How to Use a Home Brew Hydrometer — Gravity Readings Explained
Quick answer
Float the hydrometer in a sample in a trial jar and read the scale at the bottom of the meniscus, at eye level. Take the original gravity (OG) before adding the yeast and the final gravity (FG) at the end. Two matching readings 24 hours apart mean it's safe to bottle. ABV = (OG − FG) × 131.25.
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 around £5 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.
What do hydrometer readings mean?
A hydrometer floats higher in denser, sugary liquid. Pure water reads 1.000. A typical beer kit wort starts around 1.038–1.050 and drops to around 1.006–1.012 once the yeast has turned the sugar into alcohol.
- Original Gravity (OG) – taken before you add the yeast. It shows how much fermentable sugar, and so how much potential alcohol, is in your brew.
- Final Gravity (FG) – taken when fermentation looks finished. It shows how much sugar is left.
- Attenuation – how much of the sugar the yeast has used. Higher attenuation means a drier, less sweet drink.
Tips for an accurate reading
- Clean and sterilise the hydrometer and trial jar first, for example with Young's Cleaner/Steriliser, then rinse.
- Give the hydrometer a gentle spin as you lower it in, to knock off air bubbles that would push it up.
- Read the scale at eye level, at the bottom of the meniscus.
- Note the temperature too. Most hydrometers are calibrated for 20°C, so a much warmer or colder sample will read slightly out.
- Take a reading before adding the yeast, again after 5–7 days, then on two days in a row. Matching readings mean it's safe to bottle with carbonation drops.
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. Prefer not to do the sums? Our ABV calculator works it out for you.
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.