Alcohol Content (ABV) & Units Calculator

Homebrewing ABV Guide

What's the key thing to understand about The ABV Formula?

ABV (%) ≈ (OG − FG) × 131.25, where OG and FG are specific gravity readings. The simplified Milligan formula: ABV = (OG − FG) × 131.25 is accurate for most beers and wines (under 15% ABV). For high-gravity beers and wines, the more precise formula is: ABV = ((1.05 × (OG − FG)) / FG) / 0.79 × 100. Example: OG 1.060, FG 1.010 → ABV = (0.050) × 131.25 = 6.56%. This formula accounts for the fact that yeast converts sugars to alcohol and CO₂ rather than directly measuring alcohol.

What should I know about Using a Hydrometer?

A hydrometer measures the density of liquid relative to water (specific gravity). Water = 1.000. Adding sugar increases density above 1.000 (typical wort: 1.040–1.080). As yeast ferments sugars into alcohol, density decreases toward 1.000 or below. Take readings at the same temperature (usually calibrated for 20°C — apply temperature correction otherwise). Hydrometer readings are affected by bubbles — degas the sample by shaking or warming slightly before measuring final gravity.

What should I know about Typical Gravity and ABV Targets?

Session beer (3–4% ABV): OG 1.030–1.040. Standard ale (4–5% ABV): OG 1.040–1.050. Strong ale (5–7% ABV): OG 1.050–1.070. Strong beer / barleywine (8–12% ABV): OG 1.080–1.110. Table wine (10–13% ABV): OG 1.075–1.095. High-gravity wine (14–17% ABV): OG 1.100–1.130. The yeast strain determines maximum attenuation — wine yeast (up to 18% ABV) can ferment much further than typical beer yeast (8–10% ABV).

Is Fermentation Complete?

Fermentation is complete when: gravity readings taken 2 days apart are identical, and the reading matches the expected FG for the recipe/yeast. Do not bottle based on time alone — bottling too early traps residual CO₂ production, causing bottles to over-pressurise (bottle bombs). Common FG ranges: most ale yeast 1.008–1.015, wine yeast 0.990–1.005, Belgian/saison yeast 1.004–1.008. A stuck fermentation (gravity stops moving above expected FG) may indicate temperature problems, nutrient deficiencies, or a stressed/inactive yeast strain — try gently rousing the fermenter or raising the temperature by 2-3°C before giving up on it.

What should I know about the Formula Behind ABV From Gravity?

This calculator's abv_pct output is built on the standard homebrewing formula ABV = (OG − FG) × 131.25, where OG and FG are the original and final specific gravity readings taken with a hydrometer. Specific gravity measures the density of the liquid relative to pure water, which is defined as exactly 1.000; a sugary pre-fermentation liquid is denser than water (giving a reading above 1.000), and as yeast consumes sugar and produces alcohol (which is less dense than water), the gravity reading falls. This simplified formula works well for the gravity ranges typical of beer and most wine, but it becomes progressively less accurate at very high original gravities (strong barleywines, some meads, and fortified-style wines), where more precise formulas that account for the non-linear relationship between gravity and alcohol content are generally preferred by brewers aiming for a very exact figure. For the vast majority of standard beer and wine batches, though, the formula this calculator uses gives a reliable, widely trusted estimate.

What should I know about Apparent vs Real Attenuation?

The attenuation output in this calculator describes how much of the available sugar has been converted during fermentation, but it's worth understanding that there are two related figures brewers talk about: apparent attenuation and real attenuation. Apparent attenuation is calculated directly from the uncorrected hydrometer readings as (OG − FG) divided by (OG − 1), expressed as a percentage, and it's what most homebrew calculators (including the gravity-based approach this tool uses) report. The reason it's called "apparent" rather than "real" is that alcohol itself is less dense than water and slightly lowers the measured final gravity reading beyond what the actual remaining sugar content alone would cause — meaning apparent attenuation typically reads a few percentage points higher than the true (real) percentage of sugar actually consumed. For most practical homebrewing purposes, apparent attenuation is perfectly adequate for judging whether fermentation has progressed as expected for a given yeast strain, since brewers compare it against the yeast manufacturer's typical attenuation range for that strain rather than against an absolute real-attenuation figure.

What do I need to know about Temperature Correction for Hydrometer Readings?

Hydrometers are calibrated to give an accurate reading at a specific reference temperature, commonly 20°C (68°F), and taking a reading at a different temperature without correcting for it introduces a real error into the gravity reading and therefore the calculated ABV. Liquid expands as it warms, becoming slightly less dense, which makes a hydrometer read slightly lower than the true gravity at temperatures above the calibration point, and slightly higher than true at temperatures below it. The error is small at temperatures close to the calibration point but becomes more significant the further the reading is taken from it — a reading taken at 30°C instead of 20°C, for example, can be off by roughly 0.002-0.003 in specific gravity, enough to shift a calculated ABV by a few tenths of a percent, which matters when precision is wanted. Correction tables (widely available for homebrewing) or a simple temperature-correction formula can adjust a reading taken at any temperature back to what it would read at the calibration temperature. In practice, the simplest approach is to let the sample cool to close to the hydrometer's stated calibration temperature before taking a reading at all, avoiding the need for correction and giving the most reliable result without extra calculation.

What's the Difference Between the Simple and More Precise ABV Formulas?

The simple formula, ABV = (OG - FG) × 131.25, is accurate for typical table-strength beers and ciders (roughly up to 10-12% ABV) and is the formula most home brewing hydrometers and calculators default to. At higher starting gravities (strong ales, barleywine, or high-gravity meads), the simple linear formula starts to drift from the true ABV because the relationship between gravity drop and alcohol produced is not perfectly linear across the full range — more precise formulas (such as the commonly used alternative ABV = 76.08 × (OG - FG) ÷ (1.775 - OG) × (FG ÷ 0.794)) correct for this non-linearity and are generally preferred for high-gravity brews. For most home brewing at typical beer and cider strengths, the simple 131.25 formula is close enough that the difference rarely matters in practice.

Worked Example: ABV From Original and Final Gravity Readings

A cider has an original gravity of 1.050 and ferments down to a final gravity of 1.000. Using the simple formula: ABV = (OG - FG) × 131.25 = (1.050 - 1.000) × 131.25 = 0.050 × 131.25 ≈ 6.56%. This is a fully dry-fermented cider with no residual sweetness (FG at or near 1.000 typically indicates the yeast has consumed essentially all the fermentable sugar). Comparing to a beer with OG 1.055 and FG 1.014 (leaving some residual sweetness and body, typical of many ale styles): ABV = (1.055 - 1.014) × 131.25 = 0.041 × 131.25 ≈ 5.38% — illustrating how both the starting gravity and how far fermentation is allowed to drop independently determine the final ABV, and why two brews with similar starting gravities can finish at quite different strengths depending on how attenuative (sugar-consuming) the yeast strain and fermentation conditions were.

What's the Difference Between Apparent and Real Attenuation?

Apparent attenuation is simply the percentage drop in gravity as measured directly by a hydrometer: (OG - FG) ÷ (OG - 1.000) × 100. Real attenuation is a corrected figure that accounts for the fact that the alcohol produced during fermentation is itself less dense than water, which makes a hydrometer reading of the actual fermented liquid read lower than the true remaining sugar content alone would suggest — in other words, a hydrometer can't fully distinguish 'sugar converted to alcohol' from 'the alcohol itself lowering the reading', so apparent attenuation slightly overstates how much sugar was actually consumed. Real attenuation, calculated with a correction factor (commonly real attenuation ≈ apparent attenuation × 0.81-0.82), gives a more accurate picture of how much of the original sugar the yeast truly fermented, which matters for comparing a yeast strain's true fermentation performance across different brews, though for everyday homebrewing, apparent attenuation (the simpler, directly measured figure) is what's normally quoted and is perfectly adequate for tracking fermentation progress.