What degrees Brix actually measures
One degree Brix is one gram of sucrose in 100 grams of solution. The scale is defined against pure sucrose in water — the reference tables are maintained by ICUMSA, the International Commission for Uniform Methods of Sugar Analysis — and both refractometers and hydrometers marked in Brix are calibrated against it.
That definition carries a warning inside it. Grape juice is not pure sucrose in water. It is mostly glucose and fructose, plus organic acids, potassium salts, pectins and phenolics, all of which bend light and add mass. A refractometer reports whatever sucrose solution would refract the same, so a 24 °Bx must does not contain 240 grams of fermentable sugar per kilogram. It contains a little less, and the rest of the reading is everything that is not sugar. Every conversion on this page inherits that fact, which is why the Brix-to-alcohol factor is a factor and not a constant of nature.
Specific gravity measures something different again: the density of the liquid relative to water. Sugar raises it, so gravity and Brix track each other closely and either can be converted to the other. Alcohol lowers it, which is what makes the finishing reading useful and what makes a dry wine read below 1.000. A hydrometer floats lower in a fermented wine than it does in water.
The three numbers this calculator moves between therefore answer three different questions. Brix tells you how ripe the fruit is. Grams per litre tells you how much sugar you are actually handling, which is the number you need if you are chaptalising or back-sweetening. Potential alcohol tells you what the label will say.
The three conversions, and where the 0.55 to 0.59 factor comes from
Brix to gravity. The relation used throughout brewing and winemaking software is SG = 1 + B ÷ (258.6 − B × 227.1 ÷ 258.2). It is a fitted approximation to the sucrose density tables, not a derivation, and it is good to a few ten-thousandths of a gravity point over the range you will ever pick fruit at. It inverts in closed form: B = 258.6d ÷ (1 + 0.8796d) where d = SG − 1, so the calculator round-trips exactly whichever scale you enter.
Gravity to alcohol. ABV = (OG − FG) × 131.25 is the standard workshop formula. The 131.25 is empirical: it absorbs the density of ethanol, the mass of carbon dioxide that leaves the vessel, and the fact that gravity is a proxy for sugar rather than a measurement of it. It is accurate enough for table wines and drifts high above roughly 12% ABV, which is one reason a professional cellar measures finished alcohol by distillation or by ebulliometer rather than by hydrometer.
Brix to potential alcohol. Work it from the chemistry. Gay-Lussac's equation says one molecule of glucose gives two of ethanol and two of carbon dioxide, so 180.16 g of sugar gives 2 × 46.07 = 92.14 g of ethanol — a yield of 0.511 g per gram. One percent alcohol by volume is 10 mL of ethanol per litre, and ethanol weighs 0.789 g/mL, so 1% ABV is 7.89 g of ethanol per litre and needs 7.89 ÷ 0.511 = 15.4 g of sugar. Real yeast keeps some carbon for cell walls and glycerol, which pushes the practical requirement to about 16.8 g of sugar per litre per 1% ABV — the figure European enrichment rules are written around.
Now put Brix back in. At 24 °Bx the gravity is 1.10106 and the reading corresponds to 24 × 1.10106 × 10 = 264.3 g/L. A factor of 0.59 predicts 14.16% ABV, which implies 264.3 ÷ 14.16 = 18.7 g of Brix per 1% ABV against 16.8 g of actual sugar. The gap, 18.7 ÷ 16.8 − 1 = 11%, is the non-sugar solids the refractometer counted. That is the whole content of the 0.55-to-0.59 range: pick the low end when the fruit is high in acid and extract, the high end for clean juice and a complete ferment.
Worked example: a 24 °Bx Cabernet must fermented dry
You pick at 24 °Bx and the wine finishes at −1.6 °Bx on a hydrometer. Work it through with a factor of 0.59.
- Starting gravity. 24 ÷ 258.2 = 0.09295, and 0.09295 × 227.1 = 21.109. So the denominator is 258.6 − 21.109 = 237.491, and SG = 1 + 24 ÷ 237.491 = 1.10106.
- Sugar in the juice. 24 × 1.10106 × 10 = 264.3 g/L.
- Potential alcohol. 24 × 0.59 = 14.16% ABV.
- Finishing gravity. Apply the same conversion to −1.6 °Bx: −1.6 ÷ 258.2 = −0.006197, times 227.1 is −1.4073, so the denominator is 258.6 + 1.4073 = 260.007 and SG = 1 − 1.6 ÷ 260.007 = 0.99385.
- Alcohol produced. 1.10106 − 0.99385 = 0.10721, and 0.10721 × 131.25 = 14.07% ABV.
- Apparent attenuation. 0.10721 ÷ 0.10106 × 100 = 106.1%, which is above 100 because the wine finished below the density of water.
- Cross-check. The gravity route gives 14.07% and the Brix factor predicted 14.16%. Agreement to within a tenth of a percent is as good as either method deserves; if they differed by a full point you would look for a temperature error in one of the readings.
Reading the result: what the numbers mean in the cellar
Potential alcohol is a ceiling, not a forecast. It assumes every fermentable gram is fermented, which happens only if the yeast survives to the end. Commercial wine strains are sold with a rated alcohol tolerance on the datasheet, and a must whose potential exceeds that rating is a stuck fermentation waiting to happen: the yeast dies with sugar still in the tank, and you finish with a sweet wine you did not intend and a microbial risk you did not want.
Wine grapes are normally picked somewhere around 20 to 26 °Bx, which brackets 11.8% to 15.3% potential alcohol at a factor of 0.59. Below that range the wine is thin and needs chaptalisation, where legal; above it you are into late-harvest territory, and the osmotic pressure of the sugar itself starts to inhibit the yeast before the alcohol does. Cider apples usually land between 11 and 16 °Bx, and honey must for mead is diluted to a target rather than measured off the fruit — see the mead honey amount calculator for that direction of the problem.
Apparent attenuation above 100% surprises people and is not an error. It is arithmetic: alcohol is less dense than water, so once the sugar is gone the gravity keeps falling past 1.000, and the numerator of the attenuation fraction ends up larger than the denominator. Any dry wine does this. Beer rarely does, because wort leaves unfermentable dextrins behind — that is why the beer ABV calculator normally reports attenuation in the 70% to 85% band instead.
Finally, treat the finished number as a working figure rather than a label claim. Labelling tolerances are set by regulation, and the hydrometer method drifts high on strong wines. If the number matters legally, have the alcohol measured by distillation.
Brix, gravity, sugar and potential alcohol across the harvest range
| °Bx | Specific gravity | Sugar (g/L) | Potential ABV at 0.55 | Potential ABV at 0.59 |
|---|---|---|---|---|
| 18.0 | 1.07414 | 193.3 | 9.90% | 10.62% |
| 20.0 | 1.08298 | 216.6 | 11.00% | 11.80% |
| 21.0 | 1.08745 | 228.4 | 11.55% | 12.39% |
| 22.0 | 1.09195 | 240.2 | 12.10% | 12.98% |
| 23.0 | 1.09649 | 252.2 | 12.65% | 13.57% |
| 24.0 | 1.10106 | 264.3 | 13.20% | 14.16% |
| 25.0 | 1.10566 | 276.4 | 13.75% | 14.75% |
| 26.0 | 1.11029 | 288.7 | 14.30% | 15.34% |
| 27.0 | 1.11497 | 301.0 | 14.85% | 15.93% |
| 28.0 | 1.11967 | 313.5 | 15.40% | 16.52% |
| 30.0 | 1.12919 | 338.8 | 16.50% | 17.70% |
Read the two potential-alcohol columns as a band rather than as competing answers: the low column is what a high-extract must with an incomplete ferment tends to deliver, the high column is a clean juice taken fully dry.
A refractometer stops working once fermentation starts
Ethanol refracts light far more strongly than its concentration suggests, so any refractometer reading taken during or after fermentation is badly wrong — it reads much too high, and a wine that has gone completely dry can still show 7 or 8 °Bx. Published correction formulas exist, but they are fitted to particular fermentations and disagree with each other by enough to matter. Use the refractometer for the harvest reading, where it is excellent because it needs two drops of juice, and take the finishing reading with a hydrometer, which measures density and is unbothered by what is dissolved in it.
What throws these conversions off
- Temperature. Both instruments are calibrated at a stated temperature, normally 20 °C. A warm sample reads low on a hydrometer, and juice straight off a hot press can be out by more than a full gravity point.
- Unhomogenised must. A reading taken from free-run juice sitting above crushed fruit is not the reading of the tank. Mix, then sample.
- Solids in the sample. Pulp and skin fragments make a hydrometer float high. Let the sample settle or strain it.
- Assuming Brix is fermentable sugar. It is not, and the whole 0.55-to-0.59 factor range exists to absorb the difference.
- Using the beer factor on wine, or the reverse. Wort carries unfermentable dextrins that grape juice does not, so a brewer's attenuation expectations do not transfer.
- Comparing a corrected and an uncorrected reading. If you apply a wort correction factor to the starting reading, apply the same treatment to every reading you compare it with, including the one you log at the end.
- Ignoring water additions. Any water added after the reading dilutes the sugar and the potential alcohol in exact proportion, in the same way a spirit dilutes when it is proofed down — see the alcohol dilution calculator for that arithmetic.
Key terms
- Must
- Freshly crushed juice, including skins and seeds where they are present, before or during fermentation.
- Original gravity (OG)
- The specific gravity of the juice or wort before fermentation begins. The starting point of the ABV calculation.
- Final gravity (FG)
- The specific gravity once fermentation stops. Below 1.000 for a dry wine, above it for anything with residual sugar or unfermentable extract.
- Apparent attenuation
- The fraction of the original gravity points that fermentation removed. Called apparent because alcohol distorts the density reading; the real attenuation is lower.
- Chaptalisation
- Adding sugar to a must to raise its potential alcohol. Regulated or prohibited in many wine regions, and the reason the grams-per-litre output on this page exists.
Where this fits with the other fermentation numbers
Brix is the harvest-side measurement and gravity is the cellar-side one, and most of the rest of a fermentation record hangs off the pair. Once you know the starting gravity you can size the yeast, plan the nutrient additions and predict the heat the ferment will release, all of which scale with sugar rather than with volume.
Brewers work the same conversions with different conventions. Wort gravity is usually quoted in gravity points rather than Brix, extraction efficiency becomes the dominant unknown, and the yield question turns into the one the brewhouse efficiency calculator answers: how much of the sugar theoretically available in the grain actually reached the kettle. The alcohol arithmetic at the end is identical.
Downstream of fermentation, the numbers stop being about sugar. Free sulfur dioxide additions are driven by pH and volume, which is the wine sulfite addition calculator. If you are fortifying or blending, the governing quantity is absolute alcohol rather than gravity, and the proof and dilution calculator handles that balance. And for a sweetened, low-alcohol ferment such as kombucha, the sugar you add is a feedstock for acid rather than for ethanol, which is why the kombucha sugar and starter calculator works from a ratio instead of from a Brix reading.
One habit is worth more than any of these conversions: log the reading, the temperature and the instrument every time. Almost every argument about a wine's alcohol turns out to be an argument about which reading was corrected and which was not.