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ABV Calculator (Homebrew)

You took your original gravity reading before pitching yeast, finished fermentation, and pulled a final gravity reading off the hydrometer — now you need to know how strong your beer (or cider, mead, wine) actually came out. This calculator runs the modern Brewer's Friend formula for alcohol by volume, plus apparent attenuation (how much of the sugar the yeast converted) and approximate calories per 12 oz serving. Works for any sugar-fermented beverage where you have OG and FG hydrometer readings in standard specific gravity units (e.g., 1.060 → 1.012).

How the math works

Specific gravity is the density of your wort or must compared to water (water = 1.000). Sugar makes the liquid denser; alcohol makes it lighter. The difference between OG (before fermentation) and FG (after) tells you how much sugar got converted to alcohol.

Two formulas in common use:

  • Simple: ABV ≈ (OG – FG) × 131.25. Accurate up to about 8% ABV.
  • Brewer's Friend (what this calculator uses): ABV = (76.08 × (OG – FG) / (1.775 – OG)) × (FG / 0.794). More accurate above 8% — essential for stronger beers, ciders, and meads.

Worked example: an American IPA fermented from OG 1.060 to FG 1.012. ABV = (76.08 × 0.048 / (1.775 – 1.060)) × (1.012 / 0.794) = 6.51%. Attenuation = (0.060 – 0.012) / 0.060 = 80%. Calories per 12 oz ≈ 200.

Calorie estimate uses both residual sugar (FG-derived) and ethanol content: calories ≈ ((OG – FG) × 295 + (FG – 1) × 700) × 12.

How to use this calculator

  1. Take an OG reading with a calibrated hydrometer at 60°F (or apply a temperature correction) immediately after cooling wort and before pitching yeast.
  2. Take an FG reading after fermentation appears complete and the gravity stays the same for 2-3 consecutive days.
  3. Enter both as 3-decimal specific gravity values (1.060, 1.012, etc.).
  4. The calculator returns ABV%, apparent attenuation, and calories per 12 oz.
  5. For a 5-gallon batch: multiply ABV % by 5 to estimate total ounces of pure alcohol produced — a 6.5% ABV 5-gallon batch contains about 4.2 oz of pure ethanol.

Common scenarios

American Pale Ale: OG 1.055, FG 1.012. ABV = 5.69%, attenuation 78%. About 175 calories per 12 oz. Solid balanced ale.

Imperial Stout: OG 1.095, FG 1.020. ABV = 10.34%, attenuation 79%. ~325 calories per 12 oz. The high OG means the Brewer's Friend formula matters — simple formula would give 9.84%, half a point low.

Hard cider: OG 1.050, FG 0.998. ABV = 7.04%, attenuation 104% (cider yeasts can ferment past 1.000 because the wort isn't dextrin-heavy like beer). About 130 calories per 12 oz — lower than beer because the residual sugars are gone.

FAQ

Why is my hydrometer reading off? +
Most hydrometers are calibrated at 60°F. If you measure at 70°F, the reading is about 0.001 low; at 80°F about 0.002 low. Either cool your sample to 60°F before measuring or apply a temperature correction (most hydrometers include a temp chart on the side, or use the BeerSmith app).
Refractometer vs. hydrometer for FG? +
Refractometers read sugar but get fooled by alcohol — a refractometer FG reading on fermented beer is always artificially high by 1.5-2 points. Use a correction formula (Sean Terrill's refractometer correction is the industry standard) or use a hydrometer for FG. Use refractometer for pre-boil and OG to save sample volume.
What's apparent vs. real attenuation? +
Apparent attenuation (what most people quote and what this calculator returns) is based on the difference between OG and FG. Real attenuation accounts for the fact that ethanol has lower density than water and pulls the gravity reading down independent of remaining sugar. Real attenuation = apparent x 0.81 approximately. Apparent is the convention in homebrewing.
What attenuation should I expect from my yeast? +
Typical attenuation ranges: dry ale yeasts (US-05, S-04): 75-82%. English ale yeasts (Wyeast 1968, etc.): 67-75%. Belgian strains: 80-90%. Lager yeasts: 75-85%. Champagne/wine yeasts: 90-100%. If you're way below your yeast's typical range, the wort may have un-fermentable dextrins, you may have under-pitched, or fermentation may have stalled cold.
Why are my calories estimated lower than beer industry numbers? +
Industry calorie counts often include the post-fermentation additions (priming sugar, malto-dextrins for body, lactose for sweetness). This calculator estimates calories from gravity alone. Commercial beers in the 5% ABV range typically claim 140-180 calories per 12 oz; homebrews land in the same range when measured properly.
Can I use this for distilling — to calculate output proof? +
For fermented wash (mash) ABV, yes. After distillation, alcohol concentration depends on still efficiency, cuts, and dilution — not just the wash ABV. Use a proofing hydrometer or alcoholmeter on the final spirit. Distilling at home requires a federal TTB permit in the US, by the way.
What if my OG was higher than expected? +
You either had more efficient mashing than predicted, evaporated more during the boil (lower batch volume = higher gravity), or added more fermentables. Higher OG means higher ABV potential. Adjust your recipe efficiency assumption in BeerSmith / brewing software for future batches.
What if my FG is much higher than expected? +
Stalled or stuck fermentation. Check temperature (yeasts crash cold), oxygen levels at pitch, yeast pitch rate, and wort fermentability. Often fixed by raising temp 2-4°F, gentle rousing to resuspend yeast, or pitching a fresh starter. Don't add gravity readings on top of each other — take fresh samples each time.