Original gravity (OG) is the must’s density before fermentation; final gravity (FG) is its density when fermentation is complete. Their difference estimates alcohol production, while FG helps describe residual density—but perceived sweetness also depends on acidity, tannin, alcohol, and honey.
What OG and FG actually measure
Specific gravity compares the density of a liquid with water. Dissolved honey sugars make unfermented must denser than water, so a hydrometer floats higher. As yeast converts sugar into ethanol and carbon dioxide, density generally falls. Reading before and after fermentation provides an inexpensive estimate of how much extract was consumed.
An OG of 1.100 does not mean “10% sugar” or guarantee a particular ABV. Gravity includes dissolved material, and fermentation yield varies. An FG of 1.010 does not guarantee the same perceived sweetness in every mead. Fruit, acids, tannins, glycerol, alcohol, and unfermentable material all affect the reading and the palate.
Plan strength and sweetness separately
| Planning value | Mainly tells you | Does not guarantee |
|---|---|---|
| Original gravity | Starting density and possible strength | That yeast will finish the fermentation |
| Final gravity | Ending density and approximate residual extract | Perceived sweetness by itself |
| Gravity drop | Basis for estimated ABV | Laboratory-accurate alcohol content |
| Yeast tolerance | A planning limit under good conditions | A precise stopping point |
The BJCP mead study guide emphasizes balance among sweetness, acidity, alcohol, and honey character. It recommends managing alcohol primarily with the honey in the original must and adjusting sweetness after fermentation, rather than expecting yeast to stop at an exact gravity.
Estimate ABV from the gravity drop
A common home-fermentation estimate is(OG − FG) × 131.25. For a mead starting at 1.100 and ending at 1.010:
(1.100 − 1.010) × 131.25 = 11.81% ABV
This is an estimate, not a direct alcohol measurement. NIST explains that hydrometer calculations are practical for home producers but require attention to temperature and dissolved carbon dioxide. Professional measurements may use distillation, density instruments, or chromatography.
The familiar factor is most useful in ordinary fermentation ranges. Very high-gravity mead, large additions after the OG reading, fruit solids, and unusual finishing density increase uncertainty. For labeling, taxation, or sale, use the legally accepted method in your jurisdiction rather than a hobby estimate.
Understand mead strength categories
Traditional mead terminology includes hydromel for lower-strength mead, standard mead, and sack for stronger mead. Historical BJCP guidance places hydromel roughly below 1.080 OG, standard mead around 1.080–1.120, and sack above 1.120. These are style descriptors, not fermentation instructions.
A higher OG creates a more stressful environment for yeast and does not automatically produce a better mead. Yeast strain, nutrient availability, temperature, pH, oxygen management early in fermentation, and pitch health all influence whether fermentation completes. Select a yeast and process designed for the target rather than simply adding more honey.
Use the mead honey calculatorto estimate a starting honey amount from target volume, ABV, and FG. It shows the assumed yield and target OG so you can measure the actual must and adjust before pitching. Honey composition varies; the hydrometer reading is the better record of what you actually made.
Interpret final gravity and sweetness
Older BJCP judging guidance offered rough gravity bands: below about 1.010 for dry, 1.010–1.025 for medium, and above 1.025 for sweet. Current BJCP guidance cautions that perceived sweetness is a function of residual sugar and should not be classified from FG alone.
Consider two meads at 1.015. A high-acid berry mead may taste balanced or nearly dry, while a low-acid traditional mead may taste noticeably sweet. Tannin can add structure; ethanol can contribute warmth or apparent sweetness. Use FG as one measurement, then evaluate balance with controlled bench trials.
Do not plan residual sweetness by choosing a yeast “tolerance” equal to your target ABV. Tolerance is not an on/off switch, and a fermentation can stop early from stress or continue farther under favorable conditions. A stable dry fermentation followed by a properly managed sweetness adjustment is more predictable, but stabilization requires process knowledge and should follow a trusted meadmaking source.
Take reliable hydrometer readings
- Sanitize the thief, sample jar, and anything contacting the must.
- Remove a representative sample without returning it to the fermenter.
- Degas fermented samples so bubbles do not lift the hydrometer.
- Bring the sample near the hydrometer’s calibration temperature.
- Lower the hydrometer gently and spin it to dislodge bubbles.
- Read at eye level using the instrument’s meniscus instruction.
- Apply the manufacturer’s temperature correction if required.
- Record gravity, sample temperature, date, and fermentation notes.
Check the hydrometer in distilled water at its calibration temperature. A reading offset from the expected value can be recorded as an instrument correction. Inspect for cracks and do not use a damaged glass hydrometer.
Know when final gravity is truly final
One reading is a present gravity, not necessarily final gravity. Take another corrected reading after an appropriate interval. Stable readings indicate that density is no longer changing, but a stalled fermentation can also be stable. Compare the result with expected attenuation, yeast condition, target gravity, aroma, and the fermentation plan.
Do not bottle a mead with uncertain fermentation in a sealed container. Remaining fermentable sugar can create dangerous pressure. Sweetening after fermentation adds new fermentable material unless the beverage is reliably stabilized by an appropriate method. Still, sparkling, and bottle-conditioned meads require different packaging calculations and pressure-rated equipment.
Troubleshoot unexpected gravity
OG is lower than planned
Confirm the must is completely mixed; dense honey can remain stratified at the bottom. Recheck temperature correction and total volume. If adjustment is needed before fermentation, calculate the honey addition, dissolve it hygienically, mix thoroughly, and measure again.
OG is higher than planned
Verify volume and mixing. Adding water lowers gravity but also changes acidity, nutrients, flavor concentration, and headspace. Use measured, food-safe water and recalculate the entire formula rather than diluting by guesswork.
FG remains much higher than expected
Confirm stable, temperature-corrected readings and rule out measurement errors. Review yeast tolerance, fermentation temperature, nutrients, pH, and signs of contamination. A stuck high-gravity mead needs a diagnostic process; adding more yeast blindly may not solve the underlying stress.
The refractometer and hydrometer disagree
Alcohol changes refractive index, so a raw refractometer Brix reading after fermentation cannot be treated like pre-fermentation sugar. Use a correction model that includes the original reading or rely on a properly corrected hydrometer. Even corrected hobby readings remain estimates.
Keep calculations in their proper role
Gravity lets you plan and repeat a mead, but it cannot evaluate flavor, microbiological stability, or legal alcohol content by itself. Maintain a batch record with ingredients, lot information, measurements, additions, temperatures, transfers, and packaging decisions. If producing for sale, verify licensing, labeling, taxation, and laboratory requirements where the mead will be made and sold.
A useful batch sheet keeps raw measurements as well as corrected values. Record the hydrometer model, calibration temperature, sample temperature, correction, and whether the sample was degassed. Also record every honey, fruit, water, or sugar addition after OG. Without those additions, two gravity readings may imply alcohol that was not actually produced from the original must alone.
Repeatability is the real value of gravity tracking. If one batch finishes sweeter than intended, the notes let you distinguish a higher starting density from incomplete fermentation or a measurement mistake. Change one planning variable at a time, confirm the new OG with an instrument, and judge the finished balance rather than targeting a number in isolation.