Alcohol by volume with a hydrometer

The alcohol content tells you what you are serving your guests, and it goes on the label. It also confirms that fermentation went all the way: a beer planned at 6% that stops at 4.5% has a yeast problem, not a recipe problem.

Alcohol comes from the sugars the yeast consumed. Gravity drops as fermentation proceeds, and the difference between the original gravity (OG) and the final gravity (FG) measures what was converted.

The classic mistake is to forget the priming sugar. Added at bottling, it ferments too and adds 0.3 to 0.5% alcohol: with 7 g/L of sugar, a beer calculated at 5.25% is actually 5.7%.

The tool applies the usual brewers' formula, then adds the alcohol produced by the priming sugar:

ABV (% vol) = 131.25 × (OG − FG) + sugar (g/L) / 19.5 / 0.789

The factor 131.25 is a linear approximation, accurate up to an original gravity of about 1.070. The second term converts grams of sugar per litre into alcohol: it takes about 19.5 g of sugar to produce 1% alcohol by weight, and 0.789 is the density of ethanol.

Both gravities must be read at 20 °C, or corrected for temperature.

• The formula underestimates the strength of strong beers: above 1.070, use the estimated final gravity, which applies the full formula.
• Gravities must be corrected to 20 °C: see the gravity correction for temperature.
• A hydrometer reads to within ± 0.001, about ± 0.15% alcohol.