Brewing hydrometer: A dedicated alcohol hydrometer (alcoholmeter) calibrated to 20°C is the most reliable tool for reading gravity and finished spirit ABV. A standard brewing hydrometer covers the 0.990 to 1.120 SG range needed for fermentation tracking.
What Specific Gravity Measures
Specific gravity (SG) is the ratio of the density of a liquid to the density of pure water at the same temperature. Pure water at 20°C has an SG of exactly 1.000. When you dissolve sugars in water, the resulting liquid is denser than water, so its SG is greater than 1.000. As yeast ferments those sugars and converts them to alcohol, the liquid becomes less dense because alcohol is lighter than water. The SG reading falls as fermentation progresses.
A hydrometer measures SG by floating in the liquid. The deeper it sinks, the lower the density and the lower the SG. SG readings are conventionally taken at 20°C. Readings taken at a different temperature need a temperature correction applied before you can use them accurately.
Original Gravity and Final Gravity
Original gravity (OG) is the SG of the wash or wort measured before fermentation begins. It reflects the total dissolved sugar content at the start. A higher OG means more sugar available for the yeast to consume, which leads to a higher potential ABV.
Final gravity (FG) is the SG measured after fermentation is complete. It is always lower than the OG because sugars have been converted to alcohol and CO2. The difference between OG and FG is the key figure used to calculate ABV.
Fermentation is complete when the FG has been stable across two readings taken 24 to 48 hours apart. A single low reading is not enough confirmation because a slow fermentation can look finished before it actually is.
The Simple Formula
The most widely used formula for calculating ABV from gravity is:
For example, a wash with OG 1.060 that finishes at FG 1.010 gives:
This formula is a linear approximation. It works well for gravities typical of most sugar washes and grain mashes, where the OG is below roughly 1.080. At higher original gravities the approximation becomes slightly less accurate because the relationship between gravity and alcohol content is not perfectly linear.
The Balling Formula
The Balling formula gives more accurate results at higher gravities because it accounts for the effect that dissolved alcohol has on the density of the liquid:
At lower gravities the difference between the two formulas is small. At higher starting gravities, such as OG 1.100 or above, the Balling formula produces a meaningfully more accurate result. DistilCalc's Fermentation ABV Calculator uses the Balling formula in hydrometer mode for this reason.
Potential ABV
Potential ABV is the estimated maximum alcohol content if the fermentation runs to complete dryness, meaning FG reaches 1.000. It is calculated from the OG alone using the simple formula with FG assumed as 1.000:
Potential ABV is a planning figure, not an outcome guarantee. In practice, most fermentations do not finish at exactly 1.000. Sugar washes with distilling yeast typically finish between 0.990 and 1.000. Grain mashes usually finish between 1.005 and 1.015 depending on the mash composition and yeast strain. The actual ABV will be slightly different from the potential ABV depending on where the fermentation finishes.
OG to Potential ABV Reference Table
The following table shows the potential ABV for common original gravity readings, assuming complete fermentation to FG 1.000 and using the simple formula.
| Original Gravity (OG) | Sugar content (approx.) | Potential ABV |
|---|---|---|
| 1.030 | Low | 3.9% |
| 1.040 | 5.2% | |
| 1.050 | 6.6% | |
| 1.060 | Moderate | 7.9% |
| 1.070 | 9.2% | |
| 1.080 | 10.5% | |
| 1.090 | High | 11.8% |
| 1.100 | 13.1% | |
| 1.110 | 14.4% | |
| 1.120 | Very high | 15.8% |
Note that very high OG values above 1.100 can stress yeast and may result in a stuck fermentation before the potential ABV is reached. Always match your OG to your yeast's stated alcohol tolerance.
Enter your OG and FG in SG, Brix, or Plato. The calculator uses the Balling formula and applies the Terrill correction for refractometer readings.
Brix and Plato
Brix and Plato are two other scales used to measure sugar content in solution. Both express the concentration of dissolved sugars as a percentage by weight. For practical distilling purposes, Brix and Plato give the same reading and the two terms are often used interchangeably.
As an approximation, 1 degree Brix is equivalent to a specific gravity of roughly 1.004. So a reading of 15° Brix corresponds to approximately SG 1.060, and a reading of 20° Brix corresponds to approximately SG 1.080. The relationship is not perfectly linear across the full range but this approximation is adequate for most practical purposes.
DistilCalc's fermentation calculator accepts OG and FG in SG, Brix, and Plato directly and converts between them using the ICUMSA polynomial (for Brix) and the De Clerk formula (for Plato). You do not need to convert manually.
Why Refractometers Are Unreliable After Fermentation
A refractometer measures how much a liquid bends light, a property called refractive index. Refractometers calibrated for sugar solutions give accurate Brix readings before fermentation, when the liquid contains only water and dissolved sugars.
Once alcohol is present, it bends light differently from sugar. A refractometer reading taken from a fermenting or fermented wash will show a higher value than the true gravity because the alcohol is increasing the apparent refraction. The reading is not accurate without applying a correction formula such as the Terrill correction. For this reason, most distillers use a hydrometer for final gravity readings and reserve the refractometer for pre-fermentation checks only.
Frequently Asked Questions
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