You pitch yeast into sweet wort, seal the fermenter, and then you are left with a number. Or rather, two numbers, taken a week or two apart, and a formula that turns those numbers into the alcohol percentage you will eventually brag about (or apologise for). Most beginners get the first reading wrong, take the second one too early, and then wonder why their IPA tastes like sweet tea with a hangover attached.
Understanding beer gravity and ABV is not complicated, but it is unforgiving. Get the reading right and you know exactly what you brewed. Get it wrong and you are guessing in the dark, hoping the yeast did its job. I have done both. The guessing version produced a 3.1% "IPA" that I served to friends with a straight face.
Here is the full picture: what the numbers mean, how to read them, how to do the maths, and which beginner mistakes will cost you a batch.
Key Takeaways
- Gravity measures density. Water is 1.000. Sugar pushes it up, alcohol pushes it down.
- Two readings are needed: OG (before fermentation) and FG (after).
- The standard ABV formula is (OG − FG) × 131.25. Simple, but only as good as your readings.
- Temperature skews hydrometer readings. Adjust, or measure at the calibrated temperature.
- A stable FG over two or three days means fermentation is done. One reading proves nothing.
- Most beginner errors come from reading technique and timing, not from bad yeast.
What beer gravity actually measures (and why it matters)
Gravity is the density of your liquid compared with water. Pure water at the reference temperature sits at 1.000. Dissolve sugar in it and the number climbs. Dissolve alcohol in it and the number drops, because ethanol is lighter than water.
That single fact explains the whole process. Your wort starts dense because it is full of malt sugars. As yeast converts those sugars into alcohol and carbon dioxide, the density falls. The distance between the starting point and the ending point is, roughly speaking, the amount of alcohol produced.
Original gravity meaning
Original gravity (OG) is the density of the wort before fermentation begins. It tells you how much fermentable sugar is available and therefore the ceiling on your alcohol content. An OG of 1.060 has more sugar than an OG of 1.040 and will produce a stronger beer if the yeast ferments it out fully.
You will also hear this called starting gravity. Same thing. If a recipe says "starting gravity for beer: 1.052", it means the wort going into the fermenter should read 1.052.
Final gravity and attenuation
Final gravity (FG) is the density once fermentation has finished. A typical ale lands somewhere between 1.008 and 1.014. The gap between OG and FG is what brewers call attenuation, and it depends on the yeast strain, the mash temperature, and how much unfermentable sugar is left in the wort.
Which brings up an obvious problem with the word "final". FG is only final when the yeast has stopped. More on that below.
Reading a hydrometer without fooling yourself
A hydrometer is a weighted glass tube that floats in your sample. The deeper it sinks, the lower the density. You read the scale at the point where the liquid surface meets the glass, and you read it at eye level, not from above.
Beginners lose more accuracy here than anywhere else. Three things matter:
- Temperature. Hydrometers are calibrated for a specific temperature, usually 20 °C (68 °F). A sample at 30 °C reads low. A sample at 10 °C reads high. The error is small but real, and it compounds when you calculate ABV from two readings that were both skewed the same way.
- The meniscus. Liquid climbs the glass slightly. Read the flat surface, not the ridge climbing the wall.
- Dissolved gas. Freshly fermented beer holds CO₂. Bubbles clinging to the hydrometer lift it and drag your reading down. Let the sample sit, or spin the hydrometer gently to knock bubbles off.
Use a tall sample jar, fill it to within a couple of centimetres of the top, and lower the hydrometer in slowly. If it touches the sides, your reading is worthless.
Hydrometer or refractometer?
A refractometer needs only a few drops and reads instantly, which is why a lot of brewers buy one. But it measures refraction, not density, and alcohol bends light differently than sugar does. Once fermentation has started, raw refractometer readings are wrong, and you need a correction calculation to make sense of them.
My advice: learn on a hydrometer. It is cheap, it is direct, and it never needs correcting for alcohol. Add a refractometer later for quick checks during the boil, where its speed genuinely helps.
ABV calculator versus doing the maths yourself
Online ABV calculators are fine. But you should know what they are doing, because the formula is one line:
ABV = (OG − FG) × 131.25
Worked example. OG 1.050, FG 1.010. The difference is 0.040. Multiply by 131.25 and you get 5.25% ABV. That is it.
For stronger beers the linear formula drifts slightly, and a more accurate version exists that most calculators now use. For anything under about 7% the difference is small enough to ignore in practice.
| Beer style | Typical OG | Typical FG | Approx. ABV |
|---|---|---|---|
| Standard lager | 1.045 | 1.008 | 4.8% |
| English bitter | 1.040 | 1.010 | 3.9% |
| American IPA | 1.062 | 1.012 | 6.5% |
| Belgian tripel | 1.080 | 1.010 | 9.2% |
| Imperial stout | 1.095 | 1.022 | 9.6% |
These are working ranges, not rules. A session IPA at 1.045 is a perfectly good beer. The table exists so you can sanity-check your own numbers: if your "lager" reads 1.075, something went wrong with the measurement or the grain bill.
The mistakes beginners make with gravity readings
I destroyed my first three batches' worth of data by taking the FG reading on day four because I was impatient. The number looked plausible. It was not final. The beer dropped another six points over the following week, and my recorded ABV was off by nearly a full percent.
Here is what actually goes wrong:
- Taking FG too early. Fermentation can slow down without stopping. Two identical readings 48 hours apart is the minimum proof.
- Forgetting to adjust for temperature. Most hydrometers state their calibration point on the scale. Ignore it and your OG is off before you have even pitched yeast.
- Measuring a sample with trub in it. Yeast and hop particles change the density reading. Let the sample settle.
- Confusing gravity with extract. They are related, but gravity is a density ratio and extract is a concentration of dissolved solids. Different units, different numbers.
- Not writing anything down. If you cannot remember your OG, you cannot calculate anything.
That last point sounds trivial. It is the one that cost me the most. A brew notebook, paper or digital, is the difference between brewing and hoping.
What if my final gravity is too high?
A high FG can mean a stalled fermentation, a yeast strain with low attenuation, or simply a recipe built with lots of unfermentable sugar. Check whether the reading is stable first. If it is stable and the beer tastes fine, it is probably the recipe. If it is stable and the beer tastes cloyingly sweet, you may have pitched too little yeast or let the fermenter get too cold.
Does gravity affect flavour?
Indirectly, yes. Higher-gravity worts put more stress on yeast, and stressed yeast throws off flavours you may not want. High-ABV beers also need more carbonation and often more conditioning time. But gravity itself is a measure of density, not flavour. Two worts with the same OG can taste entirely different depending on the grain bill.
One more thing worth mentioning: if you have ever searched for gravity and stumbled onto clothing brands, you are not alone. There is an apparel label called Original Gravity, and it has nothing to do with brewing. Ignore it.
A practical workflow you can trust
Here is the sequence that finally gave me consistent, repeatable numbers.
Before pitching the yeast, cool the wort to your hydrometer's calibration temperature, take a sample, and record the OG. Do it once, carefully, and write it down with the date and the temperature.
During fermentation, leave it alone. Every time you open the fermenter you risk infection, and you gain nothing.
When the airlock slows to a crawl, take a sample and note the reading. Wait two full days. Take another. If the two match, fermentation is done and that is your FG. If they differ, wait again.
Calculate ABV once, from the recorded OG and the confirmed FG, and keep both numbers in your notes alongside the recipe. Next time you brew the same beer, you will know within a point or two what to expect before you even start.
The number on the hydrometer is not the point. The point is that it is the only honest feedback your beer gives you before you drink it. Learn to read it properly and everything else, from recipe design to troubleshooting, gets easier.
