Original gravity and final gravity are terms used when brewing beer and both are measurements of specific gravity. When explaining home brew to my mom, she asked me “What is Specific Gravity?” It caught me off-guard a little because I use that term frequently at work.
So that got me thinking…when I helped out classmates in college I would always try to explain the concepts like I was talking to my mom. This way I would break down the information into easily digestible parts while still speaking respectfully. This brings us to why you are reading…
Specific Gravity…What is That Exactly?
First off, let’s define specific gravity as found in the dictionary.
So that is all fine and dandy, but let’s simplify it more. Say you go and buy 1-gallon of milk and 1-gallon of water. Your milk will weigh about 8.6 pounds and the water will weigh about 8.3 pounds. So your density is 8.6 pounds per gallon for milk and 8.3 pounds per gallon of water.
Now, to get the specific gravity you divide the two (8.6/8/3). This means the specific gravity of milk is 1.036. So you know that milk is heavier than water.
Hope that clears things up a little. Now you are probably wondering why your recipe doesn’t ask you to write down the specific gravity…
Is Original Gravity Different?
Not at all. This is a shortened version of original specific gravity which means the specific gravity of your wort prior to fermentation. What it is really measuring is the amount of dissolved sugars in your wort.
Why do you care? The original gravity is there for a couple of reasons…
- To let you know your recipe is tracking as expected
- You can calculate the alcohol content of your finished beer
Now you know where you are starting.
So Final Gravity Must Be…
I am sure you probably guessed, but it is the final specific gravity measured at the end of fermentation. At this point, you have converted the sugars to alcohol. Since alcohol is closer to water your specific gravity drops.
Why do you care? This is the final piece to give you the answer that every non-brewer will ask when they try your home brewed beer…what is the alcohol content?
(Original Gravity – Final Gravity) / 0.0075 = Approximate alcohol percent by volume
Here is an example. The last beer I made had an original gravity of 1.056 and a final gravity of 1.008. So what does that mean I get to tell my beer drinking buddies?
(1.056 – 1.008) / 0.0075 = 6.4%
Remember, this is an estimate so I will probably just tell people it is 6% and they will enjoy the beer fine. So where did I get those numbers?
How Do I Measure It?
You measure your specific gravity with a hydrometer. This is a relatively simple instrument to use…at least the one I have. It is specifically calibrated to show 1.000 when placed in water at 68 F. Some hydrometers are calibrated for 59 F so make sure you read the paperwork.
Well, that was the technical definition. Simply put, the hydrometer floats in your wort or beer and you read the line that shows touching the beer. It is really that simple!!
So I mentioned to read the paperwork with your hydrometer…this is because you can get different and slightly more accurate alcohol contents by adjusting your specific gravity for temperature. From my above example, my original gravity was 1.056 at 80 F and the final gravity was 1.008 at 62 F. Plug those into the calculator and my equation becomes:
(1.058 – 1.007) / 0.0075 = 6.8%
Similar, but changes to closer to 7%. So my beer is around 6.5% alcohol. Where do you go from here?
Use Your Newly Acquired Skill
Unless you stumbled upon this site studying for a test or trying to understand a physics problem, you want to apply your newly found skill. My recommendation is to brew some beer…what did you expect me to say?
There are many ways to use specific gravity when brewing beer, but, most importantly, you cannot use the numbers for targets. There are so many ways to affect specific gravity that the recipe values should be used as guides. I generally look to make sure my original gravity is enough above the estimated final gravity that I will be able to make beer with some alcohol.
Slight side story…I once made a batch that was meant for 1 gallon. I didn’t read the instructions (happens often) and went into autopilot to make a 5 gallon batch. It seemed to be progressing oddly during the boil, but I kept going. When I measured my original gravity it was 1.005…then I read the instructions…then I threw it out.
If you have any questions or comments, leave a note below or email me at firstname.lastname@example.org