Beer ABV Calculator
Calculate alcohol by volume from original and final gravity readings.
BSc Food Science, Certified Veterinary Nurse
Food scientist and veterinary nurse with a passion for culinary science, fermentation chemistry and animal care.
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About the Beer ABV Calculator
Calculating the alcohol content of homebrew beer requires measuring the change in density (specific gravity) of the wort between the start and end of fermentation. Original gravity (OG) is the density of the unfermented wort relative to water — a typical 5% ABV pale ale might have an OG of 1.050, meaning it is 5% denser than water. Yeast consumes the fermentable sugars and produces alcohol and CO₂; since alcohol is less dense than water, the final gravity (FG) after fermentation is lower. The difference between OG and FG indicates how much sugar was converted, which directly corresponds to the alcohol produced.
Gravity is measured using a hydrometer (a glass float calibrated to float at different depths in liquids of different densities) or a refractometer (which measures how light bends through a liquid sample — more convenient but requires a correction factor after fermentation, as alcohol affects the reading differently than sugar). Hydrometers are accurate when used correctly and calibrated at the correct temperature (usually 15°C or 20°C — always correct for temperature using calibration charts if measuring warm wort). A hydrometer and cylinder cost £5–15 and are essential homebrew equipment.
The simple ABV formula (OG − FG) × 131.25 is accurate for most homebrewing purposes for standard-gravity beers (OG below 1.080). For high-gravity beers (strong ales, barleywines, Imperial stouts with OG 1.080+), the alternate formula is more accurate because yeast attenuation behaviour changes at high sugar concentrations. Apparent attenuation — shown in the results — measures what percentage of the fermentable sugars the yeast consumed; healthy fermentation typically achieves 65–80% apparent attenuation depending on yeast strain, mash temperature, and adjuncts used.
How it works
ABV = (OG − FG) × 131.25 [Simple, standard gravity] ABV = (76.08 × (OG − FG) / (1.775 − OG)) × (FG / 0.794) [Alternate, high-gravity] Apparent Attenuation = ((OG − FG) / (OG − 1)) × 100
Where
OGOriginal gravity — density of wort before fermentation (e.g. 1.050)FGFinal gravity — density of beer after fermentation is complete (e.g. 1.010)131.25Conversion factor derived from the density of ethanol and water relationshipsApparent AttenuationPercentage of fermentable sugars consumed by yeastWorked example
Brewing a classic British Bitter:
OG = 1.042 (measured after adding water to grain wort, before pitching yeast).
FG = 1.010 (measured 10 days later when gravity has been stable for 2 days).
ABV (simple) = (1.042 − 1.010) × 131.25 = 0.032 × 131.25 = 4.2%.
Apparent attenuation = ((1.042 − 1.010) / (1.042 − 1)) × 100 = (0.032 / 0.042) × 100 = 76.2%.
This is within the expected range for most ale yeasts (70–80%), confirming healthy fermentation.
If FG had remained at 1.020 after 2 weeks: attenuation = 52%, suggesting a stuck fermentation — caused by insufficient pitching rate, temperature drop, or nutrient deficiency.
Tips to improve your result
- 1.
Always take two FG readings 48–72 hours apart before calling fermentation complete. If gravity is still dropping, fermentation hasn't finished. Packaging before fermentation is complete causes over-carbonation — in bottles, this is a safety hazard (bottle bombs); in kegs, it's a pressurisation and foam problem.
- 2.
Temperature matters for gravity readings. Hydrometers are calibrated at a specific temperature (usually 15°C or 20°C on the scale). Measuring wort at 25°C introduces an error of approximately 1.5–2 points (0.0015–0.002 SG). For best accuracy, cool samples to calibration temperature or use the correction formula: add 0.00097 for every 5°C above calibration temperature.
- 3.
Refractometers are convenient for measuring OG but give inaccurate FG readings because the Brix-to-SG conversion doesn't account for the presence of alcohol. If using a refractometer for FG, apply the correction formula: FG = 1.001843 − 0.002318474 × OG − 0.000007775 × OG² − 0.000000034 × OG³ + 0.00574 × Brix + 0.00003344 × Brix² + 0.000000086 × Brix³.
- 4.
High-gravity beers (OG > 1.080) require careful yeast management: a higher pitching rate (1.5–2× the standard amount), adequate oxygenation of the wort, and sometimes nutrient supplementation. Under-pitching in high-gravity worts produces stressed yeast that generate off-flavours (fusel alcohols, esters) and may not fully attenuate, leaving residual sweetness.
- 5.
For accuracy, sanitise your hydrometer and sample vessel. Unsanitised samples can introduce wild yeast or bacteria that continue fermenting after collection, giving a falsely low reading. Always take samples from the middle of the fermenter — not the surface (where CO₂ is still outgassing and affecting density) or the bottom (where sediment affects readings).