calculating Original Gravity from Specific Gravity

From my understanding, "Specific Gravity" just refers to a gravity reading...at that moment and regardless of when you do it. i.e. first runnings, second runnings, during fermentation, etc.

O.G. or Original Gravity refers to the reading that is taken right before you begin fermentation or the wort.

F.G. or Final Gravity refers to the reading at the completion of fermentation.

 

DarrenStory has it right. Specific Gravity is the generic term for the measurement of the liquid's density when compared to water. If your recipe gives a number for the 'specific gravity,' and if that number is greater than, let's say 1.030, then it is likely the recipe's number is meant to be the Original Gravity (OG) target for you to try to match during your brewing session (just before pitching the yeast at the 65-70 degree mark). If the gravity number given by the recipe is less than 1.030 then that number is probably referring to the Final Gravity (FG).

 

Since you're just starting, I assume you used malt extract. If that's the case, then OG is fairly easy to calculate since the mash efficiency is pretty consistent. Numbers vary (and you're only looking for a ballpark number, anyway), but I assume 37 gravity points (SG 1.037) per lb per gallon for LME, and 43 gravity points (SG 1.043) for DME. So, if your recipe for a 5 gallon batch used 5 lb of LME, the OG would be (((37 x 5lb) / 5 gal) + 1) = 1.037. If you used 6 lb of DME, the OG would be about 1.053. You then subtract FG from OG and multiply by 131 to get ABV.

As an example, if your OG was 1.055, and your FG was 1.015, the ABV would be ((1.055-1.015) x 131) or about 5.2%

 

Also remember that hydrometers are calibrated to a certain temperature (most are 60F) and that you have to adjust the hydrometer reading you get based on the temperature of your wort sample. This calculator will help you with that...

http://hbd.org/cgi-bin/recipator/recipator/hydrometer.html

 

Yeah its really not that complicated, think of gravity like you would temperature. You can take a temperature any time you want and will get a number. But that doesn't mean the number tells you anything. If you take a temperature reading while an oven is heating up you know the temperature right now, but not the temp you are cooking at. You can also take a gravity measure at any time you want and get a measure. People talk about OG and FG because these are times when the gravity measure tells you something. When you take a measure before you ferment (OG) it gives you a rough estimate of the total amount of sugar that made it into the wort. Then you pitch yeast and ferment. After fermentation you, before priming and bottling, you can take another gravity measure to get the the final gravity, this will tell you roughly how much sugar is left in the finished beer. The difference tells you the amount of alcohol. In general though, for your first beer don't sweat it, knowing what the og and fg are helpful for diagnosing problems, or describing (and planning) a recipe, but in the end the final product is the same whether you know them or not. I didn't get a decent reading until my third beer, so it can be good to practice (my samples weren't well mixed wort).

 

You can safely assume that the OG is 1.046. The FG may be a bit different due to factors outside the manufacturer's control. Use the number that the hydrometer told you and you should be good. But, as redmaw said, don't sweat it. You'll be fine.

 

In the future, make a point to do the following measurements each time:

preboil SG (measure the gravity of the wort just before you are about to boil it)
preboil volume (measure the amount of wort you have just before you are about to boil it)
post boil SG (also known as the OG)
post boil volume
volume into the fermenter (also known as the Batch Size)

Those 5 brew day measurements are the basic key ones to determining your efficiencies / analyzing your brew day

Then after fermentation is complete, your stabilized / terminal gravity measurement should be recorded (also known as the FG) as should your final volume into bottles.

 

OP, Did you mean calculate ABV from SGs?

Really only a couple of good reasons to take any set of gravity readings:

1. To make sure your bottled beer doesn't explode
2. To estimate the Alcohol amount (ABV)

there are other reasons for beer geeks , but generally, that's it.

 

Noobies really don't need to worry about that. Only the OG (post boil SG) and Batch Size are important. Also, knowing your starting volume you can use proportional reasoning to figure out everything else with your OG and Batch Size, because the amount of sugars in the wort do not change during the boil.

 

Point appreciated but still the advice I gave will give the op a good conceptual framework that will help him as his brewing prowess evolves.

 

OG is what you get before you ferment, FG is what you get when it's all said and done, and alcohol has been produced.

So you should have had around 1.046 before you pitched yeast, and if you used extract for the majority of your fermentables, then you can pretty much calculate that it close, if your volumes and stuff were correct.

 

Sure, an extract brewer doesn't need to check it before, during and after, but it would be good advice to get in the habit of doing so. Would save them alot of headaches trying to figure it out when they move to more advanced brewing.

 

Read this: http://www.howtobrew.com/appendices/appendixA.html Then read the rest of that site. You're welcome.