Original Gravity (OG) & Final Gravity (FG) Calculator

 

About The Calculator


To make the calculator easier to use, default values have been set for the average brewhouse efficiency, yeast attenuation and batch size (5 gallons). If you are un familiar with these numbers and how they influence the calculations to find your beers specific original & final gravity, you can learn more by checking out the examples below or simply leave them set to their default values. For most brewers this should yield fairly accurate results as is.

Once you’ve determined the appropriate values for the above variables, simply enter in your recipes grain & adjuncts information. For ease of use, I’ve added the appropriate values for over 90 different commonly used grains & adjuncts in the drop downs. If you don’t see your grain listed, I’ve also added 3 extra sections for adding your own specialty grain information.

 

Calculating Original Gravity

Once we know the beers grain bill, we can estimate the beers Original Gravity(aka – OG). Most importantly, we have to find the beers PPG or Potential Yield value. If the PPG value for the grain you are looking for isn’t explicitly listed, you can also use the grains potential yield value. The grains potential value is usually written in specific gravity format, IE – 1.025 SG. So in this case, our PPG value for the grain will simply be 25 points.

Not sure how to find your grains Potential/PPG value? Use the STTP Grains & Adjuncts Chart.

Calculate-original-gravity-final-gravity

For Example: Lets say that I am planning a recipe that is going to be using 5 lbs of Acid Malt (1.027) and 5 lbs of Honey (1.035) in a 5 gallon batch.

27 points * 5 lbs = 135 points
35 points * 5 lbs = 175 points

Total = 310 points.

Next we need to adjust our total points to reflect brew house efficiency. While this number will vary depending on the skill and equipment of the brewer, we’ll use the average of 75% efficiency.

310 points * 75% efficiency = 232.5 points

Lastly, we need to find our points per gallon value. Pretty straight forward here, simply divide your total points for the batch by the volume in gallons of your target batch size.

232.5 points / 5 gallons = 46.5 points/gal

Original Gravity = 1.047 SG 

 

 

 

Calculating Final Gravity

Once we’ve calculated our Original Gravity value, we can now estimate our Final Gravity value by applying the average attenuation rating of our yeast strain to our Original Gravity’s (OG) points per gallon number. A yeast strain with an attenuation value of 75% means that there will be approximately 25% of the beers remaining sugars left unprocessed. Using the numbers we calculated from our OG formula above, we simply apply our points per gallon number (46.5) by the percent of unattenuated sugars (25%) in our beer and we can calculate our beers estimated FG value.

46.5 points * .25 = 11.62 points

Final Gravity = 1.012 SG

 

Calculating ABV Contribution

Since we aren’t able to simply apply the usual ABV formula to each grain/adjunct individually in our grain bill, we first need to calculate the beers total ABV.

ABV =(76.08 * (OG-FG) / (1.775-og1)) * (fg / 0.794)

Let’s substitute some example numbers.

ABV =(76.08 * (1.047-1.012) / (1.775-1.047)) * (1.012/ 0.794)

ABV = 4.64%

The ABV contribution numbers used in this calculator take the beers final ABV value and then calculates each grains total contribution to the beers gravity. Just like when calculating our beers Original Gravity, we first need to multiply each grains weight by it’s PPG or potential yield value.

27 points * 5 lbs = 135 points
35 points * 5 lbs = 175 points

From here we add up the total points value of all grains in the grain bill, and find the percent contribution of each grain.

135 points + 175 points = 310 points.

135 / 310 = ~ 43.5%
175 / 310 = 56.5%

Once we know the percent contribution for each grain/adjunct of the beer, we just need to apply these numbers to the beers total ABV percent.

.435 * 4.64% = 2.02%
.565 * 4.64% = 2.62%

Total ABV = 4.64%

It should be noted that there is no official formula for calculating the ABV contribution for each grain/adjunct in your beers grain bill. The above formula is my own best guess at finding these numbers. It does not take into account that the yeast strains may yield unique attenuation values for different types of sugars.  Please take these numbers with a grain of salt.

 

Rather work off-line? Download the OG-FG Calculator Excel Sheet!

 

Straight 2 The Pint