Beer Unit Converter

How to use this calculator

Original Gravity (OG) — The specific gravity of your wort before fermentation starts. Measured with a hydrometer or refractometer at pitching time. A typical ale wort might read 1.050. A strong stout might be 1.080 or higher. The OG tells you how much fermentable sugar was present at the start.

Final Gravity (FG) — The specific gravity after fermentation is complete and has stabilised for at least 2–3 consecutive days. A reading of 1.010 is common for a standard ale. The FG tells you how much sugar remains unfermented. The difference between OG and FG is what the yeast converted to alcohol and CO2.

Batch Volume (L) — The total volume of finished beer in litres. Used to calculate total pure alcohol produced across the entire batch.

After hitting Calculate, the output panel shows:

ABV % — Alcohol by volume. The primary result. The percentage of the total liquid volume that is ethanol.

Proof — ABV multiplied by 2. The US proof system. A 5.25% ABV beer is 10.5 proof.

Calories / 12 fl oz — Estimated kilocalories per standard 12oz (355ml) US serving. Includes both alcohol calories and residual carbohydrate calories.

Attenuation % — The percentage of fermentable sugars the yeast actually consumed. 80% attenuation means the yeast converted 80% of available sugars into alcohol and CO2. Higher attenuation = drier, more fermented beer.

OG in °Plato — Your original gravity converted to the Plato scale. Used in professional brewing and in most European and Australian brewing contexts.

FG in °Plato — Your final gravity in Plato.

Pure Alcohol (L) — Total litres of absolute ethanol in the batch. Useful for excise duty calculations and for scaling recipes.

What gravity actually measures

Specific gravity measures the density of a liquid relative to water. Water = 1.000. Wort with dissolved sugars is denser than water, so OG readings sit above 1.000: typically 1.040 to 1.070 for most ales and lagers, up to 1.100+ for barleywines and imperial stouts.

As yeast ferment sugars into ethanol and CO2, the liquid gets lighter. Ethanol is less dense than water (density ~0.789), so a higher alcohol content actually pulls the gravity down. FG readings for fully fermented beers typically sit between 1.006 and 1.018 depending on the style, the malt bill, and the yeast’s attenuation characteristics.

The gap between OG and FG is the core signal. A beer that started at 1.050 and finished at 1.010 had a gravity drop of 0.040 points. That drop is what the ABV formula converts into an alcohol percentage.

Gravity is just a proxy for sugar content before fermentation and residual sugar after. The yeast doesn't care about the number. But the number tells you everything the yeast did.

The formulas

Every output the calculator produces comes from OG and FG. Here’s each one.

The simple ABV formula (OG - FG) × 131.25 is accurate to within 0.1–0.2% for most beers in the 4–8% ABV range. The Miller formula is more accurate for high-gravity beers above 8% ABV because it accounts for the density of the ethanol itself changing the reading.

The Plato conversion multiplies the gravity drop below 1.000 by 250. So 1.050 OG = (1.050 - 1.000) × 250 = 12.5°P. The calculator uses a more precise version of this for the exact figures shown.

The calorie formula has two components: alcohol calories (ethanol contains 7 kcal/g) and carbohydrate calories from residual sugars (remaining gravity above 1.000). Both contribute meaningfully. A 5% beer and a 5% light beer can have different calorie counts if the light beer has lower residual sugars (lower FG).

Understanding each output

ABV — Alcohol by volume

ABV is the standard global measure of alcoholic strength. It’s a percentage: 5.25% ABV means 5.25ml of pure ethanol per 100ml of beer.

Most session ales sit between 3.5–4.5% ABV. Standard lagers and IPAs typically run 4.5–6.5%. Imperial stouts and barleywines reach 10–14%. The UK’s Misuse of Drugs Act threshold for “low alcohol” is below 0.5% ABV. Beer below 0.05% ABV can be labelled “alcohol-free” in most markets.

Proof

US proof is exactly 2 × ABV. A 5.25% beer is 10.5 proof. The proof system comes from 18th century British taxation, where gunpowder mixed with spirits was used to “prove” alcohol content. If the powder ignited, the spirit was “proved” at roughly 57% ABV (now 100 proof in the UK system). The US adopted a simpler 2× system that has nothing to do with gunpowder but stuck around anyway.

Attenuation

Attenuation tells you how thoroughly your yeast worked.

Apparent attenuation (what this calculator shows) is calculated from gravity readings and is the standard homebrewing figure.

Real attenuation corrects for the fact that ethanol is less dense than the sugars it replaced. Real attenuation is always lower than apparent attenuation by about 20% of the apparent figure.

A beer stuck at 70% attenuation when you expected 80% either hit its limit due to unfermentable dextrins (which is fine if intentional) or fermentation stalled early (which needs troubleshooting: temperature, yeast health, nutrients).

Calories

Beer calories come from two sources: ethanol and residual carbohydrates.

Ethanol contributes 7 kcal per gram. Carbohydrates contribute 4 kcal per gram. In most standard beers, alcohol accounts for 60–70% of the total calories, with residual sugars accounting for the rest.

This is why a “light” beer isn’t just lower ABV — it also needs lower residual sugars (lower FG) to meaningfully cut calories. A 4% beer with lots of residual body (FG 1.016) can have more calories than a drier 4.5% beer finishing at 1.008.

Plato scale

°Plato measures the percentage of dissolved solids (primarily sugars) by weight. 13°P means 13g of dissolved solids per 100g of solution.

Professional brewing uses Plato everywhere because it scales linearly with dissolved sugar content. Specific gravity doesn’t. Commercial recipes, yeast manufacturer specs, and most brewery software work in Plato. If you’re adapting a professional recipe for homebrew or vice versa, the Plato conversion in the calculator output is what you need.

The relationship between Plato and specific gravity is close to linear at normal brewing gravities:

So 1.060 OG ≈ 15°P, 1.080 OG ≈ 20°P, 1.100 OG ≈ 25°P. These are approximations. The calculator uses the full polynomial for exact values.

Real-world examples

American pale ale

A homebrewer pitches an American ale yeast into a 23L batch of pale ale wort. OG measured at 1.054. After 12 days, FG stabilises at 1.011.

Imperial stout — high gravity brewing

A brewer making an imperial stout. OG 1.095. FG 1.022 after extended fermentation with a high-gravity yeast.

Diagnosing a stuck fermentation

A brewer expected their wheat beer to finish at 1.010 but it’s sitting at 1.018 for 5 days.

Scaling for excise duty

A small UK craft brewery produces 500L of a 5.5% ABV pale ale per batch. HMRC charges excise duty on pure alcohol volume.

What the numbers tell you about fermentation health

Gravity readings aren’t just for calculating ABV. They’re diagnostic tools throughout the brewing process.

OG lower than target means you didn’t extract enough sugars from the grain (low mash efficiency), your grain bill was light, or your batch volume is higher than planned. Boil longer to concentrate the wort, or accept a lower-ABV beer.

FG higher than expected means fermentation didn’t complete. Check temperature (most ale yeasts work best at 18–22°C), verify yeast viability, consider repitching with fresh active yeast. Don’t package a beer that hasn’t hit terminal gravity — residual fermentation in bottles causes overcarbonation and potentially exploding bottles.

FG lower than expected means your wort was highly fermentable (more simple sugars, less complex dextrins) or your yeast attenuated exceptionally well. The beer will be drier and slightly higher ABV than planned. Usually fine.

Attenuation significantly below strain expectations is the primary signal for stuck fermentation. Every yeast strain has a published attenuation range. White Labs WLP001 (California Ale): 73–80%. Wyeast 3724 Belgian Saison: 76–84%. If your measured attenuation falls more than 5 points below the strain’s expected minimum, something went wrong during fermentation.

A gravity reading mid-fermentation tells you nothing useful except that fermentation is happening. Take your OG at pitching and your FG only after the reading has been stable for 48 hours. Everything in between is noise.

Common measurement mistakes

Not temperature-correcting the hydrometer. Hydrometers are calibrated to 60°F (15.6°C) or 68°F (20°C) depending on the manufacturer. Measuring wort at 80°C gives a falsely low reading. Either cool the sample to calibration temperature or use a temperature correction chart. The error is about +0.002 per 10°C above calibration.

Measuring FG too early. Seeing three or four days without airlock activity doesn’t mean fermentation is done. Some slow-fermenting strains and high-gravity beers ferment for 2–3 weeks. Take readings 48 hours apart. If both match, you’re done.

Contaminating the hydrometer sample. Always pull a sample using a sanitised wine thief or turkey baster. Dipping an unsanitised hydrometer directly into the fermenter is a contamination risk for the entire batch.

Using a refractometer for FG without correction. Already mentioned in the formula section but worth repeating. Refractometers are excellent for OG. They give falsely high readings for FG because alcohol refracts light differently than sugar. The common “refractometer correction” formulas reduce the error, but a clean hydrometer reading is more reliable.

Rounding OG and FG to 3 decimal places. A reading of 1.050 vs 1.051 is a 0.001 SG difference, which translates to 0.13% ABV. That’s within the margin of most homebrew measurements. Don’t stress about the last digit, but don’t round aggressively either. Report to 3 decimal places (1.050 rather than 1.05 or 1.0).

Style benchmarks for gravity and ABV

Saison’s uniquely high attenuation is a signature of the style. Belgian saison yeasts produce distinctive phenolic esters and ferment very dry. A saison finishing at 1.002 is achieving 96%+ attenuation, which is exceptional and intentional.

Barleywines finish high (1.016–1.030) despite high alcohol because the large grain bill contains significant unfermentable dextrins. High FG in a barleywine reflects the intended full-bodied sweetness, not incomplete fermentation.

The bottom line

OG and FG are two numbers. From them, this calculator derives seven: ABV, proof, calories per serving, attenuation, OG in Plato, FG in Plato, and pure alcohol volume for the batch.

ABV is what most people care about. Attenuation is what tells you whether fermentation went as planned. Plato is what bridges homebrew readings to professional recipe specs. Calories matter if you’re tracking them or marketing the product. Pure alcohol volume matters for duty calculations and batch records.

Take your OG at pitching with a sanitised hydrometer at calibration temperature. Take your FG after two identical readings 48 hours apart. Plug both in. The rest is arithmetic.