It’s been almost a week since I kegged my beer and I am finding that waiting is the hardest part. I am anxious to let the nectar flow and find out how it tastes… but patience is a virtue.
So in the mean time lunch breaks at work are spent reading brewing blogs, forums and books in an attempt to continue absorbing as much information as I can.
One point that I‘ve found comes up often is the question of working out the alcohol content of your home brew.
It is a valid question, but I’ve actually found it pretty hard to pin down a precise answer – as different brewers seem to favour different formulas.
However, one constant in the alcohol maths is the main tool you use when gathering the numbers – of course, I’m talking about the hydrometer.
As you’ll no doubt be aware the hydrometer is used before fermentation (to find the original gravity – OG) and after fermentation (to find the final gravity – FG).
It is the difference between these readings that form the building blocks you require when working out how much alcohol has been produced by the yeast – when they’ve been snacking on the sugars in your beer.
The formula I have settled on for predicting the ABV (alcohol by volume), and one that I am fairly confident works well, is as follows:
ABV% = ((OG-FG) x 105) x 1.25)
Full disclosure: I’ve yet to find information explaining why 105 and why 1.25, but there we are.
With my recent extract home brew, the OG was 1.043 and the FG settled out at 1.012. Using the above formula this means that I have a beer on my hands (ok, in my pressure keg… don’t worry I’ve not contaminated the beer by sticking my hands in there) of around 4.06%.
ABV Reference Table
The reason I am happy with this formula (aside it being fairly straight forward – I should have paid more attention in maths at school!) is that in John J Palmer’s book “How to brew” I found a reference table for working out alcohol content.
Now, although this table is a general overview and should be taken with a pinch of salt, it does give you a nice reference point for you to check whether your formula conforms – the formula I use does.
Based upon John J Palmer’s table, I have made my own (apologies, it is rather crude due to width issues):
This is based upon table 10 in How to Brew: Everything You Need to Know to Brew Beer Right for the First Time by John J Palmer – page 98.
This table uses the formula stated earlier in the post and is for reference only – don’t take it as gospel.