How to Read a Cider Fermentation Gravity Log

Record the starting point

Before pitching yeast, measure a representative sample of well-mixed apple juice and record its original gravity (OG), temperature, date, and any additions. Apple juice composition varies, so a reading is more useful than assuming a standard starting strength.

Use the hydrometer according to its calibration temperature and correction instructions. Sanitize sampling equipment and do not return the sample to the fermenter.

Look for a pattern over time

Take readings at sensible intervals with a clean instrument. A falling gravity suggests sugar is being consumed, but one reading does not show whether fermentation is complete. Airlock bubbles can slow or stop for reasons unrelated to a stable final gravity.

When activity appears finished, compare repeat readings separated by several days. Follow a reliable cider process before packaging, especially if you plan to sweeten or carbonate.

Worked example: estimate strength

Suppose a cider starts at OG 1.050 and later measures FG 1.004. The common home-fermentation estimate gives (1.050 − 1.004) × 131.25 ≈ 6.0% ABV. This is an estimate, not a lab result. If another reading several days later is still 1.004, that supports a stable reading; it does not by itself establish that every packaging or safety condition has been met.

Keep the record useful

Write down instrument, temperature, gravity, date, and any changes to the batch. If a reading stops changing earlier than expected, check the measurement method and temperature before assuming a stuck fermentation.

The cider ABV calculator uses the same gravity-difference estimate; review its limitations before relying on the result.