If you’ve ever looked at a recipe and seen numbers like OG 1.058, FG 1.012, wondered what they mean and whether they actually matter — they matter enormously. Original gravity and final gravity are two of the most useful measurements in homebrewing. They tell you how much sugar went in, how much the yeast converted, whether fermentation is actually complete, and how strong your finished beer is. Here’s everything you need to know.
What Is Gravity in Beer?
In brewing, “gravity” refers to the density of your liquid relative to pure water. Pure water has a specific gravity (SG) of 1.000. Wort — the sugary liquid before fermentation — is denser than water because of dissolved sugars, so it reads above 1.000. The more sugar, the higher the gravity.
Gravity is measured on a scale where most homebrew wort falls between 1.030 (very light) and 1.120 (very strong). The four digits after the decimal are often called “gravity points” — a wort at 1.058 is sometimes referred to as “58-point wort.”
Original Gravity (OG): The Starting Line
Original gravity (OG) is the gravity of your wort before you pitch yeast. It tells you:
- How much fermentable sugar is available for yeast to consume
- The theoretical maximum alcohol your beer could produce
- Whether your mash efficiency was on target (for all-grain brewers)
A pale ale might have an OG of 1.048–1.056. An imperial stout could be 1.090–1.110 or higher. The BJCP Style Guidelines list OG ranges for every recognized style, and hitting your OG target is one of the more satisfying moments in brewing precision.
Measure OG after chilling your wort and before pitching yeast — this is the cleanest measurement, free of CO₂ bubbles from fermentation.
Final Gravity (FG): The Finish Line
Final gravity (FG) is the gravity of your beer after fermentation is complete. Yeast has consumed most of the fermentable sugars, so FG is lower than OG. A beer that started at 1.052 might finish at 1.010–1.014.
The FG tells you:
- Whether fermentation is truly complete
- How much residual sweetness remains in the beer
- The beer’s body and mouthfeel (higher FG = fuller body)
FG is influenced by yeast health, fermentation temperature, yeast strain attenuation, and mash temperature. A mash at 148°F (64°C) produces a more fermentable wort that finishes lower; a mash at 158°F (70°C) leaves more unfermentable dextrins and finishes higher.
How to Use a Hydrometer
A hydrometer is a simple glass instrument that floats in wort or beer and reads specific gravity based on how high or low it bobs. Here’s how to use one accurately:
- Fill a sanitized hydrometer test cylinder or tall glass with enough liquid to float the hydrometer without touching the bottom.
- Gently lower the hydrometer into the liquid and give it a slow spin to release any air bubbles clinging to the glass tube.
- Read the gravity at the bottom of the meniscus — the curved surface of the liquid at the hydrometer’s stem. The reading is where the flat surface of the liquid intersects the hydrometer scale.
- Correct for temperature. Most hydrometers are calibrated to 60°F (15.5°C). If your sample is warmer, your reading will be artificially low. Add approximately 0.001 for every 10°F above calibration temperature, or use an online correction calculator.
Taking a sample from a fermenting vessel carries minor contamination risk — always return the sample to the sink rather than back to the fermenter if you’ve used a test cylinder that wasn’t sanitized.
For a more practical comparison of measurement tools, see Brewing Thermometers and Hydrometers: A Quick Guide.
Using a Refractometer
A refractometer is a handheld optical device that reads gravity from just a few drops of liquid — far more convenient for mid-boil checks. However, refractometers read falsely low in the presence of alcohol, so they’re only reliable for pre-fermentation measurements. How to Brew by John Palmer includes correction formulas for using refractometer readings post-fermentation, though a hydrometer remains the simpler choice for FG measurements. Brewer’s Friend provides a free online refractometer correction calculator for post-fermentation readings.
How to Calculate ABV
Once you have your OG and FG, calculating alcohol by volume is straightforward. The most common homebrewer formula is:
ABV = (OG − FG) × 131.25
Examples:
- OG 1.052, FG 1.012 → ABV = (0.040) × 131.25 = 5.25%
- OG 1.072, FG 1.014 → ABV = (0.058) × 131.25 = 7.61%
- OG 1.096, FG 1.018 → ABV = (0.078) × 131.25 = 10.24%
This formula is accurate enough for homebrew purposes up to about 8–9% ABV. For very high-gravity beers, the more precise formula used by the Siebel Institute and commercial breweries accounts for the non-linear relationship between gravity and alcohol, but the difference is minor for most homebrewing contexts.
The American Homebrewers Association notes that home ABV calculations are estimates — variations in residual CO₂, measurement temperature, and other factors mean your result might be off by ±0.3–0.5%.
What Is Apparent Attenuation?
Apparent attenuation is the percentage of sugars the yeast consumed during fermentation:
Attenuation % = ((OG − FG) / (OG − 1.000)) × 100
A beer that went from 1.056 to 1.012 has an apparent attenuation of about 79%. Most ale strains attenuate 72–80%; highly attenuative strains like some Belgian yeasts can hit 85–90%. Low attenuation (below 70%) often signals incomplete fermentation — either the yeast gave up early due to stress, poor health, temperature, or the wort contained a lot of unfermentable sugars.
Is Fermentation Actually Done?
Gravity is the only reliable indicator that fermentation is complete. The old trick of watching airlock bubbles works as a rough guide but is not definitive — fermentation can be finished while CO₂ is still off-gassing slowly, and airlock activity can continue from temperature changes long after yeast have finished working.
The rule: take two gravity readings 48 hours apart. If they match your expected FG, fermentation is complete. CraftBeer.com echoes this guideline as the industry standard for determining readiness to package. If gravity is still dropping, wait.
Gravity and Recipe Design
Understanding gravity gives you real control over your recipe design. If you want a dry, crisp pale ale, design for a low FG (1.008–1.012) by mashing at the lower end of your temperature range and choosing a highly attenuative yeast. If you want a full-bodied stout with some residual sweetness, mash higher (154–158°F / 68–70°C) and target FG 1.014–1.020. The BJCP Style Guidelines provide FG targets for every style as useful benchmarks.
The Brew Professor Takeaway
Original and final gravity are not intimidating lab measurements — they’re just the two numbers that bracket every fermentation, telling you where you started, where you finished, and how strong your beer turned out. A good hydrometer and a little basic math give you more meaningful information about your beer than any other single tool in your kit. Take the readings, do the calculation, and you’ll never have to guess whether your beer is ready to bottle or how much of it to drink in one sitting.
