Nearly every time I share a homebrew with someone–after they bow down at my feet and praise me for making such a glorious creation–inevitably, they ask me:  “How do you determine how much alcohol is in this work art?”  Seriously, I am posed this question at least monthly.  As I explain and begin throwing out essential terms such as Original Gravity, Hydrometer, and Flux Capacitor, I often see that deer in the headlights look.

In fact, I have spoke to several home brewers who, while familiar with the terms, have no clue what they mean, or how to properly use a hydrometer.  I know I didn’t some years ago!  So, let’s clear it up.

What is gravity when we talk about beer (and other liquids)?  Specifically, what is specific gravity?  Simply put, it is the density of water:  a measurement of the density of water.   Just like there is a scale to determine key temperatures of water (120 degrees Fahrenheit/ 100 degrees centigrade) there is a scale for density.  The specific gravity of water at a given temperature is (i.e 60 degrees)  1.000.

Obviously beer is not just water, and neither is pre-beer (or wort, un-fermented sugar water).  If a brewer is making beer, starts with water (1.000 on the scale) and adds sugar extracted from malted barley, the specific gravity of the water (now a solution) will rise.  But how does one know how dense this new solution (wort) is?  That’s where the hydrometer comes in.

I once spent half a summer in Hawaii.  I spent nearly everyday in the Pacific, surfing, swimming, snorkeling, and practicing my underwater dance routine.  After 6 weeks of bliss, I headed home.  With an overnight layover in LA, I took a dip in the hotel pool.  Unaccustomed to swimming in water that lack salt, I sank like lead.  Why?  The Pacific ocean was more dense than regular H20 and had provided my body with buoyancy.  A hydrometer is like my body, only much less sexy.  It is placed in a solution like wort, and, depending on the density of the solution it will float lower or higher.  There is a scale on the hydrometer that the brewer observes.  If the surface of the wort crosses the 1.040 mark on the hydrometer, the brewer knows that his/her beer has a potential for 5.1% alcohol after fermentation.

But a beer never ferments back to 1.000, and thank God…that would be one nasty beer.  There would be no residual sweetness left to balance out the bitter hops.  So after fermentation the brewer takes a second reading.  Why a second reading?  Well, fermentation is basically the consumption of sugar by yeast with alcohol as a byproduct.  After fermentation, the wort gets closer to density (gravity) that the water was before sugar was dissolved in it.  Let’s suppose the wort fermented down to 1.010.  The brewer can see that there is approximately 1.030 of alcohol in the brew (3.6% ABV).  A more detailed formula is used by many:  1.040 – 1.030 =.03 X 129 = 3.87% ABV?  Ive met a couple of home brewers (and one mead enthusiast) who assumed that the first reading of the hydrometer equaled the final ABV of the product.  The mead brewer (who brewed his stuff ridiculously sweet) was sorely disappointed to learn that his mead wasn’t as boozy as he’d assumed.

Make sense?  I hope so.  I tried to make this simple and easy to understand, knowing full well that I could write a chapter on measurements of alcohol in beer, scales, tools used, etc.   Cheers!

(Oh yeah…if you know anything about the first picture in this article, you will know that the brewers behind that beast must have had a really big hydrometer.  I wonder if it’s true what they say about brewers with big hydrometers….)