How to Calculate ABV: The Standard Formula, Worked Out

How Do You Calculate ABV?

To calculate ABV (alcohol by volume), subtract your beer's final gravity from its original gravity and multiply the result by 131.25:

ABV = (OG − FG) × 131.25

Original gravity (OG) is the density of your wort before fermentation, measured with a hydrometer. Final gravity (FG) is the density after fermentation is finished. The difference between the two tells you how much sugar the yeast converted to alcohol.

Here it is with real numbers. Say your hydrometer read 1.050 before you pitched the yeast, and 1.010 two weeks later:

• 1.050 − 1.010 = 0.040
• 0.040 × 131.25 = 5.25% ABV

That's the whole calculation. The rest of this guide explains where those numbers come from, how to measure them accurately, and when the simple formula needs an upgrade.

What Original and Final Gravity Actually Mean

Gravity readings are the measurement backbone of homebrewing, so it's worth being precise about what they are.

What is original gravity (OG)?

Original gravity is the specific gravity of your wort before fermentation begins — its density compared to pure water, which reads 1.000. Because dissolved malt sugars are denser than water, wort reads higher: a typical pale ale might start at 1.048, a big imperial stout at 1.090 or more.

Brewers often shorthand this as "gravity points": 1.050 becomes "50 points." A higher OG means more sugar available for yeast, which means more potential alcohol.

What is final gravity (FG)?

Final gravity is the specific gravity of the beer after fermentation is complete. It's lower than OG because yeast has eaten much of the sugar, and alcohol is actually less dense than water. Most ales finish somewhere in the 1.008–1.016 range, though this varies with recipe and yeast.

FG never drops all the way to 1.000 in normal beer — unfermentable sugars and proteins remain, and that residual gravity is part of what gives beer body and sweetness.

The Standard Formula, Step by Step

Here's the full process from measurement to number:

1. Measure OG on brew day. After the wort is cooled and mixed, draw a sample and take a hydrometer reading before pitching yeast. Write it down — you can't recover this number later.
2. Ferment. Give the yeast time to do its work, typically one to two weeks for a standard ale.
3. Measure FG. Draw a sample and read the hydrometer again. Then read it again two or three days later — if the number hasn't moved, fermentation is done.
4. Subtract: OG − FG.
5. Multiply by 131.25. The result is your ABV as a percentage.

Why 131.25? It's a constant derived from the chemistry of fermentation: it approximates how much alcohol is produced per unit of gravity drop, folding in the density difference between alcohol and the sugar it replaced. It's an approximation, but for beers of ordinary strength it's accurate to within about a tenth of a percentage point — better than most hydrometer technique.

How to Use a Hydrometer for Beer

The formula is only as good as the readings you feed it. A hydrometer is a weighted glass float: the denser the liquid, the higher it rides. Here's how to get a trustworthy reading.

1. Sanitize everything that touches the sample — thief or turkey baster, test cylinder, the hydrometer itself. Anything touching cooled wort or unfinished beer must be sanitized.
2. Fill a test cylinder with enough liquid to float the hydrometer freely, without it touching the bottom or sides.
3. Spin the hydrometer gently to knock loose clinging bubbles, which push it artificially high.
4. Read at eye level, at the liquid surface. The liquid climbs slightly where it meets the glass (the meniscus); read at the bottom of that curve unless your hydrometer's instructions say otherwise.
5. Correct for temperature. Most hydrometers are calibrated at 60°F (15.6°C) or 68°F (20°C) — the calibration temperature is printed on the paper scale inside. A warm sample reads lower than its true gravity. A few degrees off is negligible, but a hot sample can skew a reading by several points, so use a temperature-correction chart or calculator for anything well above calibration temperature.
6. Don't return the sample to the fermenter. Taste it instead — sampling as you go teaches you what beer tastes like at every stage.

One caution on the alternative tool: a refractometer works fine for OG, but once alcohol is present it distorts the reading. Refractometer FG numbers need a correction calculation — a plain hydrometer is the simpler instrument after fermentation.

Worked Examples Across Common Beer Strengths

Run the formula a few times and it becomes automatic. Each row below is (OG − FG) × 131.25:

Two things worth noticing. First, the gravity drop drives ABV, not the OG alone — a beer that starts at 1.060 but finishes high at 1.020 has the same 5.25% ABV as our pale ale. Second, small measurement errors matter: misreading either gravity by 0.002 shifts the result by about a quarter of a percentage point. Careful readings are what make the math meaningful.

When the Simple Formula Isn't Enough

The 131.25 constant assumes the relationship between gravity drop and alcohol is linear. It nearly is at ordinary strengths, but it drifts as beers get bigger. For high-gravity beers — roughly 1.070 OG and above — many brewers switch to a longer formula:

ABV = 76.08 × (OG − FG) ÷ (1.775 − OG) × (FG ÷ 0.794)

For the imperial stout above (OG 1.080, FG 1.015), the simple formula gives 8.5% while this one gives about 9.1% — a meaningful gap on a strong beer. For anything under about 1.070, the difference is small enough that the standard formula is the sensible default: easier to remember, easy to check in your head, and accurate where most homebrew lives.

Either way, remember that homebrew ABV is an estimate from two density readings, not a laboratory analysis. Quoting "5.25%" is honest; quoting "5.253%" is false precision.

Why Stable Gravity Readings Are a Safety Check

Your FG reading does double duty. Beyond the ABV math, identical hydrometer readings taken two to three days apart are the only reliable proof that fermentation is finished — and that matters because bottles are sealed pressure vessels. If you bottle while yeast is still consuming sugar, fermentation continues inside the capped bottle, pressure builds beyond what the glass can hold, and bottles can gush or burst. Airlock activity stopping is not proof; a stable gravity is.

So before you prime and cap, confirm stable readings first. Our guide to how to bottle homebrew covers the full safe-bottling sequence, including priming sugar amounts and conditioning timelines.

Common ABV Calculation Mistakes

• Forgetting to record OG. Once fermentation starts, the number is gone. Write it on the fermenter, in a notebook, anywhere permanent.
• Reading a warm sample without correcting. Wort fresh off the kettle at even 80–90°F will read noticeably low on a 60°F-calibrated hydrometer.
• Trusting the airlock over the hydrometer. Bubbling can stop early or continue from dissolved CO₂; only gravity readings confirm the finish.
• Using a refractometer for FG without correction. Alcohol skews the reading — your beer will look weaker than it is.
• Bubbles on the hydrometer. Foam and CO₂ bubbles lift it and inflate the reading; spin it before you read.
• Multiplying by the wrong constant. You'll see 131, 131.25, and 132 in different sources; they're all versions of the same approximation. Pick 131.25 and be consistent.

Where This Fits in Your Brew Day

If you're still planning your first batch, the two gravity readings slot naturally into the process — OG right after chilling on brew day, FG before packaging. Our walkthrough of how to brew beer at home shows exactly where they land in the full sequence, and the rest of our fermentation and gravity guides go deeper on the measurement side.

Take two honest readings, subtract, multiply by 131.25. That's the standard formula — and now you've seen it worked out.