Two formulas, and why nobody agrees which one to print
The formula almost every homebrewer meets first is alcohol by volume equals original gravity minus final gravity, times 131.25. It is easy to do in your head, it is close enough for ordinary strength beer, and it is wrong in a predictable direction once the wort gets strong. A beer that goes 1.050 to 1.010 comes out at 5.25% by that formula. The alternate form, which is fitted rather than derived and appears in most brewing software, gives 5.34% for the same pair. Nine hundredths of a point is not worth arguing about.
Push the gravities up and the gap opens. A barleywine at 1.100 finishing at 1.020 is 10.5% by the simple multiplier and 11.58% by the alternate. That is more than a full point, which is the difference between two beers you would pour differently. The direction is worth remembering: the simple multiplier reads low on strong beer, not high. The constant 131.25 is calibrated around ordinary gravities and does not account for the fact that a strong wort has more dissolved solids affecting the density that the hydrometer is actually reading.
The alternate form used here is
ABV = (76.08 x (OG − FG) / (1.775 − OG)) x (FG / 0.794)
That is an empirical fit, not physics. It is better behaved across a wide gravity range and it is the one most calculators quietly use when they show a single number. This page shows both, because a single number implies a precision that neither formula has.
Attenuation, apparent and real
Apparent attenuation is the fraction of the original gravity points the yeast removed: (OG − FG) / (OG − 1.000), expressed as a percentage. The 1.050 to 1.010 beer above attenuated 80% apparent, which is at the upper end of what a clean ale yeast does with a normal mash.
It is called apparent because the hydrometer is lying to you, and it is lying in a useful, consistent way. Ethanol is less dense than water, so a fermented liquid reads lower than its actual sugar content would suggest. The real extract is what is genuinely still dissolved, and it is estimated from the two gravities in Plato with the standard relation RE = 0.1808 x OG°P + 0.8192 x FG°P. Real attenuation always comes out lower than apparent, typically around 80% of it. For the example beer the apparent figure is 80% and the real figure is 65%.
Which one you want depends on the question. Apparent attenuation is what yeast manufacturers publish and what you compare against a strain's stated range, so it is the one to use when you are asking whether the yeast did its job. Real attenuation is what feeds the calorie estimate, because calories come from what is actually left in the glass.
The readings, which are where the error lives
Every number on this page inherits the error in two measurements. Three things routinely spoil them.
| Problem | What it does | What to do |
|---|---|---|
| Sample not at the hydrometer's calibration temperature | Warm sample reads low, cold sample reads high, by a few thousandths across a normal range | Cool the sample, or turn on the correction above and enter both temperatures |
| Refractometer used after fermentation started | Reads badly high — ethanol bends light differently from sugar | Use a hydrometer for the final reading, or a published dual-reading correction |
| Wort not fully mixed when the OG sample was drawn | Top-up water or sugar syrup sitting in a layer gives a reading from the wrong part of the vessel | Stir hard, then draw the sample |
The refractometer problem is the single most common source of a wrong homebrew number. A refractometer measures how much the sample bends light, and it is calibrated assuming the only thing dissolved is sugar. The moment there is alcohol in the sample that assumption fails, and the instrument reads high — often enough to make a finished beer look stuck. There are correction formulas that take the original Brix and the current Brix and estimate a true final gravity, and they help, but they carry their own error and they need the pre-fermentation reading to be from the same instrument. A refractometer is excellent for tracking pre-boil and post-boil gravity where the sample is a few drops. It is the wrong tool for the reading that decides your ABV.
Calories, and what they are not
The calorie estimate uses the standard relation built from alcohol by weight and real extract: [(6.9 x ABW) + 4.0 x (RE − 0.1)] x FG x 3.55 for a 12 oz serving. For the 5.3% example that returns about 164 calories, which is where an ordinary pale ale sits. Alcohol carries most of it — roughly 100 of those calories in that beer are from the ethanol rather than the residual sugar, which is why a dry strong beer has more calories than a sweet weak one.
The figure is an estimate from two density readings and nothing else. It cannot see adjuncts added after fermentation, lactose, fruit, or anything else that changes the extract without changing the gravity in the way the model expects. This site does not make health claims about alcohol in either direction; the calorie number is here because people ask for it, and it belongs in the same category as the ABV — a working estimate for your own notes.
Next steps
If the beer is short of the gravity you planned, the problem usually happened upstream of the yeast. The strike water calculator covers mash temperature, which sets how fermentable the wort was in the first place, and the brewing water and boil-off calculator covers the volume error that shifts every gravity reading in a batch. If the beer finished where you wanted, the priming sugar calculator is the next number you need. For the general question of what alcohol does to a body over hours rather than in a glass, the blood alcohol calculator and the alcohol calories page cover different ground.
Questions people ask
Which ABV formula should I actually use?
Below about 1.070 original gravity it does not matter — the two agree within about a tenth of a point, which is smaller than the error in your hydrometer readings. Above that, use the alternate form, because the simple multiplier reads low on strong wort and the gap grows with gravity. At 1.100 down to 1.020 the difference is over a full percentage point. What matters more than the choice is consistency: pick one, use it for every batch, and your batch-to-batch comparisons stay meaningful even if the absolute number is off.
My refractometer says the beer is still at 1.020 but it tastes finished. What is wrong?
Almost certainly nothing is wrong with the beer. A refractometer reads high once alcohol is present, because it infers sugar content from how the sample bends light and ethanol bends light more than water does for the same concentration. A finished beer can read two or three gravity points high, sometimes more. Take a hydrometer reading of a degassed sample at a known temperature and use that. If you only have a refractometer, look up a dual-reading correction that takes your original Brix and current Brix together — it will get you close, but it inherits error from both readings and from the correction itself.
Why is my apparent attenuation higher than the yeast manufacturer says?
Attenuation is a property of the wort as much as the yeast. A mash held low produces more fermentable sugar and a beer that finishes drier than the same yeast would give from a mash held high, so a strain rated at 75% can easily reach 82% on a thin, low-mashed wort. Simple sugar additions push it further, since they are fully fermentable and the strain has nothing to leave behind. Published attenuation ranges are measured on a standard laboratory wort. Treat them as orientation, not a specification.
Does temperature correction matter enough to bother with?
Usually it changes the result by a fraction of a percent of ABV, which is inside the noise. It becomes worth doing when the two readings were taken at very different temperatures — a hot post-boil sample and a cold conditioned one — because then the error does not cancel and it lands entirely in the difference between them, which is the number the whole calculation depends on. The correction here is the standard density relation referenced to whatever calibration temperature is printed on your hydrometer, and it is reliable between roughly 50 and 100 degrees F.
Can I use this to work out how much alcohol I made in total?
Enter the batch volume and it will multiply through, giving pure alcohol volume and full servings in the batch. Bear in mind that home fermentation of beer, wine, cider and mead is governed by federal rules and by separate state rules, and both set conditions and limits that differ by state and change over time. This site does not state those rules, and it does not cover distillation at all — distilling spirits at home in the US is a federal matter with its own licensing regime and is outside the scope of everything here. Check current federal guidance and your own state before assuming what you are allowed to make.