Understanding ABV: How Alcohol Content Is Calculated from OG and FG

Every homebrewer eventually asks the same question: how strong is this batch? You can’t measure alcohol directly without lab equipment, but you can measure it indirectly — by tracking how much sugar the yeast ate. That’s what original gravity and final gravity are for, and a short formula turns those two readings into an ABV percentage.

What gravity actually measures

Specific gravity is the density of your liquid compared to water (which is 1.000). Sugar is denser than water, so unfermented wort full of sugar reads high — say 1.050. As yeast ferments that sugar into alcohol and CO₂, the density drops, because alcohol is less dense than water.

• Original Gravity (OG) — the reading before fermentation, when sugar content is at its peak.
• Final Gravity (FG) — the reading after fermentation, once the yeast has finished.

The difference between them tells you how much sugar was converted, which is directly proportional to how much alcohol was produced.

The formula

The standard homebrew approximation is:

ABV % = (OG − FG) × 131.25

So a beer that started at 1.050 and finished at 1.010:

(1.050 − 1.010) × 131.25 = 0.040 × 131.25 = 5.25% ABV

The 131.25 factor bundles together the chemistry of converting sugar mass into ethanol volume. It’s an approximation — at higher gravities a more refined formula is slightly more accurate — but for typical beers it’s well within a tenth of a percent. The ABV calculator uses both the simple and the more precise formula so you can compare.

Why your ABV is mostly decided up front

Here’s the insight that changes how you brew: OG sets the ceiling. The amount of fermentable sugar you start with caps how much alcohol you can possibly make. FG just tells you how much of that potential the yeast realised. If you want a stronger beer, you add more fermentable sugar at the start (more grain or a higher-gravity recipe) — you can’t meaningfully boost ABV after the fact.

That’s also why a “stuck fermentation” (FG higher than expected) leaves you with a sweeter, weaker, under-attenuated beer: the yeast quit before converting all the sugar.

A quick reference

OG	FG	ABV
1.040	1.010	~3.9%
1.050	1.010	~5.3%
1.060	1.012	~6.3%
1.070	1.014	~7.4%
1.090	1.018	~9.4%

The rest of the recipe

Strength is one dial; balance is another. Bitterness offsets the sweetness of the malt, and the IBU calculator estimates how bitter your hop additions will make the beer so it doesn’t finish cloying. And once fermentation is done, the Priming Sugar calculator tells you exactly how much sugar to add at bottling for the right level of carbonation — too much is dangerous, too little is flat.

Frequently asked questions

Do I need a hydrometer or a refractometer? Either works for OG. Refractometers are convenient for small samples, but their FG readings need a correction once alcohol is present — a hydrometer is more straightforward post-fermentation.

Should I correct for temperature? Yes — hydrometers are calibrated to a specific temperature (often 20 °C / 68 °F). A reading taken on warm wort needs a small correction for accuracy.

Why is my ABV a little off from the label of a clone recipe? Mash efficiency, yeast attenuation and measurement error all shift the result. Track your own OG and FG and trust your numbers over the recipe’s estimate.

Pop your two gravity readings into the ABV calculator and you’ll know your batch’s strength to the decimal.