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EDIT: The coke bottle I am referring to in the question is 1.25L.

Why are soft drink bottles round, but milk cartons square?

The milk carton is more utilitarian, being more space efficient, and having a handle for ergonomics. Why then have soft drinks not followed this design?

Is it a matter of aesthetics?

Could it also be to allow the user to drink direct from the bottle more easily?

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Note that the coke is under pressure, whereas the milk is not. The roundness of the coke bottle helps equalize the pressure across the surface. If you where to package coke in a milk bottle, the bottle would deform due to the pressure. –  Dan D. Jul 20 '13 at 13:36

6 Answers 6

up vote 16 down vote accepted

Basically, coke with its gaseous nature, exerts more pressure per volume than Milk and to balance it out, round shape fits it well for coke, because round bottles take the pressure evenly, while square bottles get the corners chipped due to uneven pressure.

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+1 ah, the physics. I forgot about that. This is the correct answer. –  obelia Jul 20 '13 at 17:44
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...and to give the other half of this: the reason milk cartons aren't round is that they pack into a smaller volume. (Milk used to be delivered in round glass bottles, which were round both to resist impact and because making square glass bottles is more of a pain in the neck.) –  Alex Feinman Jul 22 '13 at 13:42
    
Alex brings up an excellent point; making a rectangular paper-based container is much simpler than making a round paper-based container (fold it and glue it). I'm honestly not sure about which is easier for plastic; I suspect that with a mold it wouldn't matter much. –  Brian Jul 22 '13 at 20:20
    
There is no problem with coke in a square bottle similar to a milk carton but try filling a milk carton with coke and giving it a shake... –  bendataclear Oct 9 '13 at 15:08

In addition to what is already being said --

The coke bottle design with the round and slenderness was based on the design principle called anthropomorphic design (a tendency of humans to find forms that appear humanoid or exhibit human like tendencies to be very appealing)

When coke bottle with its slender feminine contours was launched in 1915 it was a considered a breakthrough from the square bottles of soda. It was popularly referred to as Mae west bottle. Like obelia said youth , vitality , sex appeal are all part of grabbing attention and genetic predisposition gave it an edge.

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If you think of the functionality:

Milk cartons

  • They are used to pour the liquid into somewhere else
  • People don't really drink directly from the container. People pour the milk into a coffee, cereal, etc.
  • The handle allows the user to pour it more precisely.

Soft drinks

  • People usually drink directly from the container - Specially the drinks under 500 ml.
  • They need to be resistant and also have a reliable lid because people will put it in their bags, etc.

Moreover,physically:

  • You are able to fit more liquid in a square container than in a round one.
  • Square containers are easier to pack and stack
  • Round containers are more ergonomic to hold than the square ones
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Actually in the situation I'm referring to, the coke bottle is 1.25L. I'll update the question. –  Rich Jul 20 '13 at 13:27
    
Also, for the physical points you made, milk cartons are easily fitted into fridge doors. That wouldn't work if they were round. –  Brendon Jul 20 '13 at 14:19
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@Brendon: It would still work, but they'd be less space efficient. Where you get problems is when you put drinkable containers with tiny legs in a refrigerator with a wire rack (rather than solid surface). Such legs inevitable slide into the gaps within the rack, tipping over the drink. Thus, Gatorade rarely tips over, but Mountain Dew often does. –  Brian Jul 22 '13 at 20:17

Milk is milk, but there are hundreds of different kinds of soft drinks. Milk is a staple, a commodity, a utilitarian foodstuff while soft drinks are a lifestyle, helps define one's identity and increases the attractiveness and sex appeal of it's consumer.

The aesthetics and branding of soft drink packaging has much more impact on its sales that that of milk.

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Just because it is the norm, it doesn't mean that somebody made it so because they thought it was superior design. It can be just convention.

Coke bottles are plastic today, but used to be glass until a few decades ago. Glass bottles were invented round, because they were made by manually blowing liquid glass, and it is very hard to blow a non-round bottle. In later times, processes for non-round bottles were invented, but they are typically used with expensive drinks (whiskey, some liqueurs) where a production cost for the packaging is not going to have significant impact on the product price. Normal glass bottles for wine, beer, soft drinks and milk (!) stayed round in the industrial age. And when coca-cola became popular, it just used a variation of the standard.

By now, the original constraint for creating a round bottle is long gone, as it is not even made of glass any more. But by the time Coca cola switched to plastic bottles, the existing bottle shape was already an iconic part of the brand. So they have a very good reason to not switch to a canister-style bottle.

As for the milk, the public has long accepted the switch from glass to non-glass packaging. I don't know whether it was possible to create food-safe (some of the baddies used in plastic production are fat-soluble), non-transparent (light degrades milk but not cola) and light-weight containers shaped like a milk bottle at the time the switch took place, but the manufacturers opted for certain materials (like lined heavyweight paper) and for the shapes most efficiently done with these materials. As they were not bound to a conventional shape any longer, they could start exploring functional changes to their design without alienating their customer base. By the way, the old-fashioned milk bottle still has iconic value, and gets used by premium brands for this reason, even though the containers are functionally inferior.

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It's worth mentioning that the diameter of the bottle is determined by the shape and size of an average adult human hand.

Also holding a round shape is much more comfortable than holding a square container. We tend to poor milk in a glass or cup whereas (thanks to advertising) we're encouraged to grab the bottle and drink from it.

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