A hydrometer is one of the simplest and most useful tools in home brewing, yet it’s often misunderstood. Whether you’re making wine, beer, cider, or mead, a hydrometer tells you how fermentation is progressing and helps you avoid bottling too early.
Used correctly, it removes guesswork from brewing.
What a Hydrometer Measures
A hydrometer measures specific gravity (SG) which is the density of liquid compared to water. Sugar increases density, while alcohol lowers it. By measuring gravity before and after fermentation, you can tell:
- How much sugar was present at the start
- Whether fermentation is still active
- When fermentation is finished
- Your approximate alcohol content
Taking Your First Reading (Original Gravity)
Before fermentation starts, take an original gravity (OG) reading.
- Fill a trial jar with your wort, juice, or must
- Gently lower the hydrometer into the liquid
- Spin it slightly to remove bubbles
- Read the scale at liquid level (not the curved surface)
Write this number down you’ll need it later.
Checking Fermentation Progress
As yeast converts sugar into alcohol, the gravity drops. You can take readings during fermentation but avoid opening the vessel too often to reduce contamination risk.
Knowing When Fermentation Is Finished
Fermentation is considered finished when:
- Gravity readings stay the same for 2–3 days
- The reading matches the expected final gravity
Airlock activity alone is not reliable. This is one of the biggest reasons brewers end up with over-carbonated bottles or corks pushing out.
Common Hydrometer Mistakes
Some common issues to watch for:
- Reading too warm – Hydrometers are calibrated at a set temperature (usually 20°C)
- Reading the meniscus wrong – Always read at liquid level
- Bottling too early – Never bottle based on time alone
Taking an extra reading costs nothing and can save a batch.
From Demijohn to Bottling
Once gravity is stable, you can safely rack from the demijohn and move to bottling or secondary fermentation. For a guide on Racking Click here and for a guide on bottling Click Here
Don’t Forget to Sanitise Your Hydrometer
Because a hydrometer comes into direct contact with your brew, it should always be sanitised before use. It’s easy to overlook, especially when you’re only taking a quick reading, but skipping this step can introduce unwanted bacteria or wild yeast.
Before taking a gravity reading:
· Sanitise the hydrometer
· Sanitise the trial jar
· Avoid putting the hydrometer directly into your demijohn or fermenter
If you want a full guide on how to do this click here
Final Thoughts
Testing Equipment doesn’t make brewing more complicated; it makes it more predictable. Whether you’re brewing wine, beer, or mead in Ireland, it’s one of the best tools you can own for avoiding mistakes and improving consistency.
If you’re serious about repeatable results, learning to use a hydrometer properly is time well spent.