Despite the amount of brew software that’s out there, it’s undoubtedly useful to be able to do your own brewing calculations.
I’ve been finding out how, and you can too if you want. All you have to do is keep reading!
Volume, efficiency, gravity. These are ambiguous terms.
So after muddling along for long enough, I’ve spent the last few brews getting a handle on the numbers myself.
It turns out that basic brewing maths is not too tricky. And as with most things, spending time looking at it properly clears up a lot of doubts.
Home Brewing Calculations
My idea here is to use an example recipe to describe calculations for water volume, gravity, alcohol, colour and bitterness.
I planned it as a series, but in the event decided there’s just too much overlap between each ingredient for that to make sense.
It’s more logical, I think, to look at everything in one lengthy hit.
Before I go any further, I should point out why I’ve put off learning about this until now. It’s because most of the information out there is in ounces, quarts and gallons.
Much preferring metric units myself, I’ve always found it difficult to follow along. I just don’t have an in-built understanding of, for example, what a cup is.
So this guide is largely for those of you who like metric units, although the principles are the same for everyone.
Apologies if you work in ounces and gallons but I think it’s fair to say you’re already well served elsewhere.
Starting at the End
In my opinion brewing equations are much more useful if they let you work backwards.
Instead of working out how much bitterness 40 g of Kent Goldings gives, it’s more useful to know how many hops you need to get 35 IBUs.
So that’s how I’ve presented the information where appropriate.
OK. Let’s say you’re trying to brew a strong ale with these characteristics:
The rest of this page explains how to work out what you need of each ingredient.
In keeping with the backwards theme, the best place to start is with how much beer you want to end up with.
How Much Beer To Make
To account for spillages and siphoning losses it’s not a bad idea to add an extra litre to your finished target if you’re concerned about an exact volume.
So let’s make 15 litres of beer, planning for 16.
Target batch size: 16 litres
Before you can work out how much water to start with, you need to settle on the grain bill.
Planning Malt and Fermentables
The gravity of beer is usually given as a decimal, 1.085 in the case of the example pale ale.
It describes the density of wort relative to pure water, but it doesn’t tell you how much malt you need.
For that there are gravity points.
Thinking About Gravity Points
It took me a while to get gravity points, misguidedly focussing on gravity instead. Of course gravity is important, but once you shift to gravity points the various gravities (pre-boil, original, final etc.) make a lot more sense.
Gravity points describe the actual amount of sugar in your beer. Unlike gravity, which changes throughout brew day, gravity points stay the same. They are an absolute number that doesn’t change once you’ve extracted the sugar from the malt (unless you add raw sugar).
Here are some illustrations that explain the difference.
First, these two beers have the same gravity, represented by the density of the dots. Because of the different volume the total gravity points (the number of dots) is different.
The beer on the right has less sugar.
In the next example, the beers have the same number of gravity points. The different volume gives the one on the right a higher gravity.
The amount of sugar required to make both beers is the same.
It’s easy to work out how many gravity points you need, if you know your target original gravity and the target batch size:
So for the 1.085 strong pale ale:
Every fermentable you add to the mash contributes gravity points. How many is determined by the extract potential.
White sugar (sucrose) to all intents and purposes contributes 100% of its weight, and is the reference for all other ingredients. It has an extract potential of 100%.
Malt on the other hand, even after mashing, is not 100% sugar. This is obvious, of course, because you throw the husks away afterwards.
You can use a malt analysis sheet, like this one from Briess Malt, to find out the extract potential of a particular malt.
Looking at that chart, find the Extract % next to the malt you’re going to use. FG and CG stand for fine and coarse grain, and describe the size of the crushed malt the % represents.
For the two malts we’re interested in:
Pale ale malt: 80%
Crystal 60 malt: 76%
The pale ale malt should give us slightly more sugar than the crystal, per weight.
Extract potential is a theoretical maximum that you’ll never get from a home brew set-up, nor probably would you want to.
This is where efficiency comes in.
The percentage of potential extract you actually get from your mash is called efficiency.
A good working number is 70%, although as you brew more you start to know how your system works and can adjust the number accordingly.
Now if we go back to sucrose, 1 pound of it contributes 46 points of sugar when added to one gallon of water. Another way of saying this is that it gives 46 ppg (points per pound per gallon).
All else derives from this, so before going any further let’s convert it to pkgl (points per kilogram per litre) with the conversion factor 8.345.
So 1 kg of white sugar adds 384 gravity points to 1 litre of water.
Now you can calculate the pkgl of anything:
But let’s get back to planning the recipe. Combining the above (extract potential, efficiency and the white sugar reference point) you get the equation for gravity points from any malt:
The gravity points in a beer is the total of each malt’s contribution.
As I mentioned above, I prefer to work backwards so rearranging the equation I get:
All that’s necessary now is to work out the number of gravity points that each grain provides.
Of the 1360 total 10% are from crystal 60 malt and the rest are pale ale. That means:
Pale ale malt: 1224 points, 80% potential extract
Crystal 60L: 136 points, 76% potential extract
You can now work out the amount required of each:
We now have a grain bill:
90% Pale ale malt: 5.69kg
10% Crystal 60L: 0.67kg
OTHER MALT CALCULATIONS
At this point you can check what the likely beer colour is and how strong the beer will be.
Colour and strength don’t feed directly into the bitterness calculations, but you might as well get them right now. Then you can tweak the grains, if necessary, before going on.
Predicting Beer Colour
I’m going to skim over this, because by all accounts predicting beer colour is approximate at best.
Personally, I’m not especially bothered as long as it’s roughly in the ball park of what I’d expect from a particular beer.
The metric unit of beer colour is EBC, but the equations are all based around the Standard Reference Measurement (SRM).
There’s a straightforward conversion to EBC which can be done at the end.
To predict beer colour you need to know how many Malt Colour Units (MCUs) you have. These’re similar to gravity points, in that they’re a function of the amount of malt and its colour potential.
The total MCUs are worked out like this:
2.205 converts kg to pounds, 0.264 litres to gallons. I looked at taking those factors out and running the equation through as metric but it seemed to make things more complicated.
If you know an easier way of doing it please let me know!
On the same malt analysis chart you used for the gravity points, get the colour rating in Degrees Lovibond.
Pale ale malt: 3.5°L, 5.69kg
Crystal 60L: 60°L, 0.67kg
Then calculate the MCUs.
Simply add the contributions of each malt to get a total of 31 MCUs.
There are several ways of turning this into a prediction of colour. But cutting straight to the chase, here’s the Morey version:
Which for the strong ale gives:
To convert SRM into EBC multiply by 1.97:
At this point you might decide to change the crystal malt variety to lighten the beer, for example.
Bear in mind that there are so many variables in beer colour such as length of boil, amount of hops, age of beer, etc., that an approximation is the best you can hope for.
If you want to know more, it’s explained clearly and in more detail in an appendix of How To Brew.
Predicting Alcohol Levels
This is a fairly straightforward matter, not worth spending much time on because of its approximate nature.
You can predict the final gravity, the amount of sugar left after fermentation, from the attenuation of the yeast. It seems reasonable to use 75% for every calculation, as this is a rough estimate anyway.
Let’s say our ale uses Windsor yeast:
Predicting Water Volumes
Before moving onto hop bitterness, you need to look at water.
From a practical point of view you want to know how much water to start with. But you also need an estimate of the boil volume to use in the bitterness calculations.
The basic calculation for starting volume is straightforward:
S=Starting volume (to be determined)
B=Target batch size (as explained above)
G=losses to Grain (rate of e.g. 1.1 litre/kg)
E=losses to Evaporation (rate of e.g. 2.1 litre/hour)
H=losses to Hops and break (e.g 1 litre)
You should determine these rates for your own brewing system, but I’ve put my current assumptions in brackets as an example.
Losses to grain and evaporation need a little work before you feed them in:
So for the ale:
Start volume: 26 litres
Planning Hop Additions and Calculating Bitterness
While you could specify hops by weight alone (as in some old home brewing books), the chances are you’ll struggle to be consistent between batches, or to compare one beer with another.
That’s because alpha-acid content, responsible for bitterness, varies from hop to hop.
The first step up from a simple weight is the AAU. This takes into account the alpha acid content:
It’s usually in ounces but it works just fine as a metric value, so long as you don’t interchange without converting.
For example, 40g Nugget with 12% alpha acid content:
If you’re brewing to someone else’s recipe you generally try to match the AAUs. Say your Nugget hops only have 10% AAU, you can work out how many to use like this:
48 g of 10% alpha acid hops is the same as 40 g with 12%.
That’s all very well, but what about when you’re deciding how many hops to use in a recipe built from scratch?
To match relative bitterness levels across a variety of beers you need to be more accurate.
Unfortunately, there’s no simple equivalent of extract potential for hops.
The amount of alpha acid (the main cause of bitterness) that wort gets from the hops is called utilisation. This is dependent on many things such as length of boil, gravity of boil, age of hops, format of hops and more.
Many of these factors don’t work in a linear way so calculation is complicated. In other words, brewing software’s so useful because bitterness is a pain to work out.
Hop Utilisation and IBUS
Beer bitterness is measured in International Bitterness Units (IBUs). This can be predicted in advance if you know the alpha acid content, the amount and the utilisation of your hops.
The basic formula for IBUS is:
%A = Alpha Acid %
U = Utilisation (decimal)
W = Weight in grams
V = Volume (final) in litres
The good news is that the IBU is a metric unit, defined as 1 milligram of hop iso-alpha acid per litre of wort.
Utilisation is not completely understood and there are many proposals for determining it. This excellent article from Zymurgy runs through many of them.
Most often I see recommended the Tinseth method, which is what I’ve started using. It’s described here by Tinseth himself.
The important thing to realise is that for each combination of boil length and wort gravity the utilisation of the hops is different.
In a nutshell, utilisation increases with boil length…
…and decreases with wort gravity…
…although, as ever, advice is always changing and there’s now a theory that high gravity isn’t necessarily the cause of decreased utilisation.
You can use Tinseth’s formula to calculate utilisation manually, or look up the relevant combination in his table. It’s at the bottom of the description.
Boil Length and Boil Gravity
Before we get back to the ale recipe, you’ll see in the table that you combine boil length with wort gravity to get utilisation.
Boil length is self-explanatory: it’s how long you boil the hops.
Gravity is of the boil. To work this out, we need to bring back some information from before: gravity points and boil volume.
I think John Palmer uses the volume at the start of the boil, but Tinseth says to use an average so that’s what we’ll do.
You can work out the average boil volume by going back to the water calculations.
G=losses to Grain
E=losses to Evaporation
For our ale:
With this we can get the boil gravity:
Divide The IBUs
Before working out the weights, you need to make an executive decision about when to add the hops.
For the example, let’s say you decide:
Fuggles: 5% AA, 60 mins, 40 IBUS
Kent Goldings: 4.5% AA, 30 mins, 25 IBUS
Kent Goldings: 4.5% AA, 5 mins, 15 IBUS
Total 80 IBUs
Half the IBUS are added by the Fuggles at the start. The rest are spread between two additions of Kent Goldings that will give flavour and aroma as well.
Now we’re able to go back to the tables and look up the utilisation % for each addition.
Fuggles: 60 mins, 40 IBUS, 0.169
Kent Goldings: 30 mins, 25 IBUS, 0.130
Kent Goldings: 5 mins, 15 IBUS, 0.036
Note that 1.085 isn’t in the table, so I interpolated between the values given.
Rearranging the IBU equation we can work out the amount of each hop addition:
(V is target batch size, not boil volume).
Fuggles 60 minutes
Kent Goldings 30 minutes
Kent Goldings 5 minutes
Fuggles: 76g, 5% AA, 60 mins
Kent Goldings: 68g, 4.5% AA, 30 mins
Kent Goldings: 148g, 4.5% AA, 5 mins
Correction For Pellets
These hop weights are for leaves.
When using pellets, it’s usually agreed you need less of them but there’s no set correction factor.
Randy Mosher recommends reducing the quantity by 25% when using pellets.
Fuggles pellets: 57g, 5% AA, 60 mins
Kent Goldings pellets: 51g, 4.5% AA, 30 mins
Kent Goldings pellets: 111g, 4.5% AA, 5 mins
If IBU calculations are estimates, you may be thinking why bother?
Even if the beer doesn’t actually have 80 IBUS, or whatever (you’re not going to measure it), it is useful to know that it has 5 more than your last batch.
Now we have the final recipe:
Target batch size: 16 litres (to get 15 litres of beer)
Start volume: 26 litres
Pale ale malt: 5.69kg
Crystal 60L: 0.67kg
Fuggles: 76g, 5%, 60 mins
Kent Goldings: 68g, 4.5%, 30 mins
Kent Goldings: 148g, 4.5%, 5 mins
These are home brewing calculations as I’ve come to understand them. Please bear in mind I’m a mere home brewer, far from an expert brewing mathematician or scientist.
Test them out and see if they work on your system. If not, tweak them!
Finally, I’ve built a spreadsheet based on these sums that works everything out automatically and can easily be changed.
Of course I could, and do, use brewing software instead. But doing it has given me a good feel for what’s going on when I adjust recipes.
If you can I’d highly recommend doing the same.