Specific Gravity in home brewing is one of those things that makes us feel like mad scientists. That’s the great thing about home brewing, there is so much more to it than just slapping a batch of beer together. I’m a firm believer that as a homebrewer, beer makes you smarter. You can talk to your mates about specific gravity and sound really clever with how you apply science to brewing.
As usual, I’m going to keep this simple and aimed at the beginners, so let’s get into it.
What is Specific Gravity?
Specific Gravity or (SG) which it’s commonly abbreviated to, is the measure of density of a liquid compared to water. In the homebrew world you will often hear people saying that SG is the amount of sugar in beer, but in fact this is not exactly correct. For example sea water which obviously has salt in it would give you a SG of 1.025, as compared to fresh water which is 1.000.
When it comes to measuring the specific gravity of wort, sugars in water are generally what you’re measuring. However in some cases with beer, the things that can give you a slightly false SG reading are materials suspended in your beer or wort, These such things can be proteins, hop oils or un-fermentable starches.
Why do I need to measure Specific Gravity in Home brewing?
Typically a homebrewer requires to measure the specific gravity of their wort just before pitching the yeast. This is known as the Original Gravity and will be referred to as OG.
After fermentation has finished, another specific gravity reading is taken. This reading is known as the Final Gravity (FG).
How to calculate Alcohol By Volume? (ABV)
Once you have both the Original Gravity (OG) and Final Gravity (FG) you can use the formula below to calculate ABV, or simply click here to use the Brewers Friend ABV Calculator.
ABV= (OG – FG) x 131.25
5.25% = (1.048 – 1.010) x 131.25
If bottle priming, add 0.5%
You will need to know the ABV for when sharing your home brew with friends, because quite often you will get asked “What percentage is this?”
There are various stages during an all-grain brew when the brewer will take gravity readings to calculate brew efficiency. I won’t go into this today any further, because Kevin would like to talk to you about efficiency in his all-grain posts.
Confirming Fermentation has Finished
The other main reason for taking gravity readings is to ensure your beer has finished fermenting and is ready to bottle or keg. To ascertain that beer is ready to bottle or keg you will need gravity readings over 2 or 3 consecutive days that are exactly the same. If these readings are exactly the same then it’s safe to say your beer has finished fermenting and this is your Final Gravity (FG). This of course is only the case if your beer has been sitting at the recommended temperatures that your yeast requires to ferment.
How to measure Specific Gravity?
The 2 common instruments to measure SG are using a Hydrometer and a testing flask or a Refractometer. But today I’ll just give you the steps using the Hydrometer.
1. Remove the airlock or loosen the fermenter lid so none of the water gets sucked back into the fermenter. This is where the 2 or 3 piece airlocks come in handy because all you need to do it pull the top section off.
2. Fill the testing flask with wort or beer to about 3/4 full. You will learn after a few goes what volume you’re testing flask needs to float the hydrometer.
3. Put the airlock back on or tighten the fermenter lid back on as soon as you have taken your sample.
4. Gently lower the hydrometer into the flask of wort and let it float. Don’t just drop it in because you will smash it.
5. Give the hydrometer a spin to clear off any attached bubbles and improve the accuracy of the reading.
6. Get down at eye level with the top of the wort and read the hydrometer from the top of the meniscus. Make sure you write this reading down in the following format 1.XXX.
Tip: Your hydrometer should have written on the side what temperature it’s been calibrated to. Try take your reading close to this temperature if you can. There is a good temperature adjustment calculator on Brewers Friend if your wort is not close the required temperature.
7. Remove the hydrometer, wash it and store it away.
Warning: Do not wash glass hydrometers in hot water, because you can break them or cause the indicator inside to dislodge.
8. This step is vitally important! Taste a sample of the sample. I think it’s a good practice to taste your samples along the way just to understand what’s going on.
I have heard a lot of people say to take the gravity reading from the bottom of the meniscus. If this is you can you care to explain why in the comments, as we would love to put this debate to bed.