Why is alcohol measured in percentages?

ABV works for all types of packages. It could be helpful to know that a can of beer contains 18.6ml of alcohol, sure, but what about a bottle of whiskey or vodka? Is it better to know that it’s 40% alcohol or 300ml? Should a bar publish that a keg has 4.2 liters of alcohol in it?

With ABV you can compare alcohol across volumes.

I haven't seen anyone really answer the why of it, which is that the industry developed using a floating glass tool called a hydrometer which measures the specific gravity, or density, of liquids.

When you boil the wort to prepare for fermentation, you end up with a sugary liquid that is denser than water or alcohol. Water has a specific gravity of one, and the specific gravity of the wort is increased by everything you dissolved into it. You would float a glass hydrometer in it and lets say you get a reading of 1.055.

After fermentation the yeast has converted much of the sugar to alcohol and decreased the specific gravity. You measure a second time, and multiply the difference by a constant factor to get ABV. let's say after fermentation you got a reading of 1.015.

1.055 - 1.015 = 0.04
0.04 * 131.25 = 5.25% ABV

We label with ABV because that was how it was calculated, and remained the same regardless of the quantity served.

There is a similar process for distilling as well. Before these methods people didn't know the exact amounts, which led to fun things like navy and admiral strength.

Edit: also the 131 figure really should vary based on temperature since it is derived from the ratios of the density of ethanol and water. The higher the ABV the more important it is to factor temperature, and distilling requires more sensitive measurements and tools. But for beer, using 131.25 is fine and has about 0.2% error up to around 10% ABV.

X% of the cans volume tells you the exact amount. You can know with very simple math. The reason why is because historically, alcohol drinks are taxed based on the concentration, or percentage, of alcohol. Beer would be taxed at a lower rate than say whisky. It was all based on the percentage, or "proof", of the beverage, not the actual amount. That way a small and large jug of whisky would be taxed at the same rate.

There are medications that measure the active ingredient as a percentage too, like topical creams. I think it's done to make the relative strength easier to compare when the volume is varied. Labelling a keg of beer or a barrel of wine with the actual volume of alcohol it contains doesn't make it easy to tell the amount that would be in a single glass.

I feel like it would be useful to know exactly how much alcohol is in a can or a bottle

They always list the total volume of the liquid so you can easily get the absolute alcohol content with the percentage.

I don't know, but sugar is usually in "g / 100g" so technically in percentage

Should be relatively easy to figure out if you know both the total amount of liquid and the percentage of alcohol.

100ml bottle that's 40% ABV would be 40ml of alcohol.

As a consumer, I think it makes more sense to have the percentage by volume. Firstly, it makes the general pattern of alcohol content much clearer so we get a better intuition of what we're getting into. Beers generally hover around 5%, wines at 10-15%, and spirits at 20% and above.

Building on that, people don't generally remember the volume of pure alcohol they can handle, but they do remember, for example, that three glasses of wine is the most that they should drink. If you're presented beer, then along with your intuition of their relative alcohol content, you can easily approximate that you can handle about twice as much volume since it's about half the alcohol by volume. And similar if you have spirits, then you can reduce that volume to half or less.

How would you estimate it given a volume of alcohol instead? I know I can handle three glasses of this one particular kind of wine. If I have another type of wine with a different alcohol content and different bottle size, I'll have to math out how much alcohol will be in my glass: (Volume of glass)*(total alcohol context)/(Volume of wine bottle). Most people would give up before even trying to come up with this formula. Now how much alcohol did I have the last time when I had three glasses of wine? The bottle said something like 80ml of alcohol. But how big was it? That's two large numbers that you need to remember, and since they're so different from one bottle of wine to the next, you likely don't remember any of it.

Secondly, it affected flavour. In general, you can taste how much alcohol is in a certain drink and that depends on the relative volume of alcohol, not the absolute amount.

Since krellor answered the main question:

I feel like it would be useful to know exactly how much alcohol is in a can or a bottle.

• By volume: multiply ABV by the volume of the drink.
• By weight: do the maths above, and then multiply the result by 0.789 g/ml.

RL example: the beer in my fridge is 350ml, 4.7% ABV. Doing the maths this is roughly 16.5ml of alcohol, or 13.0g.

Since everyone pretty much nailed the ABV% answer I’ll take a crack at the other. I’m guessing that the reason they don’t list other ingredients, like what food producers are required to do, is because most countries are concerned about the alcohol content more than anything else that’s in it.

Australia does this right. Everything has a percentage.

Nutritional panels have:

• Serving size
• Servings in the pack
• Energy, sugar, fat salt. etc per 100ml or 100g, and per serving.

Alcoholic beverages have "standard drinks" per bottle which factors in ABV and volume.

I can quickly see that a drink has 9g sugar per 100ml and know it's 9% sugar. Easy.

A percentage is going to be more useful to a consumer than a volume anyway, especially for bottles that are consumed in part. 40% tells you a drink is going to be strong, without any further math based on whatever size drink you have poured yourself. If you wanted to know the volume alcohol of a poured drink, you'd have to linearly interpolate according to the size of glass you poured yourself, which is one step beyond just multiplying the alcohol percentage by the volume of the glass.

So in conclusion, labeling the volume of alcohol on a bottle is only practical for beer bottles and prepackaged cocktails, etc. If you do want to know the volume of alcohol consumed, simply multiply the volume of drink consumed by the ABV%.

Don't most consumables list their volume?

1liter of beverage at 5% ABV is 50ml of alcohol. For example

Labeling alcohol content by %ABV or %ABW is more versatile than absolute measure. Its also trivial to figure the absolute content given a specific volume.

In the US at least, there are regulations for packaging and labeling enforced by a federal agency. (I think that agency is Alcohol, Tobacco and Firearms (ATF), but I could be wrong.) The label must include the %ABV or proof, and must also specify the volume. Every distiller and brewer must file a label for approval of every product and pay a free before the product can be packaged and sold. The fees are not trivial, and the approval takes time.

In the US, you can purchase liquor in bottles from 50mL all the way up to 1.75L, beer from 8oz (customary) to 750mL, and wine from 333mL up to a magnum (which volume I don't know off the top of my head), all with many sizes in between

Allowing for labeling the alcohol content in %ABV means the producer can package the same product in different sized units using the same approved label across the line. It saves on fees and administrative overhead.

edit to add: Another nuance of alcohol production and distribution in the US is bonding. There's a federal tax on all alcohol products made for consumption that is paid by the producer when the product leaves the production facility. Product is psckaged and held "in bond," a specific area of the facility where the product awaits sale and shipment. When it leaves Tue area, it must he recorded and taxes paid. It's serious business, and the ATF can sometimes pull licenses even for minor infractions.

To keep the math and record keeping easy, the producer tracks the number of units releaeed from bond, and usually reports and pays the duties on it quarterly. Since the duties are assessed by absolute volume, the duties are calculated by %ABV times times number of units.

There are special rules for labelling for alcohol given the obvious risks of consuming more than you realised.

Here alcohol must show the percentage alcohol content, but alcohol designed to be consumed as-is (without mixing) also states the number of standard drinks.

The number of standard drinks is the same as the "amount of alcohol" you mentioned.

Other ingredients like sugar aren't shown as a percentage because manufacturers don't want to show it, and it's not required by law because although there are risks of excess consumption, they're not "immediate risk to life" type risks.

You wouldn’t take a shot of beer with 5% ABV

Nor would you take a pitcher of vodka

To add to this, %ABV is what's actually measured by distillers when distilling, unless they're doing it entirely by smell/taste. The first and last parts will have less alchohol since everything evaporates off at different temperatures. (the tail is mostly water. the head, IIRC is mostly methanol or whatever else.)

The total amount of alchohol changes depending on how it's packaged or sold off, or if you're pouring a double, so people will need to do math anyhow; unless it's a single serving package. (and even then... they might not drink it all. blashphemy, I know.)

I doubt it will be of any benefit. How would it help if you knew that bottle of liqueur contans 125ml of pure alcohol? Or that bottle of Sauvignon Blanc contains 98ml? Or a can of beer 15ml?

I suppose you need to multiply. And you need to factor in all of that additional info anyways. How much wine do you drink? The whole bottle? How much alcohol can you tolerate? (What do you weigh...) And how much sugar is in the beverage, because that also affects things and is extra work for your liver, too.

But it's a good question. And sugar, caffeine etc are in fact measured in percentage. At least in europe you always have a measurement per 100ml or 100g written on the packaging, which is kind of the percentage. Interestingly enough it's not written on wine, liqueur etc.

I think alcohol can't be kept pure in normal conditions, it would turn into a gas. So it is diluted by necessity. Knowing the full volume of the thing is certainly better than knowing only the quantity of alcohol without knowing the degree of dilution.

It is not the only product to be indicated this way. All diluted chemicals are. I think syrup is also?

Just thinking. Maybe there's a non linear relation between the uptake and the amount of alcohol. As for other products, they usually have a nutritional information table per 100g that you can thus read as percentages.