If you know your boiling losses and the pre-boil specific gravity, you can estimate the post-boil original specific gravity (OG) of your beer with reasonable accuracy. The formula for calculating the approximate post-boil OG is:
Post-boil gravity points = (Pre-boil volume x pre-boil gravity points) / Post-boil volume
Specific gravity “points” (or GP) are the portion of the specific gravity reading to the right of the decimal point multiplied by 1000. For example, a specific gravity of 1.050 is 50 points.
Using the pre-boil and post-boil volumes from the example above, with a pre-boil specific gravity of 1.036, the projected post-boil OG would be 1.049.
(7.5 gallons x 36 GP) / 5.5 gallons = 49 GP (equivalent to SG 1.049)
(28.3 Liters x 36 GP)/20.8 Litres = 49 GP (equivalent to SG 1.049)
Calculating the post-boil OG at the beginning of the boil is valuable because this is the easiest point to make adjustments. If the calculated OG differs appreciably from the target for the recipe, additional extract or water can be added.
To increase the gravity of a 5-gallon (19 L) batch by approximately 1 GP at the end of the boil, add 2.5 oz. (72 g) — roughly one-half cup/118 mL — light dried malt extract, or approximately three-quarters cup (177 mL) liquid extract at the start of the boil.
To decrease the gravity of the same batch by approximately 1 GP, add 24 fl. oz. (710 mL) water prior to boiling.
CHANGING THE BOILING TIME:
You can also manipulate the OG by changing the boiling time. In the above examples, a 9-minute increase in the boiling time will raise the OG by approximately 1 gravity point, while decreasing it by 9 minutes will lower it by approximately 1 point. The approximate change in the post-boil volume per 9 minutes will be 22 fl. oz. (640 ml)
FIGURING OUT YIELDS FROM BREWING INGREDIENTS
Approximate yields of brewing ingredients:
(article courtesy of Delfalcos)
With one pound (453g) of each of the following ingredients dissolved in water to make up a total volume of one gallon (3.79 Liters), you should expect the following specific gravities:
|Dried malt extract||1.042|
|Malt extract syrup||1.036|
|Brewery Grade Corn Syrup1.036||1.036|
|Rice Syrup Solids||1.042|
|Brewer's Pale Malt||1.025 - 1.030 (depends on your efficiency!)|
|Munich malt||1.022 - 1.027 (depends on your efficiency!)|
|Wheat malt||1.025 - 1.030 (depends on your efficiency!)|
|Black malt/Chocolate malt||1.010|
Okay, how do you use this information? Easy! Simply multiply the last two digits on the gravity for each ingredient in your recipe by the number of pounds, add these amounts together and divide this total by the number of gallons.
For example: Let's say we have a pale ale recipe that calls for two 3.3 lbs. (1500g) cans of light malt extract (hopped or unhopped makes no difference), a pound (453g) of crystal malt and a half pound (227g) of pale malt. What should we expect the original to be? First, let's multiply the weight in pounds of the malt extract times 1.036 (actually, let's drop the 1.0 part to make the arithmetic easier). That equals 237.6. Let's add this number to the value from the crystal malt (1 X 15 =15) and the value from the pale malt (1/2 X 25 = 12.5). Altogether that is 237.6 + 15.0 + 12.5 = 265.1. Divide this sum by the number of gallons in this recipe (5) (18.9 Liters) and you get 53.02. This means that the original gravity should be approximately 1.053. These same ingredients brewed into a six gallon recipe should yield a brew with an original gravity around 1.044(265/6 = 44 or, as we would say, 1.044). On the other hand these same ingredients would produce a beer with an O.G. of about 1.066 if brewed in only four gallons (265/4 = 66 or 1.066). These numbers are only approximations, but they can be quite helpful when trying to formulate recipes. Obviously, as more refined and processed ingredients (e.g. malt extract, sugars, honey) are replaced with less refined ingredients (grains), then these numbers are much more variable. The yields will greatly depend upon your efficiency in the mashing and sparging processes. Still this chart should prove to be a helpful tool.
CORRECTING ORIGINAL GRAVITY BY ADDING SUGAR
NOTE: Use the Imperial measurements in the article and convert to the final result to Metric.
NOTE: The type of sugar you add can depend on the style of beer you are making. (see Brewing with Sugar)
To increase the specific gravity of wort:
Add corn sugar/invert sugar/ to increase the gravity. To calculate the amount needed, take an initial gravity reading, then subtract that from the specific gravity you wish to begin with. The difference will determine approximately how much sugar to add (use table below).
Example: If your current gravity is 1.075 (24.5 oz. sugar per gal), and the desired gravity is 1.095 (31.0 oz. sugar per gal) then [ 31.0 – 24.5 = 6.5 oz.] So 6.5 oz. of sugar per gallon that must be added to bring the gravity up to 1.095
Hydrometer Conversion Chart:
|Specific Gravity||Brix||Alcohol Potential||Sugar Per oz/gal|
CORRECTING ORIGINAL GRAVITY BY ADDING DME
Once you’ve missed your OG, and looked at potential causes, you are usually faced with the problem of what to do with your existing batch? Fortunately it is not hard to adjust the OG of wort as it goes into the fermenter by using some water or some dry malt extract. We usually keep a few pounds of high quality light DME around just in case. Brew Shop has the Breiss range, and Williams Warn sell an excellent neutral LDME. Use a high quality DME so that the flavour of your beer is not adversely affected.
Note: There is a simple Adjust Gravity Tool in "BeerSmith" which will do the calculation for you. I’ve included the calculations in English units below (as it is easier to calculate) but you can convert metric volumes to English pretty easily (1 gallon = 3.785 liters, 1 kg = 2.2 lbs)
If your gravity is too low, the calculation for the amount of dry extract to add is:
• Calculate the difference between your target and actual OG, then multiply by 1000. For example if you were targeting 1.056, but only hit 1.048 this would give us (1.056-1.048) x 1000 = 8 points
• Now we need to raise our gravity by 8 points which means we need to add 8 points/gallon of dry malt extract (DME) equivalent. Assuming a 5 gallon batch size, we need a total of 40 points of DME.
• DME has a potential of 1.046 which means it contributes 46 points/lb added, so we simply take the 40 points and divide it by 46 to get 0.9 lbs of DME to add.
CORRECTING ORIGINAL GRAVITY BY DILUTION WITH WATER
If the gravity is too high, we can dilute it by adding water: (BOIL WATER FIRST TO SANITIZE)
[To reduce the specific gravity (if sugar level is too high): 2 quarts of water will decrease a 5 gallon must approximately by 0.010]
• This time we’ll assume our target was 1.056 but we overshot and came in with a gravity of 1.064, again using a 5 gallon batch. We’ll use the fact that the number of points times volume should be a constant to do the dilution.
• So we start by taking our starting points of 1.064 = 64 gravity points, and multiplying by our original volume of 5 gallons: 64×5 = 320 points
• Now we divide by our target points which is 1.056 = 56 points which will give us the target volume: 320 / 56 = 5.71 gallons
• Since we started with 5 gallons, we need to add 0.71 gallons of water to dilute our gravity to achieve the target of 1.056