Homebrew ABV Calculator - Alcohol by Volume from OG and FG

Same wort, two readings - and the gap between them is your alcohol

ABV (Alcohol by Volume) is the percentage of alcohol in your finished beer. You cannot measure it directly at home. Instead, you measure the gravity of the wort before fermentation (OG) and after (FG). The difference tells you how much sugar the yeast converted into alcohol.

How to calculate ABV - step by step

Choose your gravity unit - SG (Specific Gravity, e.g. 1.050) or Plato (e.g. 12.5P). Hydrometers typically read SG, refractometers read Plato.
Enter Original Gravity (OG) - the reading you took before pitching yeast, after cooling the wort to 20C / 68F.
Enter Final Gravity (FG) - the reading after fermentation is complete (stable for 2-3 days).
Pick a formula - standard for beers under 8% ABV, Balling for stronger brews.
Read results - ABV, apparent attenuation, calories per 330 ml, strength classification and contextual tips.

Before and after fermentation - what the numbers mean

Before (OG)

1.050

12.4 Plato - sugar-rich wort

Sweet, no alcohol, no carbonation

After (FG)

1.010

2.6 Plato - most sugar consumed

5.25% ABV, 80% attenuation, ~150 kcal/330 ml

ABV by beer style - reference table

Beer style	OG (SG)	FG (SG)	ABV
Berliner Weisse	1.028-1.032	1.003-1.006	2.8-3.8%
Pilsner / Lager	1.044-1.052	1.008-1.014	4.2-5.4%
Wheat Beer	1.044-1.052	1.010-1.014	4.3-5.6%
Pale Ale / APA	1.045-1.060	1.010-1.015	4.5-6.2%
IPA	1.056-1.075	1.010-1.018	5.5-7.5%
Stout / Porter	1.048-1.065	1.012-1.018	4.2-6.3%
Belgian Tripel	1.075-1.085	1.008-1.014	7.5-9.5%
Imperial Stout / Barleywine	1.080-1.120	1.018-1.030	8.0-12.0%

Practical examples

Example 1: Classic lager
OG 1.048, FG 1.010, standard formula
Result: ABV 4.99%, attenuation 79.2%, ~148 kcal/330 ml

Example 2: West Coast IPA
OG 1.065, FG 1.012, standard formula
Result: ABV 6.95%, attenuation 81.5%, ~193 kcal/330 ml

Example 3: Imperial stout (use Balling)
OG 1.100, FG 1.022, Balling formula
Result: ABV 10.7%, attenuation 78.0%, ~303 kcal/330 ml

Example 4: Plato units - Czech lager
OG 12.0 P, FG 3.5 P, standard formula
Result: ABV 4.59%, attenuation 72.1%, ~146 kcal/330 ml

Example 5: Session beer
OG 1.035, FG 1.008, standard formula
Result: ABV 3.54%, attenuation 77.1%, ~107 kcal/330 ml

FAQ - Frequently asked questions

Can I measure FG with a refractometer?

Technically yes, but the reading will be too low because alcohol changes the refraction index of the solution. You need a refractometer correction calculator (requires both OG and the current refractometer reading). The most accurate FG measurement comes from a hydrometer.

When should I take the final gravity reading?

When fermentation appears complete (no bubbles in the airlock), take a reading. Repeat after 2-3 days. If the value has not changed, fermentation is done. If it dropped, wait and measure again.

Why is my ABV lower than expected?

Common causes: mash temperature too low (fewer fermentable sugars), underpitching yeast, fermentation temperature too low, or a low-attenuating yeast strain. Check your strain's attenuation spec - the difference between 65% and 80% attenuation is enormous in terms of ABV.

What is the difference between standard and Balling formula?

Below 6% ABV, the difference is 0.01-0.05 pp - negligible. At 8-10% ABV it grows to 0.2-0.4 pp. Above 10% the standard formula underestimates by up to 0.5 pp. For imperial beers, always use Balling.

What if my FG is higher than expected?

Fermentation may have stalled (stuck fermentation). Causes: underpitching, temperature too low or too high, insufficient oxygen in the wort. Try: raise the temperature by 2-3C, gently swirl the fermenter, or repitch fresh yeast. Do not bottle beer with incomplete fermentation - risk of bottle bombs.

Can FG go below 1.000?

Yes. Beers fermented with Brettanomyces can reach FG below 1.000 because brett ferments complex sugars that Saccharomyces cannot. Apparent attenuation above 100% is not a measurement error - it is brett doing its job.

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