It’s rare that a given home brew recipe matches your brewing efficiency, hop variety or target batch size. Use these calculations to resize any recipe on brew day.
When planning a brew day, home brew recipe conversion is my first task. Usually I do these three things:
- Resize quantities
- Adjust for efficiency
- Adjust for hop alpha acid percentage
In this article I explain how to do each of them with an example recipe, taken from the International Homebrew Project.
The original recipe for a 23 litre batch asked for:
12 kg pale ale malt (70% efficiency)
East Kent Goldings (4.5%) 152g for 90 minutes
Fuggles (5.5%) 85g for 20 minutes
Fuggles (5.5%) 35g dry hopped
Resizing The Recipe
This is the most straightforward set of calculations. Quantities convert proportionally according to this equation:
Because the example recipe has an especially large grain bill, I want to brew just 10 litres.
Using the above formula I can scale the recipe quantities down:
Hops Kent Goldings 90 minutes:
Hops Fuggles 20 minutes:
Hops Fuggles dry:
This would be my recipe if everything else in my brew set-up matched that of the recipe author. That is rarely the case.
(To convert between metric and imperial units check here).
The next task is to change the malt quantity to match the efficiency of your brewing equipment. If you are in the habit of taking gravity readings each time you brew, you’ll already have an idea of your efficiency.
I won’t go into detail here, but basically efficiency describes the amount of sugar you extract during the mash versus the maximum available for extraction. It’s expressed as a percentage.
Inputting gravity readings into brew recipe calculators such as Hopville tells you your efficiency for a particular brew. In general each brewer is fairly consistent because efficiency is mainly determined by brewing method and equipment.
At the time of this brew I was averaging 60% efficiency so adjusted the malt quantities accordingly. If you don’t do these adjustments, the beer will turn out weaker than you expect (or stronger).
The efficiency adjustment formula is:
In my example (using the scaled down quantities) this looks like this:
I now know how much malt I need to achieve the original gravity of the recipe with my batch size and efficiency.
The last thing to convert are the hop additions.
As you may know, hops add bitterness to beer. The amount contributed depends on the percentage of alpha acids in the variety used.
When you buy hops they include information about the alpha acids in the form of a percentage. Similarly, recipes state the alpha acids percentage assumed when determining bitterness levels.
Often, hops you buy differ from the recipe assumption and you need to perform another brewing conversion:
I’m using 5.3% Kent Goldings and 4.8% Fuggles. These don’t match the recipe hops, so I use the formula to find out how many to use:
Use the hop weight for your batch size, not the amount of the original recipe.
Remember that if your hops have more alpha acids you use less of them, and vice versa.
My new 10 litre recipe adjusted for my hops and efficiency is:
6 kg pale ale malt (60% efficiency)
East Kent Goldings (5.3%) 56g for 90 minutes
Fuggles (4.8%) 42g for 20 minutes
Fuggles (4.8%) 15g dry hopped
Now get on and brew.