If you’re brewing beer, cider, or even experimenting with fermented coffee drinks, a hydrometer is the single most useful piece of kit you can own. It measures the density of your liquid compared to water, which tells you how much sugar is present. That might sound technical, but it’s really just a glass float with a weighted bulb and a calibrated scale. The more sugar dissolved in your wort or juice, the higher the hydrometer floats. By taking a reading before fermentation and another after, you can track exactly how much sugar the yeast has consumed — and that gives you both your alcohol content and a clear sign that fermentation is complete. Without one, you’re brewing blind.
Most home brewing hydrometers show two scales: specific gravity (SG) and potential alcohol. Specific gravity is the one you’ll use most. Pure water reads 1.000 at the calibration temperature, usually 20°C. A typical beer wort might start at 1.050, while a dry cider juice might be 1.045. The potential alcohol scale gives you a rough idea of the final strength if all sugar ferments out — handy for planning, but always rely on your own OG and FG for accuracy.
The original gravity is your baseline. For beer, take a sample after the boil has cooled to pitching temperature — usually around 18–22°C. Sanitise your sampling jar and hydrometer, then fill the jar about three-quarters full. Float the hydrometer, spin it, and note the reading. For cider, press your apples and measure the fresh juice before you add any yeast. If you’re making a coffee stout or a coffee-infused cider, your OG comes from the base wort or juice; the coffee flavour is usually added later, so it won’t affect this number. Record your OG immediately — you’ll need it for the ABV calculation.
After a few days, fermentation will be well underway. Take a sample and note the gravity. Then take another reading two or three days later. When the gravity stays the same for two consecutive days, fermentation is finished. For most beers, FG lands between 1.008 and 1.015. Dry ciders often finish below 1.000. Don’t be tempted to open the fermenter every day — each sample introduces oxygen and risk of infection. A good routine is to check at day three or four, then again at day seven, and then every two days until stable. Never bottle before the gravity is stable, or you risk exploding bottles.
Once you have your OG and FG, the maths is simple. Subtract FG from OG, then multiply by 131.25. For example, an OG of 1.050 and an FG of 1.010 gives (1.050 – 1.010) × 131.25 = 5.25% ABV. If your sample was not at 20°C, adjust your reading first. For every degree above 20°C, add 0.002 to the gravity; for every degree below, subtract 0.002. So a reading of 1.050 taken at 25°C actually means 1.056. This matters most for high-gravity brews and warm summer cider making.
Cider makers rely on hydrometers just as much as beer brewers. Fresh apple juice varies wildly in sugar, so measuring OG tells you whether you need to add sugar or dilute with water. For coffee brewing, a hydrometer is less common in everyday drip or espresso, but it becomes useful if you’re fermenting coffee-based drinks — such as a coffee kombucha or a coffee cider. In those cases, treat the sweetened coffee mixture exactly like a wort: measure OG, pitch yeast, and track FG. A few final tips will keep your readings accurate:
With a little practice, reading a hydrometer becomes second nature. It turns guesswork into precise, repeatable brewing — whether you’re crafting a dry West Country cider, a hoppy pale ale, or a bold coffee stout.
The Brewhouse Journal shares practical, down-to-earth guidance on home brewing, cider making and coffee brewing for readers across the UK.
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