohol by volume. A lighter session beer from 1.045 down to 1.008 works out to 0.037 × 131.25, roughly 4.9%. You can do it on a phone calculator faster than you can find a brewing app.
That constant of 131.25 is not arbitrary. As the American Homebrewers Association explains in its guide to taking an accurate hydrometer reading, the figure bundles together the amount of ethanol produced per unit of sugar consumed and the conversion from alcohol by weight to alcohol by volume, so you get a usable ABV straight out of the two gravity points. It is an approximation, but a dependable one at the strengths most home batches land at.
When the simple math starts to drift
That formula holds up well up to roughly 8% ABV, which covers the large majority of everyday beers. Push into big imperial stouts, barleywines, or high-gravity Belgian styles and the simple multiplication begins to underestimate the true strength, because the relationship between gravity drop and alcohol stops being quite so linear. For those, brewers switch to one of the more elaborate correction formulas, and any decent brewing calculator offers one — but for the pale ales, ambers, and lagers that fill most fridges, the 131.25 version is close enough that the error is smaller than the slop in your reading anyway.
Temperature is the other thing that quietly bends your numbers. Most hydrometers are calibrated to read true at a specific temperature — commonly 60°F, though some are set to 68°F, so check the paper slip inside yours. A sample warmer than that calibration point is less dense and reads low; a colder one reads high. For a wort sample still sitting well above room temperature, that swing is enough to throw your ABV off by a few tenths, so either cool the sample to the calibration temperature before reading or run the number through a temperature-correction calculator. Once the beer is at room temperature for the FG reading, the correction is usually small enough to ignore.
Working it into the brew day
None of this has to be fussy. My whole routine is: pull and read the OG while the wort finishes settling before I pitch, write it on the fermenter's masking-tape label, and forget about it. When the airlock slows a week or two later, I pull an FG, wait three days, pull another, and if they match I do the one subtraction and multiply by 131.25 to get the ABV before I bottle. Two readings, one calculation, and I know both how strong the beer is and — far more importantly — that it is actually finished and safe to package. For a two-minute habit, there is no better information you can get out of a batch.