Most people who drink beer have some understanding of the strength of it. It’s usually measured Alcohol By Volume (ABV), which tells you what percent of what you’re drinking is alcohol. Once you go past 8% you’re generally talking about a barley wine instead of a beer.
The other way of measuring the strength of a beer that you might hear about is the gravity. I didn’t have a clue about gravity; not what it is, what it measures or what it means. But now I do, thanks to some proper brewing, some reading, and my trusty hydrometer.
A hydrometer measures the amount of sugar dissolved in the water or wort. The more sugar there is, the denser the liquid, and the higher the hydrometer floats. Hydrometers are usually calibrated to read 1.000 in fresh water @ 20º C. There are two main readings to take during a brew: the Original Gravity (OG) and Final Gravity (FG).
The OG is taken when the wort is boiled, cooled and ready to pitch yeast into. Depending on the type of beer you’re brewing you can expect to see a reading of anywhere between 1.030 and 1.060.
Reading original gravity. It’s obscured by bubbles in this picture, but this one read at about 1.056
By knowing what the original recipe says the OG should be, and how much wort you’re meant to have before fermentation, you can figure out if you need to add-in any more water. I tend to lose a lot of water in a 90 minute boil, so my readings tend to come out higher than intended. I can’t do much about that until I get a bigger boiler, as the maximum I can get in there safely is about 28-29 litres. In the image above I was aiming for 1.053 so had to add in roughly 3 litres of water.
Once the yeast starts to work and fermentation kicks in, the sugar is converted into alcohol. This effectively thins the beer out and the gravity drops. Your recipe should tell you what the expected FG will be, so after a week or two you can draw a sample and read the gravity again. With a little luck it will have dropped to the target, or as near as possible. If it’s barely budged, you know you’ve had a failed fermentation and need some help.
The beer I’m brewing in the picture above has a target FG of 1.016. But what does that mean in terms of real-world measurements like ABV that we understand? How strong is the beer going to be??
There are loads of calculators on the internet or available as phone apps, but if you want to do it by hand, here’s the formula:
ABV = (OG – FG) * 131
It’s not perfect, but for 99% of brews it’ll give you a very good estimate to within a tenth of a percent or so. So if we plug in the numbers from my latest brew, we should be able to see what strength beer I’ll end up with:
ABV = (1.053 – 1.016) * 131 = 4.847
or rounded up:
ABV = 4.9%
Ideal 🙂