Using a Hydrometer to Estimate Alcohol Content

I don't know a simple way to measure how much alcohol is in wine or cider or beer that I have made in my basement. But I do know how to use a hydrometer before and after fermentation to estimate how much alcohol I've made.

I will explain this in non-scientific terms. It shouldn't be hard to find a more precise description of how to use a hydrometer to estimate alcohol content. But I hope this qualitative explanation is more user-friendly than most.

A hydrometer is a glass instrument that floats in liquid. A scale on the side indicates the density of the liquid, or the "specific gravity". This tells you how much sugar is in the liquid. My hydrometer also has a scale labeled "potential alcohol". I will refer to potential alcohol rather than specific gravity.


I haven't figured out captions yet. These pictures show my hydrometer, then the 6 percent potential alcohol reading in some fresh cider. Finally you can see the 11 percent reading after I added four pounds of sugar to five gallons of cider.

During fermentation, yeast eat sugar, and leave behind two by-products: carbon dioxide and alcohol. Starting the process with more sugar means the yeast will have more food to eat. It also means they will make more alcohol. Up to a point. Fermentation stops at about 12 percent alcohol no matter how much sugar you put in. Like most living things, yeast can only live in their own waste up to certain limits.

In my experience, fresh cider generally has a potential alcohol of 4-6 percent. That means, after fermentation, the finished cider would have 4 - 6 percent alcohol. By adding sugar you can increase the amount of alcohol in the final product. I find that, in a five-gallon carboy, one pound of sugar increases potential alcohol by about one percent.

Don't bother going above 12 percent potential alcohol. Like I said, fermentation stops at that point.

As fermentation progresses, the potential alcohol in the liquid goes down. When the number reaches zero, you know that fermentation is done. If my 11 percent reading goes down to zero, then I'll know that my finished cider has 11 percent alcohol. At that point the finished product will taste "dry" (I won't be able to taste any sugar).