Your Hydrometer Can't Taste Alcohol — So How Does It Read 6.2%?
First time I brewed beer, I dropped a hydrometer into the fermenter expecting it to do something clever. It just bobbed there like a fishing float. No sensor, no electronics, no little reading that said "alcohol." A weighted glass tube floating in my future IPA. And yet, five minutes of arithmetic later, I had a number: 6.2% ABV.
Here's the trick that took me embarrassingly long to understand. Nothing in a homebrew setup ever measures alcohol directly. There's no probe that counts ethanol molecules. The whole thing is inferred — from how much lighter the liquid got while the yeast was working. Sugar is heavy. Alcohol is light. Track the weight that disappeared, and you've tracked the alcohol that appeared.
That single idea is the foundation under every ABV number on a craft label, every homebrew recipe, and the formula your ABV calculator runs. Let's take it apart.
ABV (alcohol by volume) isn't measured — it's calculated from two density readings. Original gravity (OG) is how dense the liquid is before fermentation, packed with sugar. Final gravity (FG) is how dense it is after the yeast has eaten that sugar. The gap between them is proportional to the alcohol produced.
What "Gravity" Actually Means in a Fermenter
Brewers say "gravity," but they mean specific gravity — and that's just density measured against water. Pure water reads 1.000. Dissolve a pound of sugar into it and the liquid gets heavier, so it reads higher, maybe 1.050. The more dissolved sugar, the denser the wort, the higher the number. If that sounds like the same idea behind the density calculator, it is — specific gravity is density expressed as a ratio to water instead of in grams per milliliter.
Now fermentation starts. Yeast is a single-celled animal with one job at the party: eat sugar, excrete alcohol and carbon dioxide. As it churns through the dissolved sugar, two things happen at once. The heavy sugar vanishes, and in its place appears ethanol — which is actually less dense than water, around 0.789. So the liquid gets lighter from both directions. A wort that started at 1.050 might finish around 1.010.
That drop from 1.050 to 1.010 is the fingerprint of fermentation. Forty gravity points went away. Each point represents sugar that got converted, and conversion is the only thing that produces the alcohol. Measure the start, measure the end, and the difference tells the whole story.
The 131.25 Formula, and Where That Number Comes From
The standard homebrew formula — the one the American Homebrewers Association points new brewers to — is almost suspiciously simple. Subtract final gravity from original gravity and multiply by a magic constant:
Run our example. OG was 1.050, FG finished at 1.010:
In plain English: the liquid lost 0.040 in density, and history says every unit of that density loss corresponds to about 131.25 percentage points of alcohol. Where does 131.25 come from? It bakes in the chemistry of turning sugar into ethanol — the relative densities of the sugar leaving and the alcohol arriving, plus the carbon dioxide that bubbles off. It's not a fundamental constant of the universe; it's a well-fitted shortcut that lands close enough for almost any batch.
The 131.25 shortcut drifts high for strong brews. Once you push past roughly 7–8% ABV — big imperial stouts, barleywines, mead — the simple formula overstates alcohol, because the relationship between density and ethanol isn't perfectly linear. For those, brewers switch to an alternate equation that corrects the curve.
The more accurate version most calculators offer looks uglier but bends with the chemistry:
You won't be solving that on a napkin between batches. The calculator runs both, so you can sanity-check a high-gravity brew against the simple number and see how far they part ways.
Attenuation: Did Your Yeast Finish the Job?
There's a second number hiding in the same two readings, and seasoned brewers care about it as much as ABV. Apparent attenuation is the percentage of the sugar your yeast actually ate. It answers a different question than alcohol content: not "how strong is it," but "did fermentation finish, or did the yeast quit early?"
Watch the units here, because it trips people up. For our 1.050 → 1.010 batch, plug the full gravities straight in: (1 − 1.010/1.050) × 100 ≈ 3.8%. That looks far too low — and it is, because the interesting action lives in the digits after the decimal. Brewers think in gravity points, the part above 1.000. The wort had 50 points, finished with 10, so it shed 40 of its 50 points — 80% attenuation. Most ale yeasts land between 70% and 85%. A reading well below that range is a warning sign — stuck fermentation, tired yeast, or a temperature that dipped too low — and it usually means a sweeter, lower-alcohol beer than you planned.
Same wort, two outcomes: gravity drop tells the story
A fermentation that stalls at 1.025 leaves sugar uneaten — less alcohol, sweeter finish, lower attenuation.
Why the Label Number and Your Reading Disagree
Buy a commercial beer that says 5.5% on the can and your home calculation off its gravities might land at 5.9%. Both can be honest. Commercial brewers measure alcohol with lab-grade gear — distillation or near-infrared analyzers that read ethanol directly — not a $7 hydrometer doing subtraction. The gravity formula is an estimate, accurate to a few tenths of a percent for normal beers, but it's still an estimate stacked on two readings that each carry their own small error.
Temperature is the usual culprit at home. Hydrometers are calibrated to read true at a specific temperature, often 60°F or 68°F. Take a reading on warm wort straight off the boil and the liquid is less dense than it'll be cold, so the number reads low and your whole calculation drifts. Let the sample cool, or apply a temperature correction, before you trust it. Sugar additions after the first reading — priming sugar, fruit, honey in a secondary — also feed more fermentation the original OG never saw.
None of that breaks the method. It just means the number is a careful estimate, not a lab result — which is exactly what you'd expect from a glass float that can't actually taste a thing.
Brewer Questions, Answered
Can I calculate ABV from just the final gravity?
No. You need both readings. ABV comes from the change between original and final gravity, so a single number tells you nothing about how much sugar fermented. If you forgot to take an OG, you can only estimate it from your recipe's expected starting gravity — and your ABV is a guess from there.
What's the difference between specific gravity and Plato or Brix?
They measure the same thing — dissolved sugar — on different scales. Specific gravity is a ratio to water (1.050). Degrees Plato and Brix express it as roughly the percentage of sugar by weight (about 12°P for that same wort). Wine and many commercial brewers favor Plato/Brix; homebrewers usually use specific gravity. Most calculators convert between them.
Why is my calculated ABV higher than commercial beers of the same style?
The simple 131.25 formula tends to read a touch high, and it overstates more as gravity climbs. Commercial labels come from direct lab measurement, which the gravity estimate can't perfectly match. For stronger beers, use the alternate (curved) formula, and confirm your hydrometer readings were taken at its calibration temperature.
The Float That Knows More Than It Lets On
So the hydrometer never lied about not sensing alcohol — it genuinely can't. It only knows how heavy your liquid is. But because sugar turning into ethanol is the one thing that lightens a fermenting wort, that weight is a perfect informant. Two readings, a subtraction, a constant tuned by chemistry, and a bobbing glass tube tells you what's in the glass. Not bad for something that looks like it fell out of a tackle box.