Sometimes the geek inside of us needs to know how malty, how big, how boozy our beer will be when we swig it… or maybe we want to make sure that fermentation is complete…

Well, here’s where a little bit of math can come in handy…

Below are my favorite 5 home brewing formulas that I use even though I have software to do most of my needed calculations…

How To Calculate ABV In Your Beer
So how much alcohol is in your newly brewed liquid?… well this is a straight forward equation where you simply take your original gravity and subtract your final gravity. Then you multiply the result by the constant 131 to get your ABV %

ABV% = (OG – FG) * 131
OG: 1.065
FG: 1.016

(1.065 – 1.016) * 131 = 6.42% ABV

*I believe credit for this formula goes to Charlie Papazian who first published this formula in his book The Complete Joy of Home Brewing

How To Calculate Apparent Attenuation
Knowing if your beer is done fermenting is important for many reasons. I cover all of them in my home brew training program, but the most important one is to make sure that no residual sugars are left over which you’d have to account for if you are planning on bottling your beer. If your beer is not done fermenting and you add more priming sugar you may be over-carbonating your brews and get exploding bottles. If you are kegging, then you may just get sweet beer…

So to find out if you are completing your fermentations then start by looking at your yeast strain and figure out what the attenuation levels are. It will be a range around 65 to 80%…

The equation is simple. Simply take your OG points and subtract your FG points and divide by your OG points again. If you are not used to working with points simply subtract FG from OG and divide by OG minus one.

Suppose you have a yeast strain that attenuates 68 to 72%
Apparent Attenuation = (OG points – FG points)÷OG points Or
Apparent Attenuation = (OG – FG) ÷ (OG – 1)

OG: 1.050 or 50 points
FG: 1.017 or 17 points

(50 – 17) ÷ 50 = 66%

In this example, the beer would be under attenuated provided no specialty malts with non-fermentables were added.

How To Calculate How Much Yeast To Add (Pitching Rate)
This one is important to have handy. If you underpitch you can end up with stuck fermentation. If you over pitch, you may end up with lipid-producing yeast… The only equation I found for this was one used by brewers who re-use their yeast…

Yeast Pitching Rate in Billions = K * (Liters of Wort) * (Degree Plato)

Most home brewers I know, however, buy yeast which is healthier and therefore can use about half of what this equation tells you to use… Now the constant K will vary depending on how low or high of a pitching rate you want and whether you are brewing ale or lager beer.

For ale low pitching rates have a constant of about .75 and 1 for high pitch rates. Lagers start at 1 for low pitching rates and up to 1.5 for high pitching rates.

Ale Brew with high pitch rate: K = 1
Gallons: 5 (5 x 3.78541178 = 18.93 Liters)
OG: 1.052 (52 ÷ 4 = 13 Deg Plato)

1 * 18.93 * 13 = 243.36 Billion Yeast cells needed for pitching. This would be if you are re-using yeast. If you were to buy a new yeast vial then you would actually need about half which would be 243.36 ÷ 2 = 121.68…

How To Calculate AAUs From Your Hops
Consistency in brewing is the name of the game… hops is one ingredient that will mess up your consistency because one day you’ll buy hops with 7.8 AA% but a few months down the road you’ll get the same hop variety with 8.3 AA%…

That’s just the nature of ingredients… So when you come up with a recipe and want to brew the same exact beer again and again, then it’s better to go by AAUs (Alpha Acid Units)… simply take your AA% and multiply it by your weight to get your AAUs.

AAU = Weight (oz) x AA%
Hops: Centennial 9.5 AA%
Weight: .5 oz

.5 x 9.5 = 4.75 AAUs

Now suppose you buy Centennial hops again but this time they have 10.3 AA%. To figure out how to get 4.75 AAU’s again you’d divide AAUs by AA% to get the amount of hops you need.

Weight (oz) = AAU ÷ AA%

4.75 ÷ 10.3 = .46 oz

Because the hops have more alpha acids, you would use slightly less to get the same results…

How To Convert All-Grain Recipes Into Extract Recipes
You may be given an awesome all-grain recipe, but you are an extract brewer. Well, that’s not a biggie if you know how to translate all-grain into extract…

The formula is simple…
Weight of Grain x Yield = Weight of Malt Extract x Yield

Where yield (points) for grain = (46 * Extract FG, Dry Basis * Brewhouse Efficiency)
And yield (points) for malt extract = (46 * Solids)

What’s not so simple is separating specialty grains from base malts…

Easy Example:
All Grain Recipe
OG: 1.047
10 lbs pale ale malt (80% Extract FG, Dry Basis)
Brewhouse Efficieny: 65%

If we were to use Light DME (97% Solids) then we would have to solve for Extract weight using the formula above…

10 * (46 * 80% * 65%) = Extract Weight * (46 * 97%)

Gives us…

10 * 23.92 = Extract Weight * 44.62


Extract Weight = 5.36 Lbs (239.2 ÷ 44.62)

Hard Example:
All Grain Recipe
OG: 1.060
10 lbs Pale Ale Malt (80% Extract FG, Dry Basis)
1 lb Caramel Malt 60 L (73% Extract FG, Dry Basis)
1 lb Chocolate Malt 350 L (73% Extract FG, Dry Basis)
.5 lbs Carapils (73% Extract FG, Dry Basis)

First thing we would have to do is add up the yield (points) from specialty malts and subtract it from the OG points of the beer using the same formula…

Weight of Grain x Yield = total points

Caramel 1 x (46 * 73% * 65%) = 21.83
Chocolate 1 x (46 * 73% * 65%) = 21.83
Carapils .5 x (46 * 73% * 65%) = 10.91

21.83 + 21.83 + 10.91 = 54.57 points in 5 Gallons or 10.91 points per gallon.
If our OG is 1.060 or 60 points per gallon, then that means we have 49.09 points of fermentable sugars… (60 – 10.91 = 49.09)

49.09 per gallon equals to 245.45 points in 5 gallons… therefore

245.45 = Weight of Extract x (46 * 97%)


245.45 ÷ (46 * 97%) = Weight of Extract

Gives us…

5.5 Lbs Extract

So our Extract recipe would look something like

5.5 lbs DME
1 lb Caramel Malt 60 L
1 lb Chocolate Malt 350 L
.5 lbs Carapils

In essence you have to learn how to calculate how many points of sugar you get for each malt and then substitute the same amount of points from base malts with extract. So there you have it…

Do you have any formulas you like to use that I didn’t include here?

    4 replies to "Top 5 Home Brew Formulas To Use"

    • sid

      thx jorge been waiting for formulas for a long time very good thx again.

      • Jorge

        @Sid – You are welcome… they come in very handy…

    • Martin

      There is IBU total and then there is bitterness ratio. Where would I find a chart showing what this ratio should be for the beer I am brewing? For instance an American Amber should be around.60 and a Belgian trippel around .38 or an American Pale at about .72. I can calculate IBU’s for a double IPA and it fits the guidelines for style but the bitterness ratio could be over 1.0. I know I can add more grain to bring the ratio down but is there some sort of chart giving the ratios in the guide lines I am missing? What should the ratio be for a Double IPA?

      • Jorge

        There is no chart that I know of. Bitterness ratios, just like OG, FG, SRM, ABV, and IBU’s, are more of a range and not a specific number…
        Imperial IPA’s can have Bitterness ratios between .66 and 1.7!! that is of course the extremes… the best examples of DIPAs usually have Bitterness ratios somewhere in the range of .85 to 1.33…

        Amber ales are somewhere in the range of .55 and .66… Where in that range you brew your DIPA depends on your preference…

        Also… Bitterness ratios don’t mean much because it doesn’t account for the type of sugars you are fermenting and how you ferment them… it’s good to use it as a guideline, but I personally follow a different method to come up with my recipes…

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