Whenever someone drinks a beer that you made, they're bound to ask you how much alcohol it has. Maybe they're worried about whether or not they'll be able to drive later, or maybe they want to drink to forget. Either way, in this video, I'll show you how I measure the amount of alcohol in the beer I make.

As you probably already know, the alcohol content of a drink is expressed as %ABV, or Alcohol By Volume. This is exactly what you think it is - the percentage of the volume of each serving that is ethyl alcohol - sweet, delicious ethyl alcohol.

To compute the ABV, we need to understand specific gravity. Specific gravity is a unit-less measure that, in the context of brewing, compares the density of the brew to the density of water. So, the more sugar that's in the brew, the more dense it will be, and the higher the specific gravity.

At this point you should realize that the specific gravity of the brew will be different before and after fermentation, since the yeast will convert some of the sugars into alcohol. Anyway, the gravity measurement prior to fermentation is called the "Original, or Initial, Gravity" and the gravity reading taken after fermentation is called "Final Gravity".

So, the basic idea when computing the ABV is to compare the Final Gravity to the Original Gravity, so we'll know how much of the fermentable sugars were converted into alcohol. This means you'll need to take two measurements, so, if you've already started fermenting and didn't measure the original gravity, sorry, I can't help you.

So how do we go about doing this?

There are two common tools available to the home brewer. One is a hydrometer. It costs around $10 US or 9 Euros. It uses a weighted bob with a scale printed on the side. To use it, just sanitize the vessel and fill it with enough liquid to make the bob float. Then read the specific gravity directly off the scale.

The other is a refractometer, which sounds fancy, but it isn't really. After all, it's just a piece of glass and some plastic. They're not too expensive either. One like this can be purchased for about $20 US. It uses a prism to measure the amount of sugar in the liquid and report it in degrees Brix.

So, what is Brix?

Brix is a measure of the of a liquid's sugar content, with one degree of Brix equivalent to a 1% sucrose solution as a percentage of mass. Also, Brix is named after Adolf Ferdinand Wenceslaus Brix, who is apparently... dead.

To use it, place a couple of drops on the prism, hold it up to the light, and read the Brix value. Also, be sure to take the initial and final refractometer readings at the same temperature, because apparently that matters.

Now that we have our Brix value, we'll need to do some arithmetic to convert it to specific gravity.

SG = (Brix / (258.6 - ((Brix / 258.2) x 227.1))) + 1.0

Quick aside: There are actually three common scales for measuring sugar density. They are the Balling, Brix, and Plato scales, with each being a slight improvement to the one before it. However, the differences are so small that, for our purposes, they are interchangeable.

By the way, degrees Plato is not named after this guy, but after Fritz Plato, who, like Adolf Brix, is also dead.

Which one should you use. Personally, I prefer the refractometer. This is because I typically brew in small batches and it takes quite a bit of beer to fill the hydrometer far enough for the bob to float.

So, once you have your two specific gravity measurements, just plug the values into this equation:

ABV = (FG - OG) x 131.25

Or, at least that used to be the equation. Now, apparently everyone prefers this formula because it is slightly more accurate. Apparently.

ABV = (76.08 x (OG - FG) / (1.775 - OG)) x (FG / 0.794)

There's other variations on these formulas, but unless you're calculating ABV for the purpose of complying with certain legal requirements, these should work well enough.

Let's work these equations for my most recent brew. I'll have Google do the arithmetic for us.

Measuring with the refractometer, I got 16 point something degrees Brix before fermentation, so that gives me an original gravity of ... 1.06543239677

After fermentation I got a Brix reading of approximately 10. Doing some more arithmetic gives me a final gravity of ... 1.04003130566

Now that we have our initial and final gravity numbers, just plug them into the ABV formula. Not that one. This one.

It looks like this brew is a rather tame 3.6 percent alcohol by volume. Well, that's quite disappointing. Oh well.

There you have it. I wanted to keep this video short and to the point. I'll put the equations, as well as links to the equipment in the video description. If you found this video useful, please hit that "like" button. If you didn't, you don't need to do anything at all. Thanks for watching.

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