eli5 – How does the % in alcohol work?

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Is 13% of a bottle of wine 13% full of alcool or is it another formula?

In: 4

The concentration of ethanol is independent of the volume poured/left in the bottle, since it’s a [hopefully] homogeneous solution

If a wine is 13% alcohol then every sample of that wine will be 13% alcohol. Therefore in a whole 750ml bottle there will be 97.5ml of pure alcohol along with 652.5ml of water and whatever else is in the wine, and in each 6oz glass of wine there will be three-quarters of an ounce of pure alcohol.

Yes, 13% of the bottle is alcohol, the rest is what is left of the grape juice that the wine started as.

But while you’re here, there are two ways of figuring the percentage: alcohol by volume (ABV), or alcohol by weight. Because alcohol is lighter than water, 13% alcohol by volume would be only 10.3% alcohol by weight.

As long as you always use the same measure, it’s not important, but the US tends to use alcohol by weight, so their wines are listed as weaker but may be just as strong or stronger.

For the chemistry lesson: % can be calculated a few different ways, but in the case of liquids, it is just as you say: what is on the bottle is 13% ethanol, and 87% everything else (mostly water).

When trying to calculate it yourself, % is always measured as X milliliters of the ingredient in 100 ml (literally, *per cent*) of the whole substance. 1% is 1ml in 100ml; 2% is 2ml in 100ml.

So if you were wondering how much alcohol was in 100mls of this wine, the answer is 13ml. If you had a double glass, the total volume of just alcohol would be 26ml.

If you were trying to make a cocktail, you could calculate the final alcohol concentration by figuring out how much came from a shot of the liquor: (volume of the shot) x (percent of the liquor) / final volume of the drink.

Eg., an 8oz vodka tonic made from a single 1 oz shot of vodka (40% ABV) would be:

(1)(0.4)/8 = 0.05 = 5%.

Why would I bother with the math? Because medicine refers to *standard drinks* when discussing how bad alcohol is for you, but a standard drink is a lot smaller than what we are served in bar or serve ourselves at home. It’s a good idea from time to time to check just how much alcohol you are actually consuming against what is recommended (just as you should with calories).