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Original Gravity vs. Final Gravity in Homebrewing

Q: How do you calculate ABV from original and final gravity?

The standard homebrewing ABV formula is: ABV = (OG − FG) × 131.25. For example, a beer with OG 1.055 and FG 1.010 gives (1.055 − 1.010) × 131.25 = 0.045 × 131.25 = 5.9% ABV. This formula is accurate to within ±0.3% for most ales and lagers in the normal gravity range.

Q: What is attenuation in brewing?

Attenuation is the percentage of fermentable sugars consumed by yeast during fermentation. Apparent attenuation is calculated as: ((OG − FG) / (OG − 1.000)) × 100. For example, OG 1.060 and FG 1.012 gives ((0.048) / (0.060)) × 100 = 80% apparent attenuation. Most ale strains attenuate between 72% and 82%. Real attenuation is slightly lower because alcohol is less dense than water and makes the apparent reading seem lower.

Q: What is the difference between a hydrometer and refractometer for measuring gravity?

A hydrometer measures gravity directly from the buoyancy of a float in your sample. It is accurate throughout fermentation but requires temperature correction — most are calibrated to 60°F (15.6°C), and readings taken at higher temperatures read lower than actual. A refractometer measures light refraction through a small drop of liquid and is faster to use pre-fermentation. However, once alcohol is present after fermentation starts, a refractometer needs a correction formula to give an accurate gravity reading, making a hydrometer the preferred tool for final gravity checks.

Last updated: June 2026 · 8 min read

If you've ever wondered why your beer ended up weaker (or stronger) than expected, or what your hydrometer reading actually means, you need to understand original gravity (OG) and final gravity (FG). These two measurements are the foundation of homebrew quality control — they tell you how much fermentable sugar you started with, how much yeast consumed, and exactly how strong your beer turned out.

What Is Original Gravity (OG)?

Original gravity is the specific gravity of your wort before fermentation begins. Specific gravity is a unitless ratio comparing the density of your wort to the density of pure water. Pure water has a specific gravity of exactly 1.000. Wort is denser than water because dissolved sugars, proteins, and other solids add mass.

A typical American pale ale might have an OG of 1.052 — meaning the wort is 5.2% denser than water. A light lager might come in at 1.040, while a big imperial stout might reach 1.090–1.110 or higher.

Key fact: OG tells you the alcohol potential of the beer before a single yeast cell has done any work. The higher the OG, the more sugar is available, and the more alcohol the yeast can theoretically produce.

What Is Final Gravity (FG)?

Final gravity is the specific gravity measured after fermentation is complete. As yeast consume fermentable sugars and convert them to ethanol and CO₂, the wort becomes less dense — the gravity drops. Not all sugars are fermentable (dextrins, some complex proteins remain), so FG never reaches 1.000 in beer.

For a well-attenuated ale, typical final gravities fall between 1.008 and 1.015. A very dry Irish stout might finish at 1.008–1.010. A full-bodied malt-forward amber might finish at 1.014–1.016. An imperial stout could finish at 1.020–1.026.

You confirm FG by taking two readings 24–48 hours apart. If the gravity is stable and unchanged, fermentation is complete — not just temporarily paused.

The ABV Formula

The standard homebrewing formula for calculating alcohol by volume is:

    ABV = (OG − FG) × 131.25
  

Example: OG 1.055, FG 1.010

    ABV = (1.055 − 1.010) × 131.25 = 0.045 × 131.25 = 5.9%
  

This formula is accurate to within ±0.3% ABV for most standard ales and lagers. A more precise formula sometimes used for higher-gravity beers is: ABV = (76.08 × (OG − FG) / (1.775 − OG)) × (FG / 0.794), but the simpler version above is sufficient for nearly all homebrewing use cases.

Use our Hydrometer Calculator to calculate ABV automatically from your gravity readings, with temperature correction built in.

Understanding Attenuation

Attenuation is the percentage of fermentable sugars that yeast consumed during fermentation. It connects OG and FG into a single performance metric:

    Apparent Attenuation (%) = ((OG − FG) / (OG − 1.000)) × 100
  

Example: OG 1.060, FG 1.012

    ((1.060 − 1.012) / (1.060 − 1.000)) × 100 = (0.048 / 0.060) × 100 = 80%
  

Apparent vs. real attenuation: The apparent attenuation figure is slightly inflated because ethanol is less dense than water (density ~0.789 g/mL). This makes the hydrometer read a lower gravity than the actual sugar content would suggest, inflating the calculated attenuation number. Real attenuation — calculated from the actual fermentable extract consumed — is typically 3–5 percentage points lower than apparent attenuation, but apparent attenuation is the number yeast manufacturers publish in their datasheets, so it's the one you should use for comparisons.

Low-attenuating yeast (65–72%): Fuller-bodied, sweeter beers. Common in English ales, Märzen, Dunkel.
Medium-attenuating yeast (72–78%): Balanced body and dryness. American ales, Belgian tripels.
High-attenuating yeast (78–85%): Dry, crisp finish. Saisons, Belgian golden strongs, many lager strains.

Typical OG, FG & ABV by Beer Style

Use these benchmarks to verify your recipe is on-target before brew day:

Light Lager

OG: 1.040 | FG: 1.008 | ABV: ~4.2% | Attenuation: ~80%

American IPA

OG: 1.065 | FG: 1.012 | ABV: ~6.9% | Attenuation: ~78%

Dry Stout

OG: 1.044 | FG: 1.008 | ABV: ~4.7% | Attenuation: ~82%

Imperial Stout

OG: 1.090 | FG: 1.022 | ABV: ~8.9% | Attenuation: ~75%

Belgian Saison

OG: 1.058 | FG: 1.006 | ABV: ~6.8% | Attenuation: ~90%

German Hefeweizen

OG: 1.050 | FG: 1.012 | ABV: ~5.0% | Attenuation: ~76%

How to Measure Gravity: Hydrometer vs. Refractometer

Hydrometer

A hydrometer works by floating in your liquid sample — the denser the liquid, the higher it floats. Most hydrometers are calibrated at 60°F (15.6°C). If your sample is warmer than this (which it usually is right off the stove), you need to apply a temperature correction:

At 70°F (21°C): add ~0.001 to your reading
At 80°F (27°C): add ~0.002
At 100°F (38°C): add ~0.004
At 140°F (60°C): add ~0.010

The easiest approach is to chill your sample to near-calibration temperature before reading. Always take FG readings with a hydrometer, not a refractometer, since the math is simpler and more reliable.

Refractometer

A refractometer measures how much a small drop of wort bends (refracts) light. It only requires 2–3 drops of liquid, no temperature correction for near-room-temp samples, and gives an instant reading — making it ideal for pre-fermentation OG checks during the mash or at knockout.

Important limitation: Once fermentation begins, the presence of alcohol changes the light refraction in a way the refractometer's scale doesn't account for. A refractometer reading taken mid- or post-fermentation will read significantly higher than the true gravity. You can use a correction formula (the Sean Terrill correction is the most accurate), but it is much simpler to switch to a hydrometer for any reading taken after pitching yeast.

Best practice: Use a refractometer for pre-boil and OG checks (fast, minimal sample needed), and a hydrometer for all post-fermentation gravity readings (accurate, no correction formula required). Our Hydrometer Calculator handles temperature correction automatically.

What a High or Low FG Means

Final gravity is higher than expected

If your FG is stalling above its target, you may be dealing with stuck fermentation. Common causes include:

Fermentation temperature too cold — yeast become sluggish below their recommended range
Insufficient yeast pitch rate — too few cells to finish the job
Nutrient deficiency or high-adjunct wort with poor yeast nutrition
Undissolved dry hop additions raising apparent gravity

Fix options: rouse the yeast by gently swirling the fermenter, raise the temperature by 2–4°F, or pitch a small dose of a highly attenuative yeast like EC-1118.

Final gravity is lower than expected

A lower-than-expected FG is less common but can indicate:

A highly attenuative yeast strain performing at the top of its range
A high mash temperature that was lower than intended (lower mash temp = more fermentable wort)
Wild yeast contamination in rare cases

A beer finishing slightly lower than expected is usually not a problem — it simply means a drier, slightly stronger beer than planned.

Frequently Asked Questions

What is original gravity (OG) in homebrewing?

Original gravity (OG) is the specific gravity of your wort measured before fermentation begins. It tells you how much fermentable sugar is dissolved relative to pure water (1.000). A typical American ale might have an OG of 1.050 — meaning the wort is 5% denser than water. Higher OG means more sugar available for yeast and a stronger potential ABV.

What is final gravity (FG) in homebrewing?

Final gravity (FG) is the specific gravity measured after fermentation is complete. As yeast consume sugars and produce alcohol, the gravity falls. A well-attenuated ale typically finishes between 1.008 and 1.015. A higher-than-expected FG may indicate stuck fermentation; a lower-than-expected FG usually just means a drier, stronger beer.

How do you calculate ABV from original and final gravity?

The standard formula is: ABV = (OG − FG) × 131.25. For example, OG 1.055 and FG 1.010 gives (0.045) × 131.25 = 5.9% ABV. This is accurate to within ±0.3% for most ales and lagers.

What is attenuation in brewing?

Attenuation is the percentage of fermentable sugars yeast consumed. Apparent attenuation = ((OG − FG) / (OG − 1.000)) × 100. For OG 1.060 and FG 1.012, that is (0.048 / 0.060) × 100 = 80%. Most ale strains attenuate between 72% and 82%. Real attenuation is slightly lower because alcohol is less dense than water and inflates the apparent reading.

What is the difference between a hydrometer and refractometer for gravity measurement?

A hydrometer floats in a liquid sample and reads gravity directly — accurate at any stage of fermentation with temperature correction. A refractometer bends light through 2–3 drops of liquid and is ideal for quick pre-fermentation checks, but once alcohol is present it gives inflated readings and requires a correction formula. Use a hydrometer for all post-pitch gravity readings.

Put Your Gravity Numbers to Work

Use our free calculators to get precise results from every brew:

Hydrometer Calculator — Enter OG and FG for instant ABV, attenuation, and temperature-corrected readings.
IBU Calculator — Calculate bitterness units for your hop additions.
Yeast Pitch Calculator — Make sure you pitch enough yeast cells to hit your target FG.