Fermentation is where wort becomes beer, and it is the stage where measurement matters most. Gravity readings tell you how much sugar the yeast has eaten and when fermentation has finished; temperature readings tell you whether the yeast is working in the range that produces clean flavours; and airlocks keep contaminants out. Misreading these signs is the most common cause of bottles that are flat, sweet or dangerously over-carbonated. This guide explains each tool and how to use it, drawing on American Homebrewers Association (AHA) guidance.
Gravity: what it measures
Specific gravity compares the density of wort or beer with water. Dissolved sugars make wort denser; as yeast converts sugar to alcohol and carbon dioxide, gravity falls. You take an original gravity (OG) before pitching yeast and a final gravity (FG) when fermentation ends. The difference lets you estimate alcohol content, and a stable FG tells you fermentation is complete.
Using a hydrometer
The AHA describes a hydrometer as the tool used to measure gravity, or sugar density, in wort and beer.
- Clean and sanitise the hydrometer, test jar and anything used to take the sample.
- Draw a sample with a sanitised thief or siphon; do not return it to the fermenter.
- Float the hydrometer in the test jar, spin it gently to remove bubbles and read at eye level at the bottom of the curved liquid surface.
- Correct for temperature if your sample is far from the hydrometer’s calibration temperature (usually printed on it).
- Record the reading, date and temperature.
Take readings on several days towards the end of fermentation. When they stop changing, fermentation is complete. See hydrometers.
Refractometers
A refractometer measures sugar from a few drops, which is convenient during brew day. Readings are usually in Brix and need converting to gravity. Once alcohol is present, refractometer readings are skewed and need a correction formula, so many brewers use a refractometer for wort and a hydrometer for final gravity. See refractometers.
Temperature
Yeast produces different flavours at different temperatures. The AHA notes yeast is typically pitched below about 75°F (24°C); each yeast strain has its own recommended range, printed by the supplier. Fermentation also generates heat, so beer inside the fermenter can be warmer than the room.
| Method | Control | Notes |
|---|---|---|
| Cool room or basement | Low | Free; follow seasonal temperatures |
| Water bath or swamp cooler | Moderate | Damp towels and fans or ice bottles moderate temperature |
| Heat wrap or belt with controller | Good (heating) | Use only products made for fermenters, and follow electrical safety instructions |
| Fermentation chamber (fridge or freezer with controller) | Best | Controller probe taped to the fermenter wall under insulation |
See brewing thermometers, fermentation chambers and the equipment side in temperature control.
Airlocks: what they tell you
The AHA explains that an airlock allows carbon dioxide to escape the fermenter without letting contaminants in. Bubbling shows gas is escaping, but it is not a reliable measure of fermentation:
- A leaky lid or bung can let CO2 escape elsewhere, so the airlock stays quiet during active fermentation.
- Temperature changes can push gas through the airlock after fermentation has finished.
- Gravity readings, not bubbles, show when fermentation is complete.
Fill airlocks with sanitiser solution or clean water and keep them topped up. During very vigorous fermentation, foam can block an airlock; a blow-off tube into a container of sanitiser is the usual solution. See airlocks.
When is it ready to package?
The AHA tutorial on extract brewing notes that beer is generally ready to bottle after two to four weeks of fermentation but encourages brewers to confirm with hydrometer readings. Its article on sour beers adds a caution that applies more widely: a gravity drop as small as about 0.003 can produce full carbonation in a sealed bottle without any added sugar, so compare readings taken over time and only add priming sugar once they match. Slow-fermenting beers, wild yeasts and bacteria, and sweetened ciders and meads need extra care.
Signs something is wrong
| Sign | Possible cause | Action |
|---|---|---|
| No activity after a few days and gravity unchanged | Yeast not viable; too cold | Check temperature; consider pitching fresh yeast |
| Gravity stuck well above expected | Temperature too low, under-pitching, unfermentable sugars | Warm gently within the yeast’s range; check recipe expectations |
| Unusual film, colours or smells | Possible contamination | Review sanitation; do not bottle until you understand it |
| Gravity keeps drifting down slowly for weeks | Slow yeast or microbes | Wait; do not bottle until stable |
Keeping a fermentation log
A simple log turns guesswork into data. For each batch, record the date and time of pitching, yeast strain and amount, original gravity, the temperature of the room and, ideally, the fermenter, and each gravity reading with its date. Note when activity starts and slows, any dry hopping or other additions, and the final gravity. Over a few batches, the log shows how long your usual yeasts take in your space, which makes it easier to plan brew and bottling days and to spot problems early. Digital hydrometers that float in the fermenter can record gravity and temperature automatically; treat their readings as trends and confirm the final gravity with a standard hydrometer before packaging.
Frequently asked questions
Can I open the fermenter to check?
Keep openings brief and sanitise everything that touches the beer. Each opening is a chance for contamination and oxygen exposure.
Why does my final gravity differ from the recipe?
Yeast strain, temperature, mash temperature in all-grain brewing and ingredient differences all affect final gravity. Stable readings matter more than hitting the recipe’s exact number.
Equipment for each step is in our homebrewing equipment guide, and packaging in bottling and kegging. All monitoring guides are under Fermentation and Monitoring.
