Why the hydrometer beats guesswork

Every brewer has done it: stared at a bubbling airlock for reassurance and assumed fermentation must be nearly finished. Airlocks lie. A leaky lid, a drop in temperature, or a bucket that simply doesn't seal properly will all change the rhythm of those bubbles without telling you anything about the sugar still sitting in your beer. A hydrometer measures density, and density is a direct window into how much fermentable sugar remains. It is the difference between hoping your beer is ready and knowing it.

For UK brewers working with cask ales, pale ales, stouts and lagers in a garage or spare room, a hydrometer is one of the cheapest pieces of kit you can own and one of the most useful. Learn to read it accurately and you can calculate alcohol strength, catch a stuck fermentation early, and bottle with confidence rather than crossed fingers.

What original and final gravity actually mean

Original gravity (OG) is the density of your wort before fermentation begins, measured against water. Pure water sits at 1.000. A typical British bitter might start at 1.042, a stout at 1.055, a strong ale at 1.070. Those digits after the decimal point are called gravity points — 1.042 means 42 points — and they represent dissolved sugars that yeast will convert into alcohol and carbon dioxide.

Final gravity (FG) is the density once fermentation has finished. Because alcohol is less dense than water, the reading drops, often to somewhere between 1.008 and 1.015 for ales. Subtract FG from OG and you get the total points fermented. Multiply that figure by 131 and you have your approximate alcohol by volume:

• OG 1.048, FG 1.010 — that's 38 points fermented
• 38 × 131 = 4.98% ABV

If you're kegging or cask-conditioning, add roughly 0.003 to 0.005 to your FG to account for the gravity drop from secondary fermentation in the package.

How to take a reading you can trust

Accuracy starts before the hydrometer ever touches the beer. Sanitise everything — trial jar, siphon or wine thief, and the hydrometer itself — because a sample you plan to taste afterwards is a sample you don't want infected. Draw enough beer to fill the trial jar to within a couple of centimetres of the top; a hydrometer needs room to float freely and not touch the sides.

• Drop the hydrometer in gently and give it a spin to dislodge bubbles clinging to the stem
• Wait until it stops bobbing, then read at eye level, not looking down from above
• Take the reading at the bottom of the meniscus, the curve where the liquid meets the glass
• If foam or hop debris is in the way, let the sample stand for a few minutes

The difference between reading the top and the bottom of the meniscus is easily 0.002, which is enough to throw your ABV out by half a percent. It matters.

Temperature correction: the step most brewers skip

Hydrometers are calibrated at a specific temperature, usually 20°C. If your sample is warmer or cooler, the reading drifts. Warm liquid is less dense, so a warm sample reads lower than it truly is; a cold sample reads higher.

As a practical rule of thumb, for every degree Celsius above 20°C, add about 0.00025 to your reading. So a sample at 25°C that reads 1.010 is really closer to 1.011. At 30°C, add roughly 0.0025. Below 20°C, subtract in the same proportion — a 15°C sample reading 1.012 is nearer 1.011.

The correction is small but it accumulates. If you're comparing an uncorrected OG taken from warm wort against a corrected FG from a cool sample, you'll misjudge your beer's strength and its finishing point. Let the sample cool to room temperature before reading where you can, and correct when you can't.

Following fermentation from start to finish

Take your OG once the wort is thoroughly mixed and cooled, before you pitch the yeast. Then leave the beer alone for the first few days. A vigorous fermentation doesn't need monitoring, and every time you open the lid you invite oxygen and unwanted microbes in.

Once activity slows, start sampling every two or three days. If your fermenter has a tap, draw from that rather than siphoning through the top. Record each reading so you can see the trend — a steady decline tells you far more than a single number.

Fermentation is finished when you get the same reading on two or three consecutive days. At that point, many brewers give the beer another two or three days at fermentation temperature to let the yeast clean up diacetyl and other by-products before crashing or packaging. Never bottle a beer that is still dropping; you risk over-carbonated, gushing bottles.

When your readings look wrong

An unexpectedly high FG usually means the beer is still going, the temperature dropped and stalled the yeast, or it simply hasn't had long enough. Give it a gentle rouse and a few more days before reaching for anything drastic. An FG far lower than expected often points to an infection or an over-pitched, very attenuative yeast.

If your OG is wildly out, check whether your wort was properly mixed — top-up water in extract brews can stratify and give a misleading sample. And if the hydrometer reads 1.000 in plain water at 20°C, it's fine; if it doesn't, note the offset and apply it to every reading. Once you trust your numbers, you can stop guessing and start brewing with real confidence.