Working Out ABV From Two Gravity Readings
Two hydrometer readings, taken weeks apart, are what turn a fermentation from a guess into a measurement. Everything else about a batch's strength follows from them.
The formula, and what it hides
The ABV calculator uses the standard homebrew approximation:
ABV % = (OG − FG) × 131.25
A beer that starts at 1.050 and finishes at 1.010 has dropped 0.040 of gravity, which multiplies out to 5.25% ABV. One at 1.060 down to 1.012 gives 6.30%.
The 131.25 is an empirical constant, not a physical one. It bundles together the conversion from sugar consumed to alcohol produced, and it is fitted to the range most beers live in. At high strengths it drifts — a 1.090 to 1.020 fermentation reports 9.19%, and the more elaborate formulas used for barley wines and imperial stouts come out a little differently. For anything under about 8% the difference is smaller than your reading error.
Attenuation is the more useful number
Alongside ABV the calculator returns apparent attenuation — the proportion of the available gravity the yeast actually consumed:
attenuation % = (OG − FG) ÷ (OG − 1) × 100
All three of those beers above land near 80%, which is a healthy, ordinary figure. That is what makes attenuation the better diagnostic: ABV tells you how strong the beer is, attenuation tells you whether the fermentation did what it was supposed to.
What the number is telling you
Yeast strains have published attenuation ranges, and comparing your figure against the strain's is the fastest way to spot a problem.
Coming in well under the range usually means the fermentation stalled or the mash produced a less fermentable wort than intended. Coming in well over can mean an infection — wild organisms eat sugars brewer's yeast cannot — or simply a very fermentable mash. Either way it is a question worth asking before the beer goes into bottles.
Take the reading properly
Hydrometers are calibrated at a specific temperature, usually 20 °C or 60 °F. A sample read straight out of a warm fermenter reads low, and one read cold reads high. Let the sample settle to calibration temperature, or apply a correction.
Spin the hydrometer as you drop it in so bubbles release from the stem, and read at the bottom of the meniscus. A degassed sample matters too: dissolved CO2 lifts the hydrometer and flatters your final gravity.
Do not trust a refractometer after pitching
This catches everyone once. Alcohol has a different refractive index from sugar, so once fermentation begins a refractometer's reading no longer converts cleanly to gravity. The Brix converter is for unfermented wort — pre-boil checks and original gravity.
Use a hydrometer for final gravity, or an alcohol-corrected formula built for the job. A refractometer FG plugged straight into an ABV calculation will overstate the finish and understate the strength.
Two readings, days apart
The definition of “final” gravity is not a date; it is stability. Take a reading, wait two or three days, take another. If they match, fermentation is done. If the second is lower, it was not.
Bottling an unfinished beer is how bottles over-carbonate, and in the worst case burst. The priming calculator assumes the only sugar left is the sugar you add.