It's assumed that all jugs from a mould hold the same volume, but not every mould holds the same volume, so by aiming high for the initial design of the mould and then adding spacers to the side of the mould as needed you can achieve a modified mould that stores the correct amount.

DISCLAIMER: I am not a milk jug engineer, that's just how I interpreted it.

It's added after molding the jugs and the volume is checked, but before the milk is filled.

And liters

That very same article also does say however:

On top of that, the indentation allows the manufacturer to precisely control the volume that the jug can hold.

The other end of the umbilical cord goes straight to the cow. After it's cut, thats how you get cow udders!

If you jerk them the wrong way, 14 year old boys will giggle.

Believe it or not, @themeatbridge@lemmy.world is correct, just not about why. It's to adjust for differences in jug size caused by temperature.

Plastic jugs are made by blow molding, where a tube of plastic is warmed, then inflated within a mold using compressed air to create its shape. In winter, the air and environment are cooler so the plastic is also cooler and accordingly a bit less elastic while getting blown. This results in jugs that contract a bit more while cooling and are a bit smaller. To compensate, cool weather jugs have a shallower dimple. The alternative is either warming the air or warming the molds more, both of which cost more, while this actually slightly saves money by using a bit less plastic. The converse is true for summer jugs - bigger dimple, warmer air - as the warmer plastic molds more easily.

The dimple also adds a bit of structural stability, so the jugs can be made of slightly thinner plastic. These factories pump out millions of jugs, so even a $0.005 saving per jug adds up.

I actually did some work for a company that makes plastic containers, so I got it straight from them. Otherwise I'd provide a source. What I could find online that corroborates is low quality local reporting, so I didn't bother with URLs.

It's a US thing. A gallon of milk will last my family about two weeks, or less if the kids are into baking or breakfast cereal that week. I sometimes put a little milk in my coffee or tea, and I occasionally use some for making sauces or marinades. Very rarely will we throw away milk because it has spoiled, but it has happened. Maybe once a year or so, usually because of a power outage or having to travel unexpectedly.

We also have half-gallon plastic jugs which feature the same dent sometimes. When I was a kid, I remember we even had tiny pint-sized jugs for half and half, but I think that was more of a novelty.

the container isnt very hard, it's pretty thin but I've never had one break. It's sold at grocery stores in the US (idk about others) and have only ever seen this kind or the cardboard kind.
Here's an image that's essentially what the milk section at my local Walmart looks like:

collapsed inline media

It really depends how long it will last because sometimes my family will go through two a week and other times it'll stay there for a couple weeks before being thrown out. In general I'd say about one gallon a week for 5 people.

What kind of container does milk come in from where you live?

I have a family of four and we live in the Midwest in the United States; Minnesota to be specific. We will go through 5 gallons of milk in weeks time between cooking and drinking.

Pretty sure it's mostly an American thing, but I'm not well-travelled enough to be sure.

And as for rate of consumption... When I was a kid, my parents made me drink a glass of milk every day - call that 8oz, so 56oz per week. That's a little under a half gallon (which you can also buy in plastic jugs), not including other uses like tea/coffee and cereal. Altogether, we'd go through about a gallon per week.

America fuck yeah

That's fair. You can believe what you want. But that doesn't make it less true.

You're more qualified than me, I've only watched How It's Actually Made

It adds flex to the container for variations of atmospheric pressure. It's what they mean by the fill levels.

Milk containers are filled by weight. The exact volume of the liquid can vary slightly based upon the atmospheric pressure.

If the container has no flex the top will pop off or the sides will crush in with pressure changes say when a truck drives over a mountain pass to deliver the milk.