How to Calculate Your Next Grain Bill from Scratch

How to Calculate the Grain Bill

Lately I’ve been using brewing software less and less. Sure, it makes recipe formulation ‘simple’. But you tend to forget (or perhaps your never knew?) how to calculate a grain bill from scratch.

Remember when you were in elementary school math class?

Did your teacher allow you to use a calculator before you could do the calculations by hand? I bet she didn’t…

The reason for this is simple: a calculator can do a calculation for you, but you’ll never really understand why it took 2, added 2 and came out with 4.

Brewing software is exactly the same.

After you’ve learnt how to calculate a grain bill manually, your sense of understanding and creativity will improve as a result.

In this article I’ll show you how to calculate a grain bill without the use of brewing software in 10 basic steps.

Note: the steps in this article are based on the equations found in Ray Daniels’ Designing Great Beers (Amazon link), an invaluable resource for anyone interested on recipe formulation (and brewing as a whole for that matter).

1. Decide the batch size

Most homebrew recipes I come across are 5 gallons, but any volume (represented in gallons) will work.

For this batch I want to brew 5.5 gallons.

Write down: 5.5 gallons

2. Record the target gravity

The target gravity is the original gravity (the ‘OG’) of the beer that you are trying to produce.

The BJCP Style Guidelines can be a great help with this step, since they list the vital statistics for the particular style that you’re brewing.

For a Kölsch, the guidelines state that the OG should be between 1.044 and 1.050 SG.

And so for the purpose of this tutorial, I’ll shoot for an OG of 1.048.

Write down: 1.048 OG

3. Convert the target gravity into gravity units

In step 2 I decided that the recipe would begin fermenting with an OG of 1.048.

To represent 1.048 in gravity units (the ‘GU’), simply complete the following equation:

GU = (OG-1) x 1,000

After completing this equation, a target OG of 1.048 gives you 48 GU.

Write down: 48 GU

Alternatively, just take the numbers from the end of the OG reading: an OG of 1.036 is 36 GU, an OG of 1.089 is 89 GU, an OG of 1.101 is 101 GU and so forth.

4. Determine the total amount of gravity units needed

In step three I showed you that a OG of 1.048 equals 48 GU.

What this means in practice is that each gallon of post-boil wort contains 48 GU.

Therefore, to determine the total gravity units (the ‘TGU’) needed in the post-boil wort, simply multiply the batch size by the GU:

Volume (in gallons) x GU = TGU

Because I decided to brew 5.5 gallons (see step 1), and I want it to contain 48 GU (see step 3), the equation would look like:

5.5 Gallons x 48 GU = 264 TGU

So, the TGU needed for this recipe will be 264 TGU.

Write down: 264 TGU

5. Decide which grains you want to use and their proportions

If you’re using a recipe book like Modern Homebrewing Recipes by Gordon Strong, each recipe will list the grains required and their percentage of the grist.

This makes things easy: simply jot down the grains and their percentage of the grain bill.

For the purposes of this article I’ll use a basic Kölsch grist of about 95% German Pilsner malt and 5% malted wheat. 

Write down: 95% German Pilsner malt, 5% malted wheat. 

Note: this article is not intended to explain which grains come together to form a good recipe. If you’re brewing a Kölsch, check out this page for more info. If you’re brewing a different style, there are numerous books and online resources to find good recipes.

6. Calculate the amount of gravity units that should come from each grain

In step 5 I decided to use a grist consisting of 95% German Pilsner malt and 5% malted wheat.

As such, 95% of the TGU will come from German Pilsner malt, and 5% of the TGU will come from wheat malt.

To calculate the amount of gravity units needed from each grain (the ‘Ingredient Gravity’), use the following calculation:

Ingredient percentage x TGU = Ingredient Gravity 

For our German Pilsner malt addition the calculation is:

95% x 264 TGU = 250.8 GU

And for our malted wheat addition:

5% x 264 TGU = 13.2 GU

So, from this we know that the German Pilsner malt needs to contribute 250.8 GU and the malted wheat needs to contribute 13.2 GU.

Write down: Ingredient Gravity of German Pilsner malt = 250.8 GU, Ingredient Gravity of malted wheat = 13.2 GU. 

7. Convert the extract potential of each grain into gravity units

The amount of non-fermentable and fermentable sugars that a pound of grain will yield when mashed under laboratory conditions is called the extract potential (Palmer).

The extract potential of a grain is usually defined in terms of SG (for example, 1.037) or as a percentage (for example, 80%). It can often be found on your suppliers website. If your supplier doesn’t list it, check the maltster’s website.

The extract potential for each grain needs to be converted into GU by using either the method in paragraph 7A or 7B. The method used will depend on whether your supplier lists the extract potential as SG or as a percentage.

7A: Extract potential expressed as SG

If the extract potential is defined in terms of SG, then all you have to do is convert the SG into GU by following the procedure in step three.

Example: The German Pilsner malt that I am using has an extract potential of 1.037, and so its extract potential in terms of GU is 37.

7B: Extract potential expressed as percentage

When the extract potential is defined in terms of percentage (as opposed to SG), a little extra work has to be done.

Background info: A pound of sugar dissolved into 1 gallon of water will yield 100% of its weight as a soluble sugar. As such, sugar has 100% extract potential. Importantly, a pound of sugar when combined with water will yield an SG of 1.046, or 46 GU (Palmer).

Because sugar has 100% extract potential, a grain with an extract potential of 80% will yield 80% of what sugar can yield. Or, in other words, a grain with 80% extract efficiency will yield 80% of 46 GU.

Example: the malted wheat I’m using has an extract potential of 80%, and so its extract potential represented as GU will equal 80% of 46 GU (the extract potential of sugar), or 37 GU.

Because I converted the extract potential of German Pilsner malt into GU in step 7A, and malted wheat in 7B, I can:

Write down: extract potential of German Pilsner = 37GU, extract potential of malted wheat = 37GU.

Note: it just so happens that the grains being used in this example have the same extract potential; however, this isn’t always the case. It will depend on the grain being used.

8. Adjust the extract potential to take into account mash efficiency

As noted above, the extract potential of each grain (expressed as GU) calculated in step 7 is what can be expected if the the mash were conducted under laboratory conditions (read: with 100% efficiency).

But, depending on your homebrewing system, you may only achieve 70% mash efficiency. Because of this, the extract potential of each grain needs to be adjusted for mash efficiency.

Assuming your mash efficiency is 70%, the adjustment would take place as follows:

70% x grain extract potential = adjusted extract potential

So, for German Pilsner malt, the calculation would look like:

70% x 37 GU = 25.9 GU

And, for malted wheat:

70% x 37 GU = 25.9 GU

This means that when mashed at 70% efficiency, a pound of German Pilsner malt will yield 25.9 GU and a pound of malted wheat will yield 25.9 GU.

Write down: adjusted extract potential of German Pilsner malt = 25.9 GU, adjusted extract potential of malted wheat = 25.9 GU.

9. Calculate how many pounds of each grain are required

From step 8 we determined that a pound of German Pilsner malt will yield 25.9 GU and a pound of malted wheat will yield 25.9 GU when mashed at 70% efficiency.

And from step 6 we wrote down that 250.8 GU of our TGU must come from the German Pilsner malt, and 13.2 GU from the malted wheat.

So, to figure out how many pounds of each grain we need in the grain bill, the following formula can be used:

lbs needed = ingredient gravity / gravity per lb

To figure out how many pounds of German Pilsner malt the recipe requires, the calculation would be:

250.8 GU / 25.9 GU = 9.68 lbs

And for malted wheat:

13.2 GU / 25.9 GU = .51 lbs

10. Write down the resulting grain bill

From step 9 we determined that the recipe requires 9.68 lbs of German Pilsner malt, and .51 lbs of malted wheat.

So, your grain bill is as follows:

  • 9.68 lbs German Pilsner
  • .51 lbs malted wheat

Done!

You’ve now learn’t how to calculate a grain bill without the use of brewing software.

Sure, 10 steps may seem like a lot of work – but I can assure you that after doing it a few times it will become like second nature.

My closing words of advice: run through each step again using different grains with different extract potentials

On your next brew day, brew using the grain bill that you’ve calculated. Your math teacher will be proud that you put your skills to good use!

And as always, if you have any questions please post a comment below.

Cheers!

References 

Palmer, John. “Extraction and Maximum Yield.” How to Brew. Accessed August 3, 2015.

Daniels, Ray. Designing Great Beers: The Ultimate Guide to Brewing Classic Beer Styles. Boulder, Colo.: Brewers Publications, 1996.

Home brew beer fanatic and lover of all things Kölsch. Follow me on Twitter!

Join The Conversation ...

I'd love to know your thoughts on this article.
Join me on Twitter or post a comment below to join the conversation right now!


Blog Updates

Enter your email to receive email updates! By signing up you will also be automatically entered into any of my future draws!

Leave a Reply