A hydrometer is one of the most useful tools in home brewing. It helps you measure the density of your wort or beer, which tells you how much sugar is present and whether fermentation is progressing as it should.

Taking a hydrometer reading is simple, but accuracy matters. A poor sample, the wrong temperature or trapped bubbles can give you misleading results and make it harder to know when your beer is ready to package.

What does a hydrometer measure?

A brewing hydrometer measures specific gravity, often shortened to SG. This compares the density of your liquid against the density of water.

Plain water has a specific gravity of 1.000. Before fermentation, wort contains dissolved sugars from malt extract, grain or other fermentables, so it will have a higher reading, such as 1.045 or 1.050.

As yeast converts sugar into alcohol and carbon dioxide, the gravity drops. Comparing your original gravity and final gravity helps you calculate alcohol content and confirms whether fermentation has finished.

You can use our to calculate your brew. Brewing Calculators

What you need

To take a hydrometer reading, you will need:

• A brewing hydrometer
• A sanitised trial jar or hydrometer tube
• A sanitised wine thief, turkey baster or sample tap
• A thermometer
• Your beer or wort sample

Do not place your hydrometer directly into a fermenter unless the fermenter is large enough and you can remove it safely. Using a trial jar reduces the risk of contamination and makes the reading easier to see.

How to take a hydrometer reading

1. Sanitise everything that touches the beer

Anything that comes into contact with your beer after the boil should be clean and sanitised. This includes your sample thief, trial jar and hydrometer.

Sanitising protects your batch from unwanted bacteria and wild yeast that can create sour, funky or off flavours.

You can buy our Brewing Sanitisers and Cleaning Products here

2. Take a sample

Draw enough wort or beer to fill your trial jar. Try not to take too much sediment from the bottom of the fermenter, especially later in fermentation.

Avoid returning the sample to the fermenter once you have taken the reading. It is safer to taste it, discard it or use it for another measurement.

3. Check the temperature

Most brewing hydrometers are calibrated at a specific temperature, commonly 20°C. Check the paper scale inside your hydrometer or the product instructions.

A warmer sample is less dense and can make the gravity appear lower than it really is. A colder sample can make it appear higher. For best results, cool your sample close to the calibration temperature before reading it.

4. Float the hydrometer

Place the hydrometer gently into the trial jar. Give it a small spin between your fingers to dislodge any bubbles sticking to the glass.

Bubbles can make the hydrometer float higher and give you an incorrect reading.

5. Read at eye level

Look across the liquid surface at eye level and read the scale at the bottom of the meniscus, which is the curved surface of the liquid.

For example, if the liquid line sits halfway between 1.012 and 1.014, your reading is approximately 1.013.

When should you take readings?

For most beer batches, take at least two readings:

Original Gravity (OG): Take this before fermentation starts, usually after your wort has been cooled and mixed thoroughly.

Final Gravity (FG): Take this near the end of fermentation to check whether the yeast has finished its job.

A single final gravity reading does not always prove fermentation is complete. Take another reading two or three days later. If the gravity stays the same, fermentation is usually finished.

Common hydrometer mistakes

Reading the foam instead of the liquid

Foam can hide the true surface level. Let the sample settle briefly or gently remove surface bubbles before taking the reading.

Measuring a warm sample

Temperature is one of the biggest causes of inaccurate readings. Always check the hydrometer calibration temperature.

Taking a reading before the wort is mixed

If you have topped up your fermenter with water, stir or rock the fermenter carefully before measuring. Concentrated wort can sit lower in the fermenter and give a false high reading.

Bottling too early

Do not bottle based only on the number of days in the fermenter. Confirm that your final gravity has remained stable before packaging. Bottling active beer can cause excessive carbonation or bottle bombs.

Why hydrometer readings matter

Hydrometer readings help you make better decisions throughout the brewing process. They can tell you whether fermentation has started, whether it has stalled, whether your beer is ready to bottle and what alcohol level you have achieved.

Once you are comfortable taking readings, use your original gravity and final gravity in our [Link: Brewing Calculators] to estimate ABV and better understand your batch.

Frequently asked questions

Can I take a hydrometer reading during fermentation?

Yes. Use sanitised equipment and take a small sample from the fermenter. Avoid opening the fermenter unnecessarily, especially during the first few days.

What should my final gravity be?

It depends on the recipe, yeast strain and mash temperature. Many standard ales finish between 1.008 and 1.016, but stronger, darker or sweeter beers may finish higher.

Can I use a hydrometer for wine, cider or spirits wash?

Yes. Hydrometers can be used for beer, cider, wine and fermentation washes, provided you use the correct scale and follow the product instructions.