How to Use a Home Brew Hydrometer — Gravity Readings Explained

What is a hydrometer and why do you need one for home brewing?

A hydrometer measures the specific gravity (density) of your beer, wine, or cider — telling you how much sugar is present. This lets you confirm when fermentation is complete (safe to bottle), calculate your beer's ABV, and avoid the most common home brewing mistake: bottling too early. A hydrometer costs under £4 and is the single most important piece of equipment after your fermenter. See our home brew FAQ for more on why it's essential.

How do you use a hydrometer correctly?

Fill a trial jar or tall container with a sample of your beer. Float the hydrometer so it isn't touching the sides. Read the scale at the lowest point of the curved liquid surface (the meniscus) — not where the liquid climbs up the glass. Take your Original Gravity (OG) reading before pitching yeast, and your Final Gravity (FG) reading when you think fermentation is complete. When two readings 24 hours apart are identical, it's safe to bottle.

How do you calculate ABV from hydrometer readings?

ABV% = (Original Gravity − Final Gravity) × 131.25. For example: OG 1.050, FG 1.010 → (1.050 − 1.010) × 131.25 = 5.25% ABV. Most standard beer kits start around 1.038–1.050 OG and finish at 1.006–1.012 FG, producing beers of 3.5–5.5% ABV.

When is fermentation finished according to the hydrometer?

Fermentation is complete when two readings taken 24 hours apart are identical and match the expected final gravity in your kit instructions. Never bottle based on airlock activity alone — airlocks can stop bubbling before fermentation is truly complete, and bottling too early causes over-carbonated or exploding bottles. This is covered in detail in our conditioning guide and troubleshooting guide. Buy your hydrometer at BrewCo UK and browse all home brewing equipment.