The formula
potential alcohol = °Brix × 0.59 · sugar = degrees short × 17 g/l
Where it comes from
Brix measures the sugar dissolved in the must, and each degree makes a little over half a degree of alcohol when it ferments out. The exact factor depends on the yeast and the temperature, which is why it is a field: between 0.55 and 0.62 depending on which handbook you read. If the must falls short you chaptalise, which is adding sugar before fermentation; the cellar rule is seventeen grams per litre to gain one degree.
How to work it out by hand
- Read the Brix of the must with a refractometer
- Multiply by the conversion factor, around 0.59
- Subtract that potential from the strength you want
- Multiply the shortfall by 17 grams of sugar per litre
What is worth knowing
Chaptalisation is regulated and many appellations do not allow it, so check the local rules first: this calculator does the arithmetic, it does not grant permission. A must above 26 Brix causes a different problem: yeast rarely survives past fifteen or sixteen per cent alcohol, so fermentation stops with sugar left and you get a sweet wine you did not want. A refractometer also stops working once fermentation starts, because alcohol bends the light: from then on you need a hydrometer.