ABV from Gravity

What this calculator does

Two ABV formulas, run at once, so you know which one to trust.

Every homebrew app quotes ABV = (OG − FG) × 131.25 — a fast linear approximation that's fine for session beers and wrong by a meaningful margin on anything strong. This tool runs that standard number next to the accurate real-extract formula brewers actually use for high-gravity beer, plus apparent attenuation, ABW, and calories per 12 oz — all from the same two gravity readings.

Everything runs client-side. Nothing is stored, nothing is sent — change either gravity and every result updates live.

The ABV formula, explained

The math behind both numbers.

Standard ABV: fast, familiar, approximate

The formula everyone learns first multiplies the gravity drop by a constant:

It's a linearization of the true relationship between gravity loss and alcohol produced — good enough across the middle of the gravity range, where most beer lives, but it quietly bends the truth at the edges.

Accurate ABV: the real-extract method

The more rigorous approach starts by converting both gravities to degrees Plato, then estimates the real extract — how much sugar is actually left once you account for the fact that alcohol itself lowers the reading a hydrometer or refractometer gives you:

Apparent attenuation and calories, for free

Once you have OG and FG, two more numbers fall out of the same inputs. Apparent attenuation is simply how much of the original extract fermented out — (OG − FG) / (OG − 1) × 100 — a quick read on whether your yeast finished the job. Calories per 12 oz comes from combining the alcohol and residual real extract, using the standard brewing-science approximation:

The part no calculator farm writes

When the standard number is wrong.

The ×131.25 shortcut isn't broken — it's a straight-line approximation of a curved relationship, and curves punish you most at the extremes.

It drifts high on strong beer

Below about 6% ABV, standard and accurate formulas agree closely — the gap is noise. Above 7–8% ABV, the standard formula starts reading 0.2–0.5% high, and the gap widens as gravity climbs. A big imperial stout or barleywine that the standard formula calls 11.5% might genuinely be closer to 11.0–11.2% by the real-extract method. If you're labeling a strong beer, brewing to a style guideline's ABV ceiling, or comparing a big beer against a competition's stated range, use the accurate number, not the quick one.

The constant itself isn't sacred

131.25 is the figure most homebrew software has standardized on, but you'll see 131.34 and even 132 in older brewing texts and other calculators. None of them is "more correct" than the others — they're all fits to the same approximation, just from slightly different source data. Don't chase the third decimal on a linear formula; if precision matters, switch methods entirely rather than switching constants.

Refractometer FG is the #1 real-world error

This is the mistake that actually costs people accuracy in practice — more than the formula choice. A refractometer measures how light bends through your sample, and ethanol bends light differently than sugar does. Read an uncorrected refractometer FG straight off the scale on a fermented beer and it comes back too high — understating how far you actually attenuated and throwing off any ABV formula fed with that number, standard or accurate. The fix is one of two things: use a hydrometer for FG (unaffected by alcohol), or apply both a wort-correction factor and a proper alcohol-correction formula to your refractometer reading before you calculate anything downstream. A refractometer alone, uncorrected, on a finished beer is the single most common source of a "wrong" ABV this tool will ever get fed.

ABV and ABW aren't interchangeable

US labeling law reports ABV (alcohol by volume). ABW (alcohol by weight) is always a lower number for the same beer — roughly ABW ≈ 0.79 × ABV — because alcohol is less dense than water and the two percentages are measuring different things (volume vs. mass). This matters if you're ever cross-referencing a spec sheet, an old brewing text, or a state that historically labeled in ABW: don't drop one number into a field expecting the other, and don't assume a "5% beer" from a foreign or older source means the same thing your hydrometer would.

Notes from the lab