What Is a Hydrometer and How Do You Use It?

What Is a Hydrometer?

A hydrometer is a glass instrument that measures the specific gravity (SG) of your wort or beer — the density relative to water. It tells you how much sugar is in your brew before fermentation, how fermentation is progressing, and when it's safe to bottle.

How Does It Work?

A hydrometer floats in liquid. The denser the liquid, the higher it floats. Pure water reads 1.000. A typical ale wort before fermentation might read 1.040–1.050. Once fermentation is complete, the reading drops to around 1.008–1.014.

Key Terms

• Original Gravity (OG) — taken before fermentation. Shows how much fermentable sugar (and potential alcohol) is in your wort.
• Final Gravity (FG) — taken when fermentation appears complete. Shows how much sugar remains unconverted.
• Attenuation — the percentage of sugar converted. Higher attenuation = drier, less sweet beer.

How to Use Your Hydrometer

1. Sterilise your hydrometer and trial jar with steriliser before use.
2. Take a sample of wort or beer and pour into the trial jar.
3. Lower the hydrometer gently, give it a spin to release air bubbles.
4. Read the scale at the bottom of the meniscus, at eye level.
5. Note the reading and temperature (calibrated for 20°C).

Calculating ABV

ABV = (OG – FG) × 131.25

Example: OG 1.048, FG 1.010 → (0.038) × 131.25 = 4.99% ABV

When to Take Readings

• Before pitching yeast — record OG
• After 5–7 days — check progress
• On two consecutive days — identical readings = safe to bottle with carbonation drops

Pick up a Wine & Beer Hydrometer from Brewco.uk and take the guesswork out of your home brewing.