*% alcohol by volume, what does it mean

Various wine, beer, and other other alcohols usually have some print on the label that states

Some value% alcohol by volume.

What is the by volume part all about, I mean, 5% if a gallon of 5% of a pint is still 5% alcohol as far as I am concerned.


alcohol and water(the other major component of most alcoholic beverages) have different densities, so ‘5% by volume’ would be a different strength to ‘5% by weight’

Usually the convention is to express percentages of dry materials by weight and liquid by volume but there are probably exceptions.

Agree with Mangetout.

Note that 100ml of ater plus 100ml of alcohol mixed together do not yield 200ml, it’s somewhat less so I don’t know if the % by volume is calculated before or after mixing.


That would be a neat trick. Where does the missing volume go?

Originally posted by Kamandi

This page has a pretty good basic description of how it works. Basically, it boils down to that the molecules of one liquid slip into the spaces between the molecules of the other.

Well, I’ll be darned. Thanks Red.

If I remember my Chemistry right I think you “lose” 5% of the combined volume when mixing Ethanol and Water.

The ethanol molecules “fit” in the lattace of water’s hydrogen bond structure.

It’s a cool chem experiment get a 100ml graduated cylinder. Put in 50 ml of water and 50 ml of ethanol(denatured). You should get only 95ml of liquid.

I think that is why the azotrope is at 95% ethanol/5% water also.

There are three main ways to measure liquid concentration in fractions:

  1. molecular, and as a subcategory molar conentrations. Essentially, 5% of the molecules are alcohol, 95% water. Used primarily in chemistry and chemical engineering.

  2. weight. The alcohol contributes 5% of the weight of the beer, the rest water.

  3. volume. 5% of the 12 oz of beer is alcohol.

The same can of beer will give three different percentages of alcohol depending on which way you measure it.

Volume is the primary measurement of alcohol. Why that one was picked is beyond me. Like has been mentioned before, water and alcohol aren’t volumetrically additive. If you separate the 12 oz in a can of beer, you get two quantities of liquid that, if added mathematically, will equal more than 12 oz. It would seem mesuring it would be maddening.

But there was probably a historical concentration test that was in widespread use that happened to give the concentrations in volume, everyone used it, and it stuck.

Interestingly, in some states of the U.S., there are “3.2%” (pronounced “three two”) laws that have two tiers of beer (Kansas is one of them). Normal beer can be sold at liquor stores. Grocery and convienience stores can only sell 3.2% beer. Manufacturers have to make special blends that have a maximum of 3.2% alcohol. HOWEVER, the 3.2% is by weight instead of volume. So, the 3.2% is actually closer to 4.1% by normal (volume) measurments.

(Furthermore, Kansas law allows only beer and malt beverages to be sold at grocery and 7-11s, so wine cooler makers have to also make beer coolers (you have to check the fine print to see this), and they have to be under 3.2%alcohol by weight.)

Damn too late very good jobMangetout,Cooking With Gas,RedNexela. Could not have explaned better if I tried. Y’all get yer Stright Dope “Attaboy” for a job Well done.

The non-volumetric additivity and the azeotrope happen to be separate phenomenon.

The non-volumetric additivity has to do with the shape of the molecules, whereas the azeotrope has to do with the activity coefficients of each liquid through the boiling ranges.
Or something.


Thanks. I was not sure if it was a coincidence or not.

I should have looked it up first. They really have nothing to do with each other.

I think you can do the same ‘molecule packing’ experiment with water and salt.

Yes, but not just for that reason. Historically, one measures the alcohol content of beer or wine by distillation; you take a known volume of stuff, distill out the alcohol, and measure its volume: bang, % by volume (actually, it’s more complicated than that; you add water back to the original volume, determine the specific gravity, and calculate % alcohol from that).

However, % by volume also makes sense because we SELL beer by volume. It’s easier to calibrate a piston to a known volume than to adjust it to a weight.

You sound like you know what you’re talking about. Do you work in the beer or distilled spirits industry?

However, your post raises more questions than it answers (at least for me).

  1. All my experience with distillation would say that once you’ve distilled the alcohol from the water, you get two puddles of liquid, one more alcohol than water, the other more water than alcohol. To know what percentages, you need analytics–which is what you’re trying to get in the first place. Is there a analytic distillation so controlled that it “guarantees” concentrations?

  2. I thought the the alcohol in beer was fermented. Is the beer product then spiked with more alcohol (to get it up to whatever percent alcohol is usually in it). It sounds like you imply that it is (or am I reading it wrong?).

Although ‘molecule packing’ describes physically where the ‘missing volume’ goes, one could also say that missing volume ‘turned into’ greater density.

‘Have I’ used ‘single quotes’ too ‘much’?

Does the alcohol and water thing work for all alcohols, or just ethanol?

There are distillation methods that will output alcohol at the azotrope. (about 95% alcohol, 5% water)

However, alcohol content is generally measured by specific gravity, according to this. Also on How Stuff Works is Beer which is quite comprehensive.

Getting either the tops or bottoms to be relitively pure (or at azeotrope) isn’t difficult. Getting both the tops and bottoms is.

If you are doing a batch distillation, it’s easy to get azeotropic alcohol off the top by having lots of stages and reflux and taking little off the top.

When the alcohol is mostly gone, though, more water will make its way out of the top. So, if you cut it off too early, you leave some alcohol in the pot of water, giving you a too low alcohol measurment, and if too late, too high an alcohol measurment.

Specific gravity seems a more logical way of maeasuring alcohol.

That was a good like to HSW. I’ve never seen anyone address the “3.2% by weight” before.

Well, FWIW, lare wineries actually use gas chromatographs to determine % alcohol in their wines and some VERY small wineries employ specially trained people to estimate the % alcohol by taste. I assume larger beer brewers would use this as well.

No, I’m in the pharmaceutical field; I’ve run this technique on acne lotion, though :slight_smile:

Sorry, I’ll try to make this more clear. The goal in the analytical distillation is to push it far enough to get ALL of the alcohol into one puddle; you will end up with some water in it, and that’s OK, because the next step is to add water to the distillate until you’ve restored the original volume. Now, you might say “what was the point of the distillation, then?” The point is to eliminate the various solids, sugars, and flavor ingredients; when all you have is a mixture of water and alcohol, specific gravity (density, more or less) is exactly correlated to % alcohol. Beer, wine, and spirits all contain other ingredients that make the spirit hydrometer a poor tool.

No, I wasn’t saying that.