Gravity’s a measure of how much sugar is in your beer. It has a big impact on the character of your brews so it’s important to know how to measure it.
Here’s an illustrated guide explaining how it all works.
I planned to show you how to use a floating hydrometer and nothing more, but as I looked into it I found out it’s not as simple as it seems.
Calibration of the instrument is as important as taking measurements.
But before that, why bother measuring gravity in the first place?
Why’s It Worth Measuring The Gravity Of Beer
When you make beer, you prepare a sweet liquid called wort. This is usually done by mashing malted barley to extract sugars.
During fermentation, yeast convert the sugar to alcohol and gas, but they can’t usually convert all of it. There is often residual sugar in the beer that gives a sense of sweetness.
The level of sweetness is again dependent on a number of things including type of grain used, mash temperature and choice of yeast.
All of this means that small changes in ingredients or process have noticeable effects on your gravity and beer.
It’s therefore important to keep an eye on gravity if you hope to develop your brewing skills.
How To Measure Beer Gravity
Gravity is another way of saying the amount of sugar dissolved in brewing water. The more sugar, the higher the gravity.
The easiest way for a home brewer to measure gravity is with a floating glass hydrometer.
This is a long rod with a scale running vertically (see the photo at the top of the page). The hydrometer floats higher in water with dissolved sugar, because of the increased density.
Most hydrometers have three scales:
- Potential alcohol
- Specific Gravity
I’m going to focus on specific gravity.
If you float the hydrometer in water it gives a gravity reading of 1.000.
But if you float it in wort, which has sugar in it, the gravity reading is much higher.
Before taking the reading be sure to spin the hydrometer to dislodge any bubbles.
Also, check whether it’s designed to read the surface of the water, or the point where the water intersects the hydrometer. You can see in the photo of the water than the liquid rises significantly around the rod.
The eagle-eyed among will have spotted that in my photo the gravity reading of the water is 1.002, not 1.000.
There are two explanations for this:
- Gravity changes with temperature
- Hydrometers aren’t always calibrated accurately
The density of a liquid decreases with temperature, and so the gravity reading also changes. Hydrometers are calibrated for a specific temperature (mine is for 15.5°C/60°F).
You need to adjust the reading to take into account the temperature.
In this example the water is at 24°C/75°F. Using this calculator you can correct the reading:
That means the gravity is actually 0.002 higher than the measurement.
At these temperatures the difference is minimal, but measuring hot wort it has more impact:
It’s worth getting a test tube for taking wort samples. Without one it’s hard to see the hydrometer because wort is often foamy.
It’s also difficult to get a horizontal view of the scale with the hydrometer floating in the fermenter or brew pot. This can distort the reading.
Of course, don’t forget to sanitise the tube and the hydrometer before letting them touch the wort.
Calibrating A Hydrometer
After discovering that my hydrometer is giving suspect measurements, I decided to calibrate it.
Using the calculator I worked backwards from 1.000 to see that in water I should have a reading of 0.998 at 24°C.
As I’m getting 1.002, that means I’m 0.004 over and need to subtract that from my gravity readings (before running them through the temperature converter).
It’s very confusing but I think that’s the case!
This seems a large discrepancy to me, so I also wanted to check the accuracy of the rest of the scale.
Using a calibration method borrowed from BYO I took a control measurement with a higher gravity liquid.
For this to work you need to measure gravity in °Plato, which is a measure of the percentage of sucrose in a liquid.
Make a sugar solution with a gravity similar to a typical wort. I used 1.048, or 12 °Plato as many of my beers are around that mark.
I dissolved 12% (24g) sugar into 88% (176g) water.
You put the mixture in the test tube and float the hydrometer.
Remember to take the temperature and apply the correction factor.
The hydrometer here reads 1.044 at 24°C. I already know to subtract 0.004 so the reading is really 1.040.
Entering that into the calculator to adjust for temperature gives me:
So according to my test, in a solution of 1.048 my hydrometer reads 1.042. Yet more inconsistencies.
If your measurements are really far off you can make a correction graph. Here’s mine.
I’ve plotted the actual and measured gravities of the two tests and drawn a line between the two.
This lets me adjust readings from my hydrometer to the actual gravity, assuming that the scale is consistently inconsistent.
But to be frank, I don’t think I’ll bother.
In fact, I’m taking these findings with a pinch of salt.
If the hydrometer’s out, there’s a good chance my scales and thermometer are too. Perhaps if I calibrated them I’d get back on track with the gravity.
The point of all of this, is to point out that it’s pointless obsessing over very precise gravity readings.
I’m working with rough approximations of the numbers at best, and it’s a waste of time attempting to monitor any of them in much detail.
Within my system the readings are useful as relative comparisons from one batch to the next. I expect the same is true of your kit.
If you’re comparing beer with other brews, or commercial beers you’re trying to clone, it seems best to treat the numerical details as secondary to taste and other factors.
When To Take Measurements
Even though the actual numbers may or not be accurate, it’s still worth taking readings.
There are two main uses for gravity readings:
- to check efficiency
- to check fermentation activity
Before pitching the yeast, take a gravity reading to see how much sugar you’ve extracted from the grains. Comparing this with your predicted values tells you your efficiency.
If you want to control volumes, take a reading before starting the boil. Then you can adjust the boil length to make sure you end up with a beer of suitable size and gravity.
Checking Fermentation Activity
After fermentation take another reading. You’re checking that the gravity has dropped sufficiently for you to bottle your beer.
John Palmer suggests that when the final gravity is approximately a quarter of the starting gravity you’re done.
If the fermentation (after about two weeks) seems to have stopped short, you may need to pitch more yeast.
How To Calculate Percent Alcohol From Specific Gravity
Another popular reason for measuring the original and final gravities is to calculate the alcoholic strength of the beer.
There are many methods for calculating the alcohol content. A commonly used one is:
For example, if the 1.048 beer above fermented down to 1.010, I’d get this:
I don’t claim to know enough about what’s going on to offer a definitive method, but given the “accuracy” of my gravity readings I’m happy to use this rough estimate.
I’m beginning to realise that what’s most important is the consistency of my own measurements. I need to think in terms of “too strong” or “not yet fully fermented” rather than “8.3% abv” and “final gravity of 1.007”.
This has ended up a bit of long post, but I hope it’s been a useful look at the benefits and pitfalls of using hydrometers to measure beer gravity.