1 cup = 250ml per flask, 1 Pint = 500ml per flagon. 1 bottle = 1Liter. Doctors recomended drinking 2 Liters of water minimum perday. Avoid alcohol if you can.

The odd shapes are most likely made by using random large scraps from shoes or pouches, or possibly failed parts of those. Stitching holes go wrong while poking sewing holes the toe piece of a shoe? Trim the edges, slap two fails together, sew it up and it's a salable flask.

Some of them are shaped unusually because they were using left over pieces of leather to make them.  I've seen  a few that would maybe hold half a pint to a pint of liquid, to a couple that would hold over a gallon.

Great video, and I'll look into it. It might be ease of manufacture/carrying, or wanting to maximize volume (so spherical ish). If it needs to stand up on its own maybe a cylinder is better? Sometimes they might have mimicked the shape of a cows stomach as a throwback to another type of water carrying vessel? Often though, being human, they might have just made nice shapes that pleased them?

Actually reading LOTR for the first time now, I’ll keep your question in mind and keep a tab on how many times Frodo and Sam fill their water bottles.

“If you care about what they did to LOTR”.

Don’t forget what they did to The Wheel of Time as well.

Just some words for the old algorithm.

There is a channel named Townsends that is about colonial american re-enactment. He has a great video showing how to make a leather water bottle.

In reference to the bottle swinging about a lot at 11:20, there is a way to stop that depending on what clothing is worn.
If you have a belt on the outside of all clothing, unlike Kramer's waistcoat, you can wear the belt over the top of the straps and hold them in place. It's what a lot of soldiers did in the past when they had musette bags and water bottles on shoulder straps.

I love this topic, hydration is mega important and it's often overlooked in fantasy

Both a canteen and a costrel are a type of bottle. The canteen is specifically a flattened one (often a disc, but can be other shapes, such as the modern rectangular military ones, or the tiny black leather one you have) designed to be carried on your person. The difference between a costrel and a canteen, is that a costrel is a cylinder (or barrel shaped) with the opening on the lateral curved surface, rather than than on the circular part like a modern can. And as you said, typically a costrel is specifically for wine... But humans being humans, would have used it for all sorts of liquids.

I can not say much about the reason behind the shape from an historical perspective, but I can give some insight from the physical one.
The closer a bottle's shape is to the the one of a sphere, the better at retaining the temperature of the liquid inside it is. This happens because the sphere is the smallest surfice you can have for a fixed volume, and the smaller the surfice is, the less heat will be transfered between the liquid inside the bottle and the external ambient.

I strongly agree with the idea of having a smaller bottle as a "first line" kit, for those who are familiar with the "three lines" theory of combat gear. Much like carrying a 1 liter canteen on your chest rig/carrier, carrying a smaller costrel with your base larp kit as you indicated is a great idea.

Because of my own experience, I tend to carry what might be considered an excess of water (3-5 liters capacity regardless of distance). I have burned 10 liters of water in a day, carried with 5 liters of capacity between canteens and a bladder. This is a full, full day of hiking with high temperatures, but still. 5 liters of water in a day is well within reason, especially if you live in a mountainous region where you are pushing hard hills, and think there would be lots of water, but often you'll be surprised how difficult it is to find. Better to have an excess of water than not enough. You'll be amazed how running out of water will affect your morale and immediately make you feel thirstier.
Great video, cheers.

I literally just lost half my water on a hunt due to a leaking bottle before seeing this video lol. I had actually been considering bringing my Bota waterskin, but switched to a metal bottle, because (ironically) I feared the waterskin might leak on my gear.
Glad to have ye back!

For the algorithm!

If you are a musician/fidler/bard you need a bigger bottle to fit all the mead. Who knows how far it is to the next tavern/inn? Well, I do, I've been all over the world taking notes. The best one is "Gyllene Hjorten", it's in the middle of "Björkskogen", if you ever should be passing. Good food too!

Congratulations and many blessings to you.  Great to have you back!

Welcome back Kramer,hopefully you and your wife had a nice honeymoon! 👍
A flask is perfect for divine water,like aqua vita! 😁 Gives one strength and fire! 🍸

Welcome back, and congrats on the wedding!

Congratulations on getting married buddy, I wish you many fortunes in the years to come!