A hydrometer is a must-have tool in brewing. It measures the specific gravity (SG) of your wort or beer—basically, how much sugar is in the liquid. By using it before and after fermentation, you can track fermentation progress and calculate alcohol content (ABV).
Rather than dropping your hydrometer in your fermenter and risking contamination, it is best to take a sample via siphon or from the tap if your fermenter is fitted with one.
Your test cylinder should hold 100-200ml, be sure to leave enough room for the hydrometer to float freely.
Make sure everything has been thoroughly sanitised before touching the beer/wort.
Gently lower the hydrometer into the sample.
Give it a light spin to shake off any bubbles clinging to the sides (they can skew the reading).
You may see some curve in the water where it adheres to the side of the tube or the hydrometer itself. This is called the meniscus.
You should take your reading at the bottom of this.
It is a bit diificult to see in the image here, but this would be a specific gravity (SG) reading of 1.052
In brewing, we usually refer to it as Original Gravity (OG) if it's before fermentation, or Final Gravity (FG) at the end of fermentation.
Correct for Temperature.
Hydrometers are usually calibrated at 20°C (68°F).
If your sample is warmer or cooler, you may need to use a correction chart or online calculator.
Never return the sample to your fermenter—discard it or drink it for a preview taste test.
If you haven't invested in some brewing software yet, I recommend getting Beersmith. It will take all the hassle out of brewing calculations and calibrations.
If you still want to do it manually, to calculate alcohol by volume, use this formula:
Example:
If OG = 1.050 and FG = 1.010
→ (1.050 - 1.010) × 131.25 = 5.25% ABV