How to Calculate Alcohol Content: ABV Formula & Examples

Calculating alcohol content at home comes down to two numbers and one multiplication. You measure your beer’s specific gravity before the yeast goes in (original gravity, or OG) and again after fermentation finishes (final gravity, or FG), then apply ABV = (OG − FG) × 131.25. A pale ale that starts at 1.048 and finishes at 1.010 is (0.038) × 131.25 = 5.0% alcohol by volume. The equipment is cheap — a glass triple-scale hydrometer runs about $10–$15 in 2026, and an ATC refractometer about $20–$40 — and the math takes ten seconds with the free ABV calculator. This guide walks through when to take each reading, three fully worked examples, where the simple formula starts to drift on big beers, and why the number on a commercial label is allowed to differ from what you’d calculate.

Step 1: Take Two Gravity Readings

Specific gravity compares your liquid’s density to water (1.000). Dissolved sugar raises it; alcohol lowers it. Tracking the drop between two readings tells you how much sugar the yeast converted.

Original gravity (OG) is measured after the wort is chilled and thoroughly mixed, immediately before you pitch the yeast. Typical beer OGs run from about 1.035 for a light lager to 1.120 for a barleywine. Fill a test jar, spin the hydrometer gently to shake off bubbles, and read at eye level at the bottom of the meniscus.

Final gravity (FG) is measured once fermentation is done — not on a fixed day, but when two readings taken 2–3 days apart are identical. If your stout reads 1.014 on day 12 and 1.014 on day 15, it’s finished. Most beers finish between 1.005 and 1.020; sweet stouts and barleywines can sit at 1.025–1.030. Bottling on a still-falling gravity is how you get gushers, so this double-check matters for safety as well as accuracy.

Step 2: Apply the Formula — ABV = (OG − FG) × 131.25

The standard homebrew formula is:

• ABV = (OG − FG) × 131.25

The constant 131.25 folds two physical facts into one number: fermentation produces roughly 1.05 grams of ethanol for every gram of CO₂ that bubbles away, and ethanol’s density is about 0.79 g/mL versus 1.00 for water. Divide 105 by 0.80 and you get 131.25.

For ordinary-strength beer the shortcut is excellent — it agrees with more rigorous methods to within about 0.1–0.3 percentage points for anything up to roughly 1.070 OG. Enter your two readings into the ABV calculator and it applies both the standard and the high-gravity versions instantly, so you can see the spread for yourself.

Three Worked Examples

Example 1: Session Ale

OG 1.042, FG 1.010. The drop is 0.032, so ABV = 0.032 × 131.25 = 4.2%. The high-gravity formula returns 4.23% — a 0.03-point difference nobody can taste. At session strength, the simple formula is effectively exact.

Example 2: American IPA

OG 1.065, FG 1.012. The drop is 0.053, so ABV = 0.053 × 131.25 = 7.0% (6.96% unrounded). The high-gravity formula says 7.24%. A quarter-point gap is starting to show, but it’s still within the rounding you’d put on a recipe sheet.

Example 3: Imperial Stout — Where the Simple Formula Breaks

OG 1.105, FG 1.028. The simple formula gives 0.077 × 131.25 = 10.1%. That answer is meaningfully wrong. The 131.25 constant assumes a linear relationship between gravity drop and alcohol, and above about 1.070 OG that assumption falls apart. For big beers, use the high-gravity alternative:

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

Plugging in: 76.08 × 0.077 ÷ 0.670 × 1.295 = 11.3%. The simple formula understated this stout by 1.2 percentage points — the difference between a strong beer and a wine-strength one.

Beer	OG	FG	Simple formula	High-gravity formula	Gap
Session ale	1.042	1.010	4.2%	4.2%	~0.0
American IPA	1.065	1.012	7.0%	7.2%	+0.3
Imperial stout	1.105	1.028	10.1%	11.3%	+1.2

Hydrometer vs Refractometer

Both instruments measure sugar concentration; they just do it differently, and each has a catch.

	Hydrometer	Refractometer
2026 price	$10–$15 (glass triple-scale)	$20–$40 (handheld ATC model)
Sample needed	150–250 mL in a test jar	2–3 drops
Before fermentation	Direct reading	Reads °Brix; divide by ~1.04 wort correction factor
After fermentation	Direct reading	Requires an alcohol-correction formula
Main risk	Breaks; temperature-sensitive	Uncorrected FG readings run badly high

The refractometer’s catch is the big one: it measures how much the sample bends light, and alcohol bends light more than water does. Once fermentation starts, every reading is skewed upward. An imperial stout that truly finished at 1.028 might show the refractive equivalent of 1.048 uncorrected — enough to make your calculated ABV nonsense. Correction formulas (Sean Terrill’s cubic is the homebrew standard) fix most of the error, but a $12 hydrometer reading is still the more trustworthy FG. Many brewers use both: refractometer drops during the boil and active fermentation, hydrometer for the final call.

Temperature Correction Basics

Hydrometers are calibrated to read true at one temperature — 60°F (15.6°C) on most older instruments, 68°F (20°C) on many newer ones. Warm liquid is less dense, so a warm sample reads artificially low. On a 60°F-calibrated hydrometer, a sample at 80°F reads about 0.002 low, at 90°F about 0.004 low, and at 100°F about 0.006 low.

An 0.002 error on both readings can cancel out, but in practice OG is often taken on warm wort and FG on cellar-temperature beer, so the errors don’t cancel — they stack. On a 5% beer that’s easily ±0.3–0.5 points of ABV. Let the sample cool to within a few degrees of calibration temperature, or run the reading through the hydrometer temperature correction calculator before you do any ABV math. And never float a hydrometer in near-boiling wort: readings that far from calibration are meaningless, and thermal shock cracks glass instruments.

Wine, Mead, and Cider: Starting From Brix

Winemakers, mead makers, and cider makers usually start from °Brix — grams of sugar per 100 grams of liquid — instead of specific gravity. The two scales are interconvertible (17.5 °Brix ≈ 1.072 SG; the Brix calculator converts both directions), and Brix gives you a quick potential-alcohol estimate before fermentation even starts:

• Potential ABV ≈ °Brix × 0.55 to 0.59, assuming the must ferments dry
• Grape must at 22 °Brix → roughly 12.1–13.0% ABV
• Fresh apple juice at 10–14 °Brix → roughly 5.5–8.0% cider
• Mead must at 1.100 SG (about 24 °Brix) → roughly 13–14%

One quirk to expect: dry wines and meads routinely finish below 1.000 — typically 0.990–0.996 — because alcohol is less dense than water. The subtraction still works exactly the same way. A dry mead going from 1.090 to 0.994 is (0.096) × 131.25 = 12.6% ABV, and at that gravity range the high-gravity formula is again the better choice.

Why Commercial Labels Don’t Match Your Math

If you’ve ever lab-tested a commercial beer against its label, the mismatch is often legal, not sloppy. US federal rules give producers explicit tolerances:

• Beer/malt beverages: the stated ABV may be off by up to ±0.3 percentage points (27 CFR 7.65(c)), so a “5.0%” beer can legally be anywhere from 4.7% to 5.3% — though a beer labeled 0.5% or more can’t actually contain less than 0.5%.
• Wine at 14% ABV or less: tolerance is ±1.5 points (27 CFR 4.36), so a “12.5%” label legally covers 11.0–14.0%.
• Wine above 14% ABV: tolerance tightens to ±1.0 point.

Commercial labs also don’t calculate ABV from gravity at all — they measure it directly by distillation-densitometry or gas chromatography, which sidesteps every assumption baked into the 131.25 constant. Your calculated number and a lab number can honestly differ by a few tenths on the same liquid. For homebrew purposes, gravity-based ABV is plenty: competition entries, recipe scaling, and knowing whether that stout is a 10% sipper or an 11.3% one.

The Bottom Line

Measuring alcohol content reliably is a checklist, not a skill:

• Take OG after chilling and mixing, before pitching yeast
• Confirm FG with two matching readings 2–3 days apart
• Correct both readings to your hydrometer’s calibration temperature (60°F or 68°F)
• Use (OG − FG) × 131.25 up to about 1.070 OG; switch to the high-gravity formula above that
• Don’t trust an uncorrected refractometer reading once fermentation has started

One legal footnote for US brewers: federal law (26 U.S.C. §5053) permits tax-free homebrewing of up to 100 gallons per year for a one-adult household and 200 gallons for households with two or more adults, for personal use only — check your state’s rules too. When your readings are in hand, the ABV calculator handles both formulas, so the only thing left to get right is the reading itself.