Measure original gravity from a mixed, cooled sample before fermentation and final gravity from a degassed sample after readings become stable. Read the hydrometer at eye level, account for its calibration temperature, and record both values before calculating estimated alcohol by volume.
What gravity tells a brewer
Specific gravity is the density of wort or beer relative to water. Dissolved malt sugars and other extract make wort denser, so a hydrometer floats higher before fermentation. As yeast consumes fermentable sugar and produces ethanol and carbon dioxide, density falls.
Original gravity (OG) is the reading taken before fermentation begins. Final gravity (FG) is the reading after fermentation is complete. A reading taken in the middle is more accurately called present gravity. Comparing OG and FG estimates ABV and apparent attenuation.
NIST describes this as an accessible method for home producers while noting that temperature and carbon dioxide influence accuracy. Commercial or legal determinations may require validated laboratory methods; a hobby hydrometer calculation should not be used automatically for tax or labeling compliance.
Prepare and check the hydrometer
Read the paper scale and instructions before brewing day. Many brewing hydrometers are calibrated at 60°F (15.6°C) or 68°F (20°C), but you must use the value printed for your instrument. Inspect the glass for cracks, confirm the paper scale has not slipped, and choose a sample cylinder deep enough for it to float freely.
- Fill the cylinder with distilled water near calibration temperature.
- Lower the clean hydrometer without dropping it.
- Spin gently to release bubbles and keep it away from the wall.
- Read at eye level according to the specified meniscus convention.
- Record any offset from the expected water reading.
If it reads 0.998 where it should read 1.000, record a +0.002 instrument correction. A check does not replace professional calibration, but it can expose a broken or shifted scale before the OG sample is gone.
How to measure original gravity
1. Mix the wort thoroughly
Top-off water and concentrated wort can form layers. Draw the sample only after the batch is homogeneous. In all-grain brewing, a pre-boil gravity sample and post-boil OG are different process measurements; label them correctly in your notes.
2. Cool the sample safely
Hot liquid changes density and can crack a sample cylinder or hydrometer. Cool a small covered sample near the instrument’s calibration temperature. Do not pour the tested sample back into cooled wort; removing a small sample is cheaper than introducing contamination.
3. Remove foam and bubbles
Foam hides the meniscus, while bubbles attached to the stem can lift the hydrometer. Fill enough that the instrument floats, lower it gently, and spin once. Let it settle without touching the bottom or side.
4. Read at eye level
Looking down makes the meniscus appear at the wrong line. Bring your eyes to liquid level. Clear lab solutions are often read at the bottom of the meniscus, but some brewing instruments specify a different convention; use the manufacturer’s instruction consistently.
5. Correct and record
Apply the supplied temperature table or formula if the sample differs from calibration temperature, then apply any verified instrument offset. Record raw reading, sample temperature, corrected OG, time, and batch volume.
How to measure final gravity
Use a sanitized sampling valve, thief, or pipette to remove a representative sample. Avoid drawing only yeast sediment or floating debris. Fermented beer contains carbon dioxide that can cling to the hydrometer and make it float too high, so degas the sample by stirring gently or transferring between clean containers until it is mostly still.
Let foam subside, bring the sample near calibration temperature, and repeat the same eye-level reading procedure. Record it as present gravity until a later measurement confirms stability. Do not use airlock activity alone: leaking lids may show no bubbles while fermentation continues, and temperature changes can move liquid through an airlock after fermentation.
Take another reading after the interval appropriate to the recipe. If the corrected values are stable and the gravity is plausible for the yeast and style, it may be final. A high, stable gravity can also indicate stalled fermentation, so evaluate yeast health, temperature, recipe, and sensory signs before packaging.
Gravity reading error checklist
| Error | Likely effect | Correction |
|---|---|---|
| Warm sample | Density reading shifts | Cool or use the instrument’s temperature correction |
| CO₂ bubbles on hydrometer | Reading appears higher | Degas and spin gently |
| Hydrometer touches cylinder | Reading sticks or tilts | Use a wider cylinder and center it |
| Stratified top-off water | OG can appear too low or high | Mix wort thoroughly before sampling |
| Reading above eye level | Parallax error | Read horizontally at the meniscus |
| Raw refractometer reading after fermentation | Alcohol makes Brix misleading | Use an alcohol correction model or hydrometer |
Calculate estimated ABV
A widespread homebrew approximation is:
ABV = (OG − FG) × 131.25
For OG 1.050 and FG 1.010, the gravity drop is 0.040:
0.040 × 131.25 = 5.25% ABV
Enter both readings in the Beer ABV Calculator to see the standard estimate, gravity points consumed, apparent attenuation, and an alternate high-gravity estimate. The formulas can diverge as strength rises because fermentation yield is not perfectly constant.
HM Revenue & Customs publishes a factor method for beer in which the factor changes with gravity drop and strength. That legal guidance also requires the OG to be established promptly before fermentation. It illustrates why one simplified constant is useful for hobby estimates but not universal.
Calculate apparent attenuation
Apparent attenuation estimates the percentage of gravity points consumed:
((OG − FG) ÷ (OG − 1)) × 100
With OG 1.050 and FG 1.010:
((1.050 − 1.010) ÷ 0.050) × 100 = 80%
It is called apparent because ethanol lowers density. Real attenuation requires a different determination. Compare apparent attenuation with the yeast’s published range and recipe design, but do not diagnose a batch from a single percentage. Mash temperature, wort composition, yeast health, and measurement error all influence FG.
Hydrometer versus refractometer
A refractometer needs only a small sample and is convenient during wort production. It measures refractive index, often displayed as Brix. Before fermentation, conversion tools can estimate specific gravity. After alcohol appears, the raw reading no longer represents sugar in the same way.
Post-fermentation refractometer correction requires the original reading and an empirical model. A hydrometer directly responds to density but uses a larger sample. Use one primary method consistently and document conversions. Mixing uncorrected Brix and specific gravity values is a common source of impossible ABV results.
Packaging and pressure safety
Do not bottle solely because the recipe’s expected number of days has passed. Unfinished fermentation in a sealed package can create dangerous pressure. Confirm stable gravity and calculate priming sugar with a method suited to beer volume, temperature, residual carbon dioxide, and package rating.
If fermentation stops far above the expected FG, troubleshoot before packaging. Check current temperature, yeast condition, calibration, recipe fermentability, and whether the sample was degassed. Do not “fix” a stalled batch by sealing it. For commercial production, obtain appropriate laboratory testing and follow alcohol, tax, and labeling rules in the relevant market.