Original Gravity
To calculate Original Gravity (OG) for a recipe, you need to know the “potential” contribution that each fermentable ingredient in the recipe will make. This is known as the Points Per Pound Per Gallon (PPG) figure, and corresponds to the contribution that a pound of grain (or sugar) will add if dissolved in a gallon of water. The maximum potential is approximately 46, which would be a pound of pure sugar in a gallon of water.
The PPG value for pure sugar runs close to 46. Grains vary tremendously – from a low of 25 up to highs in the 40 range. Even Pale malt (which is often around 36 to 38) varies depending on the maltster. You can find the PPG value of various fermentable ingredients from the Brewers Friend database.
Before proceeding with the calculation of OG, we must first convert the weights of each ingredient from kilograms to pounds, by multiplying each by a factor of 2.2046.
The next step is to calculate the “points” for each ingredient and total them up. This is done simply by multiplying the PPG value for each ingredient by its weight.
So for a simple stout with 8 lbs of pale malt (PPG = 36) and 3 lbs of roast barley (PPG = 25) would give us:
36 points * 8 lbs = 288 points
25 points * 3 lb = 75 points
Total = 363 points.
The next step is to apply an “efficiency” factor to the calculation. The PPG values given for the grain are the maximum possible amount that 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%. A typical brewhouse efficiency number for a home system is 70-75%. In this case we’ll use 72%:
363 points * 72% efficiency = 261.4 points
Now we just divide by the “into fermenter” volume, in gallons. For example 26 litres, which is 6.9 American gallons:
261.4 points / 6.9 gallons = 37.9 points
And that is the OG estimate – 37.9 points gives us an OG of approximately 1038.
The calculation of OG is incorporated into the Designer page.