OG, FG, ABV and Attenuation Explained

Gravity is one of the brewer’s best measurements

Specific gravity compares the density of a liquid with the density of water.

Pure water is approximately 1.000 at the hydrometer’s calibration temperature.

Wort contains dissolved sugars and other compounds, so it is denser.

A wort reading of:

1.050

is commonly described as “fifty gravity points.”

Gravity allows you to follow the transformation from sweet wort into fermented beer.

Original Gravity — OG

Original Gravity is the gravity of the wort immediately before fermentation.

OG reflects the amount of dissolved extract produced by:

• malt;
• malt extract;
• sugar;
• other fermentable or soluble ingredients.

OG is affected by both extract and volume.

If you produce the expected amount of sugar but end with too much wort, OG will be low because the extract is diluted.

If final volume is too low, OG can be high because the wort is more concentrated.

That is why gravity should always be interpreted together with volume.

Final Gravity — FG

Final Gravity is the gravity after fermentation has reached a stable endpoint.

As yeast consumes sugars and produces alcohol, gravity falls.

FG reflects several interacting factors:

• original wort composition;
• mash fermentability;
• yeast strain;
• attenuation;
• fermentation health;
• simple sugar additions;
• dry hopping and hop creep.

Two beers with the same OG can finish at very different FG values.

Apparent attenuation

Apparent attenuation describes the apparent percentage of gravity extract removed during fermentation.

A common formula is:

Apparent attenuation (%) = (OG - FG) / (OG - 1.000) × 100

Example:

OG = 1.052
FG = 1.012

(1.052 - 1.012) / 0.052 × 100 ≈ 76.9%

This is called apparent attenuation because alcohol changes the density of the finished beer. It is not the same as the true percentage of all extract physically consumed.

For recipe work, apparent attenuation is still extremely useful.

ABV

A common homebrew approximation is:

ABV ≈ (OG - FG) × 131.25

Using the same beer:

(1.052 - 1.012) × 131.25 ≈ 5.25% ABV

This formula is an approximation. More advanced formulas can improve accuracy at higher gravities, and laboratory alcohol measurement is more precise.

For normal homebrew recipe planning, the approximation is usually sufficient.

Why actual FG differs from predicted FG

Software often predicts FG from the yeast’s stated attenuation range.

But yeast attenuation is not an isolated property.

Actual FG can change because of:

• mash temperature;
• grain bill;
• highly fermentable sugar;
• mash enzymes;
• yeast health;
• pitch rate;
• fermentation temperature;
• measurement error.

Manufacturer attenuation values are best treated as a realistic operating range under defined conditions, not a promise.

Hydrometer technique

For reliable readings:

1. use a clean sample cylinder;
2. avoid large bubbles clinging to the hydrometer;
3. spin gently if needed to release bubbles;
4. read at the correct meniscus convention for the instrument;
5. correct for temperature if the sample differs significantly from calibration temperature.

Hydrometers are simple instruments, but sloppy sampling can create misleading data.

Refractometers

Refractometers are excellent before fermentation because they require only a small sample.

Once alcohol is present, the reading must be corrected.

Do not look at a post-fermentation Brix value and convert it directly into specific gravity as if the sample were still wort.

Use a validated refractometer correction formula with the original reading.

Gravity points and recipe scaling

A useful concept is gravity points.

For example:

1.050 = 50 points

If you have 20 L at 1.050 and dilute to a larger volume, total extract is roughly conserved even though gravity falls.

This makes gravity and volume useful for troubleshooting corrections.

If your OG is high because final volume is low, adding suitable water may bring both volume and gravity toward target.

Diagnosing a missed OG

OG too low, volume correct

Possible causes:

• efficiency lower than expected;
• poor crush;
• mash/conversion issue;
• incorrect ingredient weight;
• measurement error.

OG too low, volume high

Likely contributor:

• too much final wort volume.

OG too high, volume low

Likely contributor:

• excessive boil-off or insufficient water.

Do not adjust equipment efficiency based on one abnormal batch before checking volume and measurements.

Gravity is a timeline, not just two points

The most useful dataset may include:

• pre-boil gravity;
• OG;
• fermentation readings;
• FG.

Those points show where the process changed.

A missed OG that was already visible pre-boil has a different cause from a missed OG caused by boil-off.