How to Use a Home Brew Hydrometer — Gravity Readings Explained

What is a Home Brew Hydrometer and How Do I Use One?

A hydrometer is one of the most important tools in any home brewer's kit — a simple glass instrument that measures the density (specific gravity) of your beer, wine, or cider relative to water. Understanding how to use a hydrometer gives you accurate control over your fermentation, lets you calculate the alcohol content of your finished brew, and tells you definitively when fermentation is complete. If you've been guessing when your homebrew is ready to bottle, a hydrometer removes all the guesswork.

How Does a Hydrometer Work?

A hydrometer measures specific gravity — the density of a liquid relative to water. Water has a specific gravity of 1.000. Wort or must (unfermented beer or wine) containing dissolved sugars is denser than water, so it has a higher specific gravity — typically between 1.035 and 1.060 for a standard homebrew. As yeast converts those sugars into alcohol (which is less dense than water), the specific gravity drops. When it reaches its final gravity — usually between 1.004 and 1.012 — fermentation is complete.

How to Calculate Alcohol Content

Calculating ABV from your hydrometer readings is straightforward. Take your Original Gravity (OG) reading before pitching yeast, and your Final Gravity (FG) reading when fermentation is complete. Then use this formula: ABV% = (OG - FG) × 131.25. For example, if your OG was 1.050 and your FG is 1.010, the ABV is (1.050 - 1.010) × 131.25 = 5.25% ABV.

When is Fermentation Finished?

Fermentation is complete when your hydrometer reading stays the same for two consecutive readings taken 24 hours apart. Never bottle based on airlock activity alone — airlocks can stop bubbling before fermentation is truly complete. Bottling too early risks over-carbonated or even exploding bottles. A quality hydrometer is an essential investment that costs less than £5 and will serve you for years. Browse all our home brewing equipment at BrewCo UK.