Brewing ABV Calculator
Alcohol by volume from original and final gravity, with ABW and attenuation.
What it does
Alcohol by volume starts as two density readings. Brewers float a hydrometer in the wort before fermentation for the original gravity, then again after for the final gravity. Yeast turns sugar into alcohol and carbon dioxide, so the liquid gets lighter, and the size of that fall maps to the alcohol made.
The tool above applies the standard homebrew factor: ABV equals the gravity drop times 131.25. A fall from 1.050 to 1.010 is 0.040, which gives 5.25 % ABV. Peer reviewed brewing protocols publish exactly this factor, and the TTB lists the official lab methods behind it.
US regulators frame the background. The TTB Beverage Alcohol Methods page catalogs the official ethanol tests, from hydrometry through gas chromatography, while the Brewers Association style guidelines tabulate OG, FG, and alcohol ranges per style. NIST adds the measurement side with its guide to alcohol testing.
The factor has limits worth knowing. It fits normal beer strengths and drifts for strong beers past about 8 % ABV, where non linear corrections or distillation apply. Temperature also shifts hydrometer readings, so samples belong at the meter calibration point before reading.
How to use
- Take the OG reading before pitching yeast and enter it.
- Take the FG reading after activity stops and enter it.
- Check that FG sits below OG. A higher FG means a misread or unfinished ferment.
- Read ABV first, then ABW and attenuation.
- Compare attenuation with the yeast strain range, often 65 to 85 %.
- For strong beers past 8 % ABV, confirm with a lab method.
Formula
- OG
- Specific gravity of the wort before fermentation.
- FG
- Specific gravity after fermentation, stable over two days.
- 131.25
- Linear factor from brewing protocols for normal strengths.
- 0.7939
- Density ratio of ethanol to water used for the ABW form.
OG and FG use specific gravity units where water reads 1.000. The 131.25 factor fits normal beer strengths up to about 8 % ABV; past that, treat the result as an estimate.
Worked example
Example: OG 1.050, FG 1.010
Original gravity 1.050, final gravity 1.010. These are also the default values, so the page opens on this answer.
- Gravity drop: 1.050 minus 1.010 = 0.040.
- ABV: 0.040 x 131.25 = 5.25 % ABV.
- ABW: 5.25 x 0.7939 / 1.010 = 4.13 % ABW.
- Attenuation: 0.040 / 0.050 x 100 = 80 %.
- Tool shows 5.25 % ABV, 4.13 % ABW, 80 %.
Reference table
| Style | OG to FG | ABV |
|---|---|---|
| Session ale | 1.035 to 1.010 | about 3.3 % |
| Pale ale | 1.050 to 1.012 | about 5.0 % |
| IPA | 1.064 to 1.013 | about 6.7 % |
| Lager | 1.048 to 1.010 | about 5.0 % |
| Imperial stout | 1.095 to 1.024 | about 9.3 % |
Limits
- Fits normal beer strengths. Strong beers need non linear corrections.
- Assumes clean fermentation. Infections and unfermentables shift readings.
- Needs temperature corrected hydrometry. Warm samples read low.
- Estimates alcohol, not flavor, body, or legal label status.
Common mistakes
- Reading FG while it still falls day to day.
- Skipping temperature correction on the hydrometer.
- Using the factor past 8 % ABV without correction.
- Mixing Plato and specific gravity scales in one calculation.
- Bottling on a high FG and getting over carbonation.
Try it yourself
Quick checks. Answers sit tucked under each question.
OG 1.040, FG 1.015?
Drop 0.025. 0.025 x 131.25 = 3.28 % ABV. Attenuation 0.025 / 0.040 x 100 = 62.5 %.
OG 1.060, FG 1.012?
Drop 0.048. 0.048 x 131.25 = 6.3 % ABV. ABW 6.3 x 0.7939 / 1.012 = 4.94 %.
Questions
What are OG and FG?
Original gravity (OG) is the density of the wort before fermentation, final gravity (FG) the density after, both read on a hydrometer or digital density meter.
Where does 131.25 come from?
A linear fit for normal beer strengths: each gravity point of drop maps to about 0.13125 % ABV. Peer reviewed brewing protocols publish exactly this factor.
How do I know fermentation is finished?
Read FG on two straight days. A stable reading near the expected range means fermentation is done; a falling reading means it is still working.
Does the formula work for strong beers?
The linear factor drifts past about 8 % ABV. Strong beers need a non linear correction or a lab method such as distillation or gas chromatography.
What is the difference between ABV and ABW?
US labeling moved from weight to volume. ABW still appears in style tables; divide roughly by 0.8 to move between them at beer gravities.
Do I correct readings for temperature?
Temperature shifts hydrometer readings. Cool the sample to the meter calibration point, often 60 F (15.6 C), or apply the meter correction chart.
Sources
- TTB: Beverage Alcohol Methods
- Brewers Association: Beer Style Guidelines (OG, FG, ABV ranges)
- NIH PMC: Hands-On Guide to Brewing and Analyzing Beer (131.25 factor)