This advanced calculator gives you the full fermentation picture from just two hydrometer readings. Beyond ABV, it reports alcohol by weight, apparent and real attenuation, and the original, apparent, and real extract in degrees Plato — the numbers brewers use to judge yeast performance and recipe efficiency.
How it works
Each gravity reading is first converted to degrees Plato (a percentage of dissolved solids by weight) using the standard cubic polynomial:
P = −616.868 + 1111.14·SG − 630.272·SG² + 135.997·SG³
From the original extract OE (from OG) and apparent extract AE (from FG) the
tool derives:
- ABV with the advanced formula
ABV = (76.08 × (OG − FG) ÷ (1.775 − OG)) × (FG ÷ 0.794) - ABW (alcohol by weight) ≈
ABV × 0.79336 ÷ FG - Real extract
RE = 0.1808 × OE + 0.8192 × AE - Apparent attenuation
(OE − AE) ÷ OE × 100 - Real attenuation
(OE − RE) ÷ OE × 100
Why real vs apparent matters
A hydrometer can’t tell sugar from alcohol — it only measures density. Because ethanol is lighter than water, your FG reads lower than the true residual sugar, which inflates apparent attenuation. Real attenuation removes that distortion, so it is the figure to use when comparing yeast strains or diagnosing whether a batch finished as designed.
Why use the advanced ABV formula?
The simple formula ABV ≈ (OG − FG) × 131.25 is accurate enough for typical
session beers (OG below about 1.060) but drifts progressively at higher
gravities. The advanced formula used here — sometimes called the Balling formula
— accounts for the non-linear relationship between alcohol concentration and
specific gravity at higher ABVs, making it better suited for strong ales,
barleywines, Belgian tripels, and wines.
For a barleywine at OG 1.100 / FG 1.022 the simple formula gives roughly 10.2% ABV while the advanced formula gives closer to 10.5–10.7%. For a typical lager at OG 1.048 / FG 1.010 the two are essentially identical.
Worked example: amber ale
Take an amber ale measured at OG 1.060 finishing at FG 1.012.
Converting to Plato:
- OG 1.060 → approximately 14.7 °P (original extract, OE)
- FG 1.012 → approximately 3.1 °P (apparent extract, AE)
Results:
- ABV ≈ 6.4%
- ABW ≈ 5.1%
- Apparent attenuation ≈ 79% (
(14.7 − 3.1) ÷ 14.7 × 100) - Real extract ≈ 5.7 °P (
0.1808 × 14.7 + 0.8192 × 3.1) - Real attenuation ≈ 61% (
(14.7 − 5.7) ÷ 14.7 × 100)
The gap between apparent (79%) and real (61%) attenuation is typical. Most neutral ale yeasts show a real attenuation in the 58–65% range on average-gravity wort. If your real attenuation comes out far below the yeast manufacturer’s stated range, suspect underpitching, low fermentation temperature, inadequate nutrients, or a wort with a high proportion of unfermentable dextrins.
Practical tips for accurate readings
- Temperature-correct your hydrometer. Most are calibrated at 60°F (15.6°C). Reading hot wort will give you a falsely low gravity — let the sample cool or apply your hydrometer’s correction factor.
- Degas the sample before taking FG. CO₂ bubbles clinging to the float make it ride higher, reading artificially high and understating attenuation. Swirl and wait, or transfer the sample back and forth between containers.
- Confirm FG has stabilised. Take readings 24–48 hours apart. If the gravity is still dropping, fermentation is still active.