Calculating Original Gravity

posted Oct 2, 2012, 11:13 AM by Richard Bryant   [ updated Oct 2, 2012, 12:03 PM ]

by: Ron Unz

When formulating your own recipes, it's important to know how much sugar your ingredients will be imparting on your beer. Grain has a maximum amount of specific gravity. When you purchase grain, the specific gravity will usually be provided (or can be found on the website where you purchased the grain) and will range from 1.038 - 1.025 depending on the type of grain and maltster. To calculate the number of gravity points, take the specific gravity, multiply by 1000 and subtract 1000. Pale malt usually has a specific gravity of 1.036. So, pale malt has 1036-1000 = 36 gravity points. An easier method for identifying the total number of gravity points is by just using the last two digits of the potential gravity.

The amount of gravity points is based on a pound of grain. To get the total amount of gravity points for a recipe, multiply the number of gravity points times the amount of grain in pounds.  For example, consider the grain bill below.

Grain        Amount (lbs)
2-row          10.00
Crystal 20    0.5
Crystal 40    0.25

The 2-row has a potential of 1.036, so a single pound of 2-row contains 36 gravity points. There is a total of 10 lbs, so 2-row will contribute a total of 360 point to the beer. Crystal 20 and 40 have a specific gravity of 1.034. Crystal 20 will contribute 17 gravity points and crystal 40 will contribute 8.5 gravity points. The total number of points for the grain bill is just the sum of all the gravity points. The example grain bill will have 385.5 gravity points.  For a list of common base and specialty malts and their specific gravities, click here (http://www.homebrewtalk.com/wiki/index.php/Malts_Chart).

The total number of points calculated in the previous section assumes you were able to extract all 100% of the sugars out of the grain. Extracting 100% of the sugars out of the grain is impossible. However, all-grain homebrew systems can extract up to 90% of the sugar out of grain. This percentage is called the mash efficiency. The mash efficiency is different for every brewer and equipment setup. It's recommend to aim for anywhere between 70-80%. For the example grain bill above, lets assume the mash efficiency is 75%. So we take the total number of gravity points and multiply it by 0.75. So the total number of gravity points for this grain bill is 289.9 gravity points. The only way to know the mash efficiency for a particular setup is by brewing on the setup, calculating the mash efficiency, and becoming familiar with your setup. Experience will allow you to pin-down your mash efficiency. A mash efficiency of 75% is a good starting place.

Now that you know the total number of  gravity points, you can calculate the OG of your beer. To calculate the OG, you need to determine volume of wort you will have at the end of the boil. Divide the total number of gravity points by the number of gallons of wort. For the example above, lets assume this is a 6 gallon batch, so the wort will have 48.2 gravity points per gallon. Take this number, add 1000 and divide by 1000, this value is your OG. The OG for the grain bill above in 6 gallons of wort is 1.048. To calculate the OG you could just tack on the first two digits of the number of gravity points per gallon (48.2) to the end of 1.0 to get 1.048.

Extract brewers can follow a similar routine, but you will have to treat the extract and steeping grains separately. Dry extract usually has a specific gravity of 1.045 per pound of extract. Liquid extract has a specific gravity of 1.035. Use 100% efficiency when using extracts. When considering the contributions from from your steeping grains, it's safe to stick with a 75% efficiency. Assume in the recipe above we replaced the 2-row with 6 lbs of dry malt extract. At 100% efficiency, the dry malt extract will add 270 gravity points.  The specialty grains will contribute another 19.1 gravity points to give a total of 289.1 gravity points. At 6 gallons, the wort will have an OG of 1.048.

If you have any questions or need help following this topic, please shoot me an e-mail at ronald dot unz at gmail dot com and I will be more than happy to help. Stay tuned for my next technical topic on calculating mash efficiencies and what to do if you miss your gravities.

Cheers! 
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