Few pieces of brewing kit reward you as handsomely as a hydrometer. For a few pounds, this simple glass float turns guesswork into measurable progress. Whether you are brewing a bitter, a cider from local apples, or a country wine, learning to read those tiny numbers will make you a more confident brewer. Let's walk through the essentials, from your first original gravity reading to estimating the strength of your finished pint.

What Your Hydrometer Is Actually Telling You

A hydrometer measures the density of a liquid compared to water. Pure water at its calibration temperature has a specific gravity (SG) of 1.000. When you dissolve malt extract, honey, or sugar into water, the liquid becomes denser, so the hydrometer floats higher and the reading increases. During fermentation, yeast converts those sugars into alcohol and carbon dioxide. Alcohol is less dense than water, so as sugar disappears, the gravity drops. By comparing the starting gravity with the finishing gravity, you can track fermentation and estimate alcohol. The scale you will use most is specific gravity, often written as 1.050 or 1050. The decimal point matters: 1.050 is not the same as 1.005.

Original Gravity: Your Starting Point

Original gravity (OG) is the gravity of your wort or must before you pitch the yeast. Take this reading once your brew is thoroughly mixed and cooled to the hydrometer's calibration temperature, usually 20°C. Sanitise your trial jar, wine thief, and hydrometer first. Fill the trial jar about three-quarters full, then lower the hydrometer gently and give it a spin to dislodge any bubbles. Read the number at eye level, where the liquid meets the scale (the meniscus). Record it straight away. A typical ale might start around 1.045 to 1.055, while a stronger stout could be 1.070 or more. If your OG is higher than expected, you may have boiled off more water; lower means you have more volume than planned. Either way, note it down.

Final Gravity and Tracking Fermentation

Final gravity (FG) is the reading you take when fermentation has finished. But how do you know it has finished? Take a reading, then wait two or three days and take another. If the number is unchanged, fermentation is done. Most beers finish between 1.008 and 1.015. The drop from OG to FG is called attenuation, and it tells you how much sugar the yeast consumed. For example, an OG of 1.050 and an FG of 1.010 means a drop of 40 points, or 80% apparent attenuation. Don't rush to bottle before the gravity is stable. Bottling early can lead to over-carbonation and exploding bottles. If your FG is higher than expected, the yeast may have stalled, or your recipe simply contains unfermentable sugars from crystal malt or lactose.

Temperature Matters More Than You Think

Hydrometers are calibrated at a specific temperature, usually 20°C, though some are set to 15°C. If your sample is warmer or cooler, the reading will be slightly wrong. Warm liquid is less dense, so the hydrometer sinks lower and reads lower than the true gravity. Cold liquid is denser, so it reads higher. For a rough correction, add about 0.001 for every 5°C above 20°C, and subtract about 0.001 for every 5°C below. So a reading of 1.040 taken at 30°C is closer to 1.042. For accuracy, use the correction chart that came with your hydrometer, or cool your sample to 20°C before reading. Never take a reading from hot wort straight from the boiler; it can warp the hydrometer and give you a false number.

Calculating Alcohol by Volume from Your Readings

Once you have your OG and FG, the maths is straightforward. Use this formula: ABV = (OG - FG) × 131. For example, if your OG is 1.050 and your FG is 1.010, subtract to get 0.040, then multiply by 131 to get 5.24% ABV. That is a perfectly respectable bitter or pale ale. For a stronger brew, say OG 1.070 and FG 1.015, the difference is 0.055, giving about 7.2% ABV. Remember that this is an estimate. It assumes a typical fermentation and does not account for small errors in your readings. Still, it is far better than guessing. Write your OG and FG on the brew sheet so you can compare batches.

Common Hydrometer Pitfalls and How to Avoid Them

Even experienced brewers make mistakes with hydrometers. Watch out for these:

• Bubbles on the hydrometer. They lift the float and give a false high reading. Spin the hydrometer or dip it in and out to knock bubbles off.
• Reading at an angle. Always read at eye level. Looking down or up shifts the apparent number.
• Ignoring temperature. As above, correct your reading or cool the sample.
• Sampling from the top. If you take a sample from the top of the fermenter, you may get weaker liquid. Gently stir or take from the middle with a sanitised turkey baster or wine thief.
• Not sanitising. A dirty hydrometer can introduce wild yeast or bacteria. Sanitise everything that touches your beer or wine.
• Dropping it. Hydrometers are fragile. Keep a spare, or treat it like a precious heirloom.

With a little practice, reading a hydrometer becomes second nature. You will soon know exactly when to dry hop, when to bottle, and how strong your brew will be. That knowledge is one of the quiet joys of home brewing.