Fermenting out and packaging
is fermentation finishedABV from gravityattenuationpriming sugarbottling calculations
Reading a finished fermentation: attenuation and alcohol from two hydrometer numbers, and the priming sugar the bottles then need.
Specific Gravity to Degrees Plato
Converts a hydrometer's specific gravity to degrees Plato, the percent of dissolved extract by weight that a refractometer and every commercial brewery report.
Apparent Attenuation
The share of the wort's original extract that the yeast has fermented away, read off the hydrometer. The number that says whether a fermentation has finished or has stalled.
Alcohol by Volume from Gravity
The standard homebrew estimate of alcohol by volume from the original and final gravity readings — the single most-used calculation in brewing, and one of the least exact.
Priming Sugar for a Carbonation Level
How much sugar to add at packaging so the yeast produces exactly the carbonation you want, allowing for the CO₂ the beer already holds and for what the sugar actually yields.
How they fit together
Brew day is over and the beer is in a vessel doing something you cannot see. Everything you learn about it for the next three weeks comes from two numbers, an original gravity and a current one, so this short set is worth more attention than its size suggests. Specific gravity to Plato is first only because half the industry speaks one scale and half the other, and a recipe read in the wrong one is out by a factor of about four. Plato is roughly sugar as a percentage by weight; SG is a density ratio. 12°P and 1.048 are the same wort.
Apparent attenuation answers the only question that matters mid-ferment: has it stopped. The word apparent is not decorative. Alcohol is lighter than water, so a fermenting beer's hydrometer reads lower than the sugar left in it would justify, and apparent attenuation consequently overstates the true figure by roughly a fifth. That is fine — every published yeast attenuation range is quoted apparent, so the numbers are comparable — but it does mean a beer reading 1.010 has real sugar in it still, and a final gravity is only final when it has not moved for three days. ABV from gravity runs off the same pair of readings.
Priming sugar is where the set turns dangerous rather than merely inaccurate. The equation wants residual CO₂, and residual CO₂ is set by the highest temperature the beer has reached since fermentation ended, not the temperature it is at now — a beer that spent a warm week at 22 °C and was then chilled holds the CO₂ of the warm week. Get that backwards and you over-prime. And prime a beer that has not actually finished attenuating and the sugar in the equation is not the only sugar in the bottle. That is how bottles become grenades, which is why apparent attenuation sits immediately before this one.