The short answer: Grams
In fact, a water-bottle's capacity is generally measured in Liters, since liters are the metric standard for measuring volume (e.g. capacity).
However, the metric Liter was created by finding the volume of one kilogram of water. In other words, a single gram of water is (at room temperature) almost exactly one milliliter. To summarize, 1g of water = 1mL
Since a milligram is 1/1000 of a gram, measuring water in milligrams would result in a bottle containing thousands of milliliters.
Milligrams would make more sense.
medicine
Food ingredients such as salt, sugar, etc.
Medications, small amounts of chemicals, and other tiny quantities are measured in milligrams.
Any unit of measure for mass such as 'ounces', 'milligrams', 'kilos' etc.
One can purchase a Nalgene waterbottle online at a number of different sites, and they can be shipped anywhere in the continental United States. The best place to buy a Nalgene waterbottle would be the Nalgene website.
Due to their small size, you would use millimeters to measure the length of an ant.
Ruler
grams, milligrams, kilograms, all the gram things
Mass. Any mass can be measured in mg. But as there are 1000 mg in one gram (the mass of a small coin) unless you want a very large number of milligrams, it will have to be to measure a small or very low density object.
To convert milliliters to milligrams, you need to know the density of the substance being measured, as milliliters measure volume while milligrams measure mass. For water, 1 milliliter is approximately equal to 1000 milligrams, so 5 milliliters of water would equal around 5000 milligrams. However, this conversion can vary for other substances based on their density.
mg is an abbreviation for milligrams, so your answer would be 3675 milligrams.