Measuring Gravity

What Is Gravity

In brewing, gravity refers to the density of a liquid relative to water. Wort is denser than water because it contains dissolved sugars. As yeast consumes those sugars during fermentation, the gravity drops. Measuring gravity tells you how much sugar is present and how much has been converted to alcohol.

Original Gravity (OG)

OG is the gravity of the wort just before pitching yeast. It indicates the total fermentable and unfermentable sugar content. A typical OG for an American pale ale is 1.050-1.060. For an imperial stout, 1.080-1.100+.

Take the OG reading after cooling and aerating the wort, just before adding yeast. Record it — you will need it to calculate ABV later.

Final Gravity (FG)

FG is the gravity after fermentation is complete. It indicates how much sugar remains (residual sweetness and unfermentable dextrins). A dry beer might finish at 1.008; a sweet, full-bodied beer at 1.018.

Fermentation is complete when the gravity reading remains stable over 2-3 consecutive days. Do not rely on airlock activity — it can stop while gravity is still dropping.

Using a Hydrometer

A hydrometer is a glass instrument that floats in a liquid sample. The deeper it sinks, the lower the gravity (less sugar). To use:

Draw a sample into a hydrometer jar (do not drop the hydrometer into the fermenter)
Place the hydrometer in the sample and spin to dislodge bubbles
Read the scale at the bottom of the meniscus (the curve where liquid meets air)
Correct for temperature — hydrometers are calibrated at 60 F or 68 F

Using a Refractometer

A refractometer measures gravity by how light bends through a drop of liquid on a prism. Advantages: requires only a few drops (no wasting beer), instant readings.

Important caveat: Refractometer readings in fermented beer are inaccurate because alcohol changes the refractive index. You must apply a correction formula or use an online calculator to convert Brix readings to true gravity in the presence of alcohol.

Calculating ABV

The basic formula: ABV = (OG - FG) x 131.25

Example: OG 1.055, FG 1.012 → (0.055 - 0.012) x 131.25 = 5.6% ABV

Attenuation

Attenuation is the percentage of sugar consumed by yeast:

Apparent Attenuation = (OG - FG) / (OG - 1.000) x 100

Example: (1.055 - 1.012) / (1.055 - 1.000) x 100 = 78% apparent attenuation

Most ale yeasts achieve 72-80% apparent attenuation. Highly attenuative strains (saison yeasts) can reach 85-95%.

When to Measure

Pre-boil — verify mash efficiency and predict OG
Post-boil (OG) — the critical baseline reading
Day 7-10 — check fermentation progress
Day 12-14 — confirm stable FG over 2-3 days
At packaging — final verification before bottling or kegging