Gravity readings are a simple yet powerful tool for every homebrewer whether they are brewing beer, mead, ciders, wine or you name it.
Although it’s possible to brew without having a clue what gravity readings are, eventually every homebrewer who wants to improve and not depend on luck should start taking the readings.
This information is super important in homebrewing as otherwise, you won’t be able to compare your brewing results to the recipe you based it on, nor will you be able to track the progress.
In this guide, I’ll help you figure out everything you should know about gravity readings such as how to take gravity readings and read them and when and why to take them.
How do gravity readings work?
Gravity readings show us how much sugars are in the wort/brew and by comparing multiple readings we can see the differences. If the gravity drops it means sugars got eaten by the yeast and turned into alcohol, so the lower the gravity the drier the brew.
So, simply put everything we observe is the start amount of sugars, the end amount of sugars in a sample, and the process between while the yeast works.
This allows us to calculate the alcohol and how many sugars are left (residual, non-fermented sugars).
The terms used:
- OG (Original Gravity) – This is the starting gravity we want to achieve before fermentation. The higher it is, the more fermentable or unfermentable sugars are there which means a higher potential for alcohol and a possibly denser body.
- FG (Final Gravity) – This is the final gravity achieved after fermentation.
- SG (Specific Gravity) – This can mean any gravity whether it’s OG, FG, or something in between. I won’t use this term in this article, but people often use this term.
Why should you take gravity readings?
As you’ll see there are plenty of good reasons for taking gravity readings.
Monitoring the fermentation
Fermentation can be tricky and it depends on many factors such as yeast strain used, temperature, yeast nutrients, amount of sugars, and alcohol percentage. It’s hard to guess whether the fermentation is complete or is it even active, so if you are guessing from an eye you can have quite inaccurate results.
Fermentation must complete before you bottle because otherwise, the homebrew might be unfinished which can create off-flavors or lack of flavors and aromas and in some cases even bottle bombs.
Gravity readings help us figure out what’s going on and it’s always accurate.
By comparing the Original gravity (OG) taken prior to fermentation with Final Gravity (FG) taken during or after fermentation a lot can be said because you’ll know what FG you should hit and if you are not there yet, the fermentation is not complete.
Checking the ABV
There are no easy ways of knowing how much alcohol is in homebrew unless there are two readings taken, the OG and FG.
I mean, there are devices that can analyze the ABV but they are quite expensive and not normally a part of a standard homebrewer’s kit.
Thus, taking these two simple readings can tell us exactly how much alcohol is in the brew. Simply, use one of the calculators available on the internet and after inserting the readings, the calculator will tell you how much ABV is there.
The calculator simply calculates how much sugars were present at the beginning prior to fermentation and how much sugar is left after fermentation as all eaten sugars must result in alcohol which is lighter.
Understanding the mash efficiency
During the mash, we extract as many sugars from the grains as we can and once this process is done we have available sugars we can ferment later.
The goal is to do this as efficiently as possible, so some standard and successful practices are to have around an 80% efficiency rate.
It’s unrealistic to ever extract 100% sugars from the grains and transfer them to the wort as some sugars will always be lost in the process.
The efficiency depends on the method used (traditional brewing method, BIAB, extract, partial mash), the quality of equipment, how well-milled are the grains, and so on.
To be able to judge the efficiency we must take the OG reading and compare it with the standards known for the number of grains we used which are either written in the recipe or can be calculated.
If you are not getting the desired efficiency you’ll immediately spot it after taking OG and be able to adjust to it.
Perhaps, the grains weren’t milled enough when they arrived from the manufacturer so you can do it on your own or contact them about it, or it was something else.
Comparing the results with the recipe and other brewers
If you are participating in social brewing challenges, sharing recipes with other brewers, or using a recipe book, gravity readings will be the best and most accurate way to tell what’s going on!
How to take gravity readings?
Taking gravity readings is easy, you’ll need a hydrometer and a testing jar (such as those you can find on Amazon).
- Fill the testing jar with water and place the hydrometer in to test the calibration. If the hydrometer works well it should be at 1.000.
- Take a sample by filling the jar approximately 2 inches below the top. It’s important that the hydrometer can float in the sample by not touching the bottom but it’s also important that the jar isn’t full to the top so the sample overflows as it could affect the readings.
- Swirl the hydrometer a couple of times in a circle to get rid of potential bubbles which could affect the readings and let it balance itself in a place. When it’s still, take the reading and write it down so you don’t forget it.
- (SITUATIONAL) Adjust the temperature – Modern hydrometers are calibrated at 68°F (20°C), so unless the sample you took is colder or warmer than that you should adjust the temperature using temperature adjusting calculators. These differences can be really meaningful when the sample is really cold or really hot as temperature greatly increases the viscosity. When sugars are heated they become more viscous, but when they are chilled they lose viscosity. This greatly impacts the gravity readings, so, for example, gravity readings taken from the 85°C wort may equal 1.020 but when the same wort cools down to room temperature (20°C ) the readings will show 1.050.
When to take gravity readings?
Gravity readings can be taken at any stage of brewing depending on what you need to calculate. Don’t get confused by all these pro-boil, post-boil, prior, and during fermentation titles, I use them for an easier explanation just to explain the possibilities, so as long as you get the purpose of what’s going on and why we need gravity readings that’s all that matters.
Pre-boil readings
Pre-boil gravity readings are useful for determining the efficiency of the mash as you’ll take such readings after mashing and sparging or after mashing and squeezing/sparging a brewing bag in case you brew in a bag. BIAB (brewing in a bag) is the easiest brewing method because it makes the process so convenient, and it offers more room for fixing mistakes because there are fewer steps involved.
In any way, the main point is to extract as many sugars from the grains as possible, and before putting a nail into the coffin by raising the temperature for a mash-out, by taking gravity readings you have the access to the efficiency information you got.
This reading is not necessary by any meaning but can be useful if you suspect the efficiency may not be where it’s supposed to be. This allows you to give your grains a second try by continuing to extract sugars until you hit the desired gravity either by extracting more sugars from existing grains or by adding fresh grains or dried malt extract as an easy solution.
Bear in mind that you can also take these readings during boiling which means you already went through the mash out. In this case, you won’t be able to return to the mashing process without starting it all over again in the other kettle and then mixing the worts.
However, if the OG is too low during boiling or anywhere near that point, you can hold on from adding any water to top it off and let the boil evaporate water. The longer the boil, the higher the OG becomes.
While prolonged boiling won’t fix drastic OG differences it can help you hit the desired OG when small losses in efficiency are present.
After all, for any greater OG differences from the desired number you can simply put dried malt extract (DME) or other ingredients full of sugars such as honey in case it fits the recipe.
Remember, it’s possible to fix the OG (increase the gravity technically) at any stage before bottling, even during the fermentation but it means more work, so it’s better to nail it from the first attempt on a brew day.
Post boil/Prior to fermentation
By post-boil/prior to fermentation readings, I mean taking the gravity reading at the last chance before pitching yeast because if you haven’t taken it before, this is the last opportunity.
Some homebrewers take their first OG at this stage because they find it easier as they don’t have to adjust for the temperature difference as the wort is chilled or they didn’t want to bother with involving extra steps during the brew day.
The point of the story is to take OG at one point after which you know nothing will change and then let the wort ferment by pitching yeast.
This is really accurate way of acquiring OG because unless you take the readings after the wort has stopped boiling there’s a chance that more water will evaporate as the OG prior to or during the boiling will differ from the OG taken straight after the boiling finished and the wort started cooling.
During fermentation
It’s a recommended practice to take a gravity reading by taking a sample from the fermenter after you suspect the fermentation might be finished.
Don’t take these readings at the beginning or before you suspect the fermentation is coming to an end because there’s no point in doing that.
These readings are supposed to tell us when the fermentation is truly final as the yeast won’t continue to eat sugars and thus decrease gravity.
If you followed the recipe exactly as it said and claimed the Original gravity, used exact mash temperature for converting sugars into fermentables, and used exact ingredients and exact yeast in theory you could just take a look at the FG and if it fits the FG in the recipe it’s done fermenting.
However, it’s difficult to have all these factors exactly as others did, so the safest choice is to take 2 samples with 3 days in between and see whether anything changed or not.
If on day 10 of the fermentation sample says 1.010 and we again take the sample on the 13 days and it was 1.010 again we are safe to conclude the fermentation is complete, yet if it continued to go below 1.010 this means the fermentation is still active.
Being extra careful with fermentation is important, but many beers will be finished within a week and just giving them a few extra days to sit in the fermenter which is a recommended practice for other reasons as well, will take care of the possible uncertainties.
Yet, high-ABV beers, meads, or wines can take longer to ferment and it will be hard to say when they are done without taking the readings on separate days. This also applies to lagers as the bottom-fermenting yeast can take weeks to fully ferment all sugars, so taking a sample is an easy way to know what’s going on.
In case fermentation may be stuck
In case you feel the fermentation is stuck and this can be done either by tasting a sample and noticing obviously sweet taste or by taking the gravity reading during fermentation. You’ll need to perhaps give it more time, add yeast nutrients or simply increase the fermentation temperature.
The instances when fermentation gets stuck are not common yet they happen, especially with more complex recipes. Understanding that gravity readings will help you monitor the fermentation is a piece of super useful knowledge.
Post-fermentation
Post-fermentation gravity readings are the final readings you took in the previous step when you were testing whether the fermentation was complete or not. If you took the readings and realized the fermentation is complete that means there is no need to take additional readings because the FG won’t change, so you can compare the OG with FG, and calculate the ABV and bottle.
Unless, of course, you are not satisfied with something such as there’s too much or not enough alcohol, not enough or too much sweetness, the beer is too bitter or you want to add some interesting flavors to enhance it.
In that case, you’ll be taking another reading to see where you stand after the additions.
If you top off the fermenter with extra water you’ll make the brew lighter which will also drop the gravity, and if you add something like lactose to back-sweeten the brew it will increase the gravity and if you add more fermentables and let it sit for some extra time it will affect the gravity once again.
In the end, you’ll want to know exactly the gravity prior to bottling as this will help you understand the recipe better and make it easy for others or yourself to replicate it next time.
Should you return the sample back?
If you take a couple of readings during the whole process of brewing you may lose a bottle of the brew you are making. However, in my opinion, it’s not worth returning the sample back unless it was taken prior to the end of the boil.
Yes, you can sanitize everything and return it safely but I just find it an unnecessary job. For such small samples I don’t want to bother opening the lid of the fermenter, accidentally dropping something in, or failing at sanitizing the testing jar which can sometimes get tricky to clean perfectly.
However, don’t throw the sample, smell it, drink it, and write down what you sense, after all even the unfinished brew may be drinkable and good to kill the thirst when chilled.
Final take on
There you go, gravity readings are necessary for any serious homebrewer and are such a simple yet powerful tool.
Personally, I take multiple readings in multiple stages for every brew I make to acquire as much information as I can.
However, gravity readings you take should be there to provide you with the information that you are looking for, so if all you want to know is the abv and amount of residual sugars take one reading prior to fermentation when the wort is chilled and one when the brew is done fermenting.
Don’t hesitate to take more than 2 readings if you feel they are necessary and that they will give you a better understanding of what’s going on.
Although brewing is an art, it’s also a science and it crosses the best of both worlds!