All booze has an “Alcohol by Volume” measure specified. It’s denoted as a percentage which is supposed to tell you “how much alcohol” there’s in the specific drink, or, alternatively “how fucked up are you going to be and how fast”. Beer is typically between 4%-10%, wine 12%-14%, vodka and whiskey 40% and so on.

But how do they measure this quantity? How do they know exactly how much alcohol is there in a bottle of beer?

### General process

The alcohol in beer is created by **fermentation**. Yeast is eating up the grains in the beer, making alcohol (ethanol) in the process.

The density of ethanol is known, so in order to tell the amount of ethanol in the beer, we measure the overall density of the beer before and after the fermentation, and then deduce the amount of ethanol in the beer.

### Density

“Density” is a measure of how “heavy” something is for a given volume. Imagine two identical boxes, one with nails and one with flowers. The box with nails will probably be heavier, so intuitively we can say that nails are more “dense” than flowers.

We use “Specific Gravity” to denote the density of beer. Specific Gravity is a unit of how dense something is relatively to some kind of “standard” density. For liquids, the standard is usually water. So water has a specific gravity of “1”. Something that’s twice as dense as water has the specific gravity “2” and so on.

### Measuring the density of beer

A tool called Hydrometer is used to measure the density of a liquid. It’s basically a tube with a weight at the bottom. You fill the tube up to a certain point with the liquid you’re interested in measuring and the hydrometer shows the density by measuring the weight of the liquid.

### The calculation

Now, since we know the density of ethanol and we have the two measures of density for the beer – before and after fermentation, we can use a simple formula to tell the Alcohol by Volume:

( ( 1.05 x ( OG – FG ) ) / FG ) / 0.79 x 100

Where “OG” is “Original Gravity” – the density **before** fermentation, and “FG” is “Final Gravity”, the density **after** fermentation.