You’ve just finished your first ever brew, the airlock has stopped bubbling, and now you’re staring at a glass tube floating in a jar of beer wondering what on earth you’re supposed to be reading. The numbers on the side don’t match anything in the recipe, and you’re not entirely sure which end is up.
A hydrometer is the single most useful tool in your brewing kit after the fermenter itself. It tells you when fermentation is done, what your alcohol content is, and whether something’s gone wrong — all for about £5. But nobody explains how to actually use one properly, so you end up guessing. Let’s fix that.
In This Article
- What a Hydrometer Actually Measures
- What You’ll Need
- How to Take a Gravity Reading Step-by-Step
- Reading the Meniscus Correctly
- Temperature Correction: Why It Matters
- Calculating ABV from Your Readings
- When to Take Readings During the Brew
- Common Mistakes and How to Avoid Them
- Hydrometer vs Refractometer: Which Should You Use?
- Looking After Your Hydrometer
- Frequently Asked Questions
What a Hydrometer Actually Measures
A hydrometer measures the density of a liquid compared to pure water. Pure water reads 1.000 on the specific gravity (SG) scale. When you dissolve sugar in water — which is basically what wort or must is — it gets denser, so the hydrometer floats higher and reads something like 1.045 or 1.060.
Specific Gravity and What the Numbers Mean
The numbers on your hydrometer scale typically run from about 0.990 to 1.170. A typical beer wort might start at 1.040–1.060, while a strong barley wine could be up to 1.100. As yeast eats the sugar during fermentation, the liquid gets less dense and the reading drops.
Original Gravity vs Final Gravity
Your original gravity (OG) is the reading before you pitch yeast. Your final gravity (FG) is the reading once fermentation is completely finished. The difference between these two numbers tells you how much sugar the yeast consumed, which is how you calculate your alcohol content. Simple as that.
Most beer recipes target a final gravity between 1.008 and 1.016. Wine typically finishes lower — around 0.996 to 1.000 — because wine yeast is more aggressive at eating sugar. If your FG is much higher than expected, fermentation may have stalled, and that’s worth investigating before you bottle.
What You’ll Need
Before you start taking readings, gather these items:
- A hydrometer — the triple-scale type is most common in the UK (shows SG, potential alcohol, and Brix). You can pick one up from The Malt Miller or Amazon UK for about £4–7
- A trial jar — the tall, narrow plastic or glass cylinder that the hydrometer floats in. About £3 from any homebrew shop. Don’t try to float it in your fermenter — you’ll lose beer and risk contamination
- A turkey baster or wine thief — for drawing samples from the fermenter without disturbing the brew
- A thermometer — your readings need temperature correction if the sample isn’t at the hydrometer’s calibration temperature (usually 20°C)
- Sanitiser — StarSan or sodium metabisulphite. Everything that touches your brew must be clean
I’ve been using the same basic Stevenson Reeve hydrometer for three years now. It cost me £5.50 and it’s never let me down — no need to spend more unless you want a precision lab instrument.
How to Take a Gravity Reading Step-by-Step
This is the process I follow every time, and it takes about two minutes once you’ve done it a few times.
- Sanitise the trial jar, hydrometer, turkey baster, and thermometer with StarSan or your preferred sanitiser
- Draw a sample from your fermenter using the turkey baster — you need enough liquid to fill the trial jar about three-quarters full
- Pour the sample into the trial jar gently to avoid creating too many bubbles
- Lower the hydrometer into the trial jar slowly, then give it a gentle spin between your fingers to dislodge any bubbles clinging to the glass
- Wait for the hydrometer to stop bobbing and settle in place — this usually takes 10–15 seconds
- Read the scale at the bottom of the meniscus (more on this in the next section)
- Note the temperature of the sample and apply correction if needed
- Record the reading in your brew log
Tips for Consistent Readings
Take the reading at eye level. If you’re looking down at the trial jar on a worktop, you’ll get a parallax error that throws the reading off. I keep mine on the edge of the counter and crouch down so my eyes are level with the liquid surface.
Give the hydrometer a spin after you drop it in. Air bubbles cling to the glass and lift it slightly, giving you a falsely low reading. A quick twist between your fingertips sorts this out.
Reading the Meniscus Correctly
This trips up more beginners than anything else. When liquid meets glass, it curves upward at the edges — that curve is the meniscus. You need to read the scale at the bottom of the meniscus, where the surface of the liquid is flat, not where it climbs up the sides of the hydrometer.
What If Bubbles Are Still Clinging?
If you’ve just transferred the sample and it’s full of CO2 bubbles, those tiny bubbles will stick to the hydrometer and push it up. Wait a minute for them to dissipate, or give it another gentle spin. If the reading seems unexpectedly low after active fermentation, bubbles are the most likely culprit.
Using a Torch for Clarity
If your sample is dark — a stout or a porter — it can be hard to see where the liquid meets the scale. Hold a small torch or your phone light behind the trial jar and you’ll see the meniscus line much more clearly. Sounds daft, but it makes a real difference on dark brews.
Temperature Correction: Why It Matters
Your hydrometer is calibrated to read accurately at a specific temperature, usually 20°C (it’ll say on the paper scale inside or on the packaging). If your sample is warmer or cooler than that, the density changes and your reading will be off.
How Much Difference Does Temperature Make?
Quite a bit, actually. A sample at 30°C will read about 0.002 lower than the true gravity. That doesn’t sound like much, but over the gap between OG and FG it can throw your ABV calculation out by half a percent. If you’re entering competitions or just want to know what you’ve actually brewed, correction matters.
The Correction Formula
The easiest approach is an online calculator, but if you want to do it by hand:
- Above 20°C: add approximately 0.0002 for every 1°C above calibration temperature
- Below 20°C: subtract approximately 0.0002 for every 1°C below
So a reading of 1.048 at 25°C becomes approximately 1.049 after correction. Most brewing apps — Brewfather, BeerSmith — do this automatically if you enter the temperature alongside the reading.
The HMRC notice on alcohol duty references specific gravity as the primary measure for determining alcohol content for duty purposes, so accuracy genuinely matters if you ever scale up to commercial brewing.
Calculating ABV from Your Readings
This is the bit everyone’s actually here for. The formula is simple:
ABV = (OG − FG) × 131.25
A Worked Example
Say you brewed a pale ale with:
- OG: 1.052
- FG: 1.012
ABV = (1.052 − 1.012) × 131.25 = 0.040 × 131.25 = 5.25%
That’s it. No complicated maths, no special tables. Just two readings and a bit of multiplication.
What If Your ABV Seems Wrong?
If your calculated ABV is much lower than expected, your fermentation probably stalled — the yeast didn’t finish eating the sugar. Common causes include temperature drops, under-pitching yeast, or not aerating the wort enough before pitching. Our guide to choosing the right brewing yeast covers strain selection that can help prevent stuck fermentations.
If it’s higher than expected, you probably had more fermentable sugar than the recipe anticipated. This happens with liquid malt extract that’s been stored warm — the sugars become more fermentable over time.
When to Take Readings During the Brew
You don’t need to check gravity every day — in fact, opening the fermenter too often increases the risk of contamination and oxygen exposure.
Pre-Fermentation (Brew Day)
Take your OG reading after you’ve cooled the wort and before pitching yeast. This is your baseline. Write it down immediately — trust me, you will forget it otherwise. If you’re using good homebrew record keeping practices, log it in your brew journal or app straight away.
Mid-Fermentation (Optional)
Some brewers like to check halfway through. If your recipe says fermentation should take 7 days, you might check on day 4. This tells you if things are progressing normally. But I skip this for most standard brews. It’s an unnecessary infection risk.
Pre-Bottling or Kegging
This is the critical one. Take a reading, then take another one 48 hours later. If both readings are the same, fermentation is done. If the second is lower, the yeast is still working — wait another couple of days and test again.
Never bottle based on a single reading. Bottling before fermentation is truly finished creates excess CO2 inside the bottle. In the best case, you get gushing, over-carbonated beer. In the worst case, you get bottle bombs — glass shattering under pressure. The Health and Safety Executive treats pressurised vessels seriously for a reason, and a glass bottle at 3+ volumes of CO2 qualifies.
Post-Conditioning (Optional)
If you’re bottle conditioning, you can open a bottle after a couple of weeks and pour the beer into a trial jar to check carbonation levels. FG should be the same as pre-bottling — the priming sugar is fully consumed and converted to CO2 rather than dropping the gravity further.
Common Mistakes and How to Avoid Them
After three years of brewing and helping mates with their first batches, these are the mistakes I see over and over again.
Not Sanitising the Equipment
Every time you draw a sample, you’re potentially introducing bacteria. Your trial jar, hydrometer, turkey baster, and anything else that touches the brew need sanitising. StarSan is the standard — two minutes of contact time and you’re safe. See our cleaning and sanitising guide for the full process.
Reading at the Wrong Temperature
I covered this above, but it bears repeating. Taking a reading at 28°C and writing it down as-is will give you a meaningfully wrong ABV calculation. Either cool your sample to 20°C first (ice bath works) or apply the correction.
Insufficient Sample Volume
If the trial jar isn’t full enough, the hydrometer touches the bottom and you can’t read it. Fill the jar until the hydrometer floats freely with at least a centimetre of clearance below.
Relying on One Pre-Bottling Reading
Two identical readings 48 hours apart is the gold standard. One reading tells you where gravity is right now — it doesn’t tell you fermentation has stopped. The difference between 1.014 and 1.012 is the difference between safe bottles and a mess on the ceiling.
Forgetting to Record Readings
You think you’ll remember your OG of 1.054. You won’t. By bottling day two weeks later, you’ll be guessing. Write it down — on your fermenter, in a notebook, in Brewfather, anywhere that isn’t your memory.
Hydrometer vs Refractometer: Which Should You Use?
Both measure density, but they work differently. If you’re trying to decide which to buy, we’ve got a full comparison in our best hydrometers and refractometers guide.
When a Hydrometer Wins
- Post-fermentation readings — refractometers need alcohol correction after fermentation, which adds complexity
- Price — a good hydrometer costs £5, a decent refractometer starts at about £15
- Simplicity — drop it in, read the number, done
When a Refractometer Wins
- Sample size — you only need 2-3 drops versus 100ml+
- Brew day readings — quick OG checks without wasting wort
- Durability — no glass to break
The Practical Answer
Get both. Use the refractometer on brew day when you’re taking quick OG readings and don’t want to waste wort. Use the hydrometer for FG readings where accuracy matters most and there’s no alcohol correction to worry about.
Looking After Your Hydrometer
Hydrometers are fragile glass instruments. One knock against the side of the fermenter and you’re fishing broken glass out of your beer — ask me how I know.
Storage
Keep it in the plastic tube it came in, upright in a cupboard. Don’t leave it rattling around in a drawer with your bottle capper and crown caps.
Cleaning
Rinse with warm water after every use. If sugar residue builds up on the scale, soak in warm water with a drop of washing-up liquid for ten minutes. Don’t use abrasive cleaners — they’ll scratch the glass and make the scale harder to read.
Calibration Check
Every few months, float your hydrometer in distilled water at 20°C. It should read 1.000. If it’s off by more than 0.001, it might be time for a new one. At £5 a pop, there’s no point wrestling with an inaccurate instrument. The CAMRA homebrew community recommends regular calibration checks as part of good brewing practice.
What If It Reads 0.998 in Plain Water?
A tiny air bubble trapped in the weighted end can cause this. Try tapping the hydrometer gently against your palm. If the reading doesn’t change, the instrument is off — apply a consistent offset to all your readings (e.g., if it reads 0.998, add 0.002 to every reading) or just replace it.
Frequently Asked Questions
Can I use a hydrometer for wine as well as beer? Yes — the same instrument works for both. Wine musts tend to have higher starting gravities (1.080–1.110) so make sure your hydrometer’s scale goes high enough. Most triple-scale hydrometers cover 0.990 to 1.170, which handles everything from session bitter to dessert wine.
How accurate is the ABV calculation from hydrometer readings? The standard formula (OG − FG) × 131.25 is accurate to within about 0.2% for most homebrew. For very high gravity brews (above 1.080 OG), more complex formulas exist, but the simple one is fine for 95% of what you’ll brew at home.
My hydrometer reads differently every time — what’s wrong? The three most common causes are temperature variation, air bubbles clinging to the glass, and not waiting long enough for it to settle. Standardise your process: same temperature (or use correction), spin to dislodge bubbles, wait 15 seconds, read at eye level.
Do I need to throw away the sample after testing? Technically you shouldn’t pour it back into the fermenter because of contamination risk. In practice, many homebrewers do pour back small samples — it’s your call on risk tolerance. If you’re paranoid (and there’s nothing wrong with that), drink the sample or tip it down the sink.
What’s the difference between SG, Brix, and Plato? They’re all measuring the same thing — sugar density — just on different scales. SG is the ratio to water (1.048). Brix and Plato measure the percentage of sugar by weight (about 12° for 1.048 SG). UK homebrewers mostly use SG. Brix is more common in winemaking and Plato in commercial brewing.