Taking readings of your home brews Original Gravity (OG) and Final Gravity (FG) is imperative to calculating the all-important Alcohol By Volume (ABV) of your beer. When we first started brewing we invested in a hydrometer but didn’t really use it because we didn’t want to lose beer on reading but had no idea the level of alcohol in our beer. If you haven’t invested in a hydrometer and beaker I suggest you get one immediately, They are not very expensive ($10-$20) it allows you to see if your hitting your targeted OG, see how the gravity is progressing from primary to secondary fermentation or Current gravity (CG) and establish your FG.
What is Original Gravity (OG)?
It is the measure of all the sugar dissolved into your unfermented wort. You take this measurement after your cool your wort but before you put in your yeast.
How to get your Target OG?
We use Brewers Friend Receipt Calculator to determine our targeted/estimated original gravity but you can also try to calculate it manually. The below information came from BeerSmith Home Brewing Blog and can help you determine your estimated OG:
To calculate OG for a recipe, you need to know the “potential” contribution that each grain or extract in the recipe will make. This corresponds to the contribution that a pound of grain or extract will add if dissolved in a gallon of water. The maximum potential is approximately 1.046 which would be a pound of pure sugar in a gallon of water.
Liquid extracts typically have a potential of around 1.036, dry extracts run around 1.044, and pure sugar runs close to 1.046. Grains vary tremendously – from a low of 1.025 up to highs in the 1.040 range. Even Pale malt (which is often around 1.036-1.038) varies depending on the maltster.
Once we have the grain bill and potentials for each extract or grain in the recipe, the next step is to calculate the “points” for each grain contribution and total them up. This is done simply by multiplying the potential points for each grain by the weight of the grain.
Recall that points are simply the fractional part of the potential – so an extract with a potential of 1.046 is simply 46 points. So for a simple stout with 8 lbs of pale malt (1.036 potential) and 1 lb of roast barley (1.025 potential) would give us:
36 points * 8 lbs = 288 points
25 points * 1 lb = 25 points
Total = 313 points.
The next step is to apply an “efficiency” factor to our process. The potentials given for the grain are the maximum possible amount you could draw from the grains if you crushed them under laboratory conditions with no losses. Real mashing processes and subsequent sparging, boiling and transferring are not ideal – so a typical brewhouse has an efficiency number far less than 100%. The brewhouse efficiency number includes all of the losses in the system into the fermenter including mashing, lautering, boiling, trub loss and transferring the finished wort to fermenter. A typical brewhouse efficiency number for a home system is 70-75%. In this case we’ll use 72%
313 points * 72% efficiency = 225.4 points
Now we just divide by the “into fermenter” volume which in this case is 5 gallons:
225 points / 5 gallons = 44.8 points/gal
And that is the original gravity estimate if we convert it back to specific gravity – 44.8 points gives us an OG of approximately 1.045
How to Calculate the ABV: (OG – FG) x 131 = ABV%