Question

Would a teaspoon be a good way to measure the capacity of a milk carton

Answer 1

No, a teaspoon is too small to measure the capacity of a milk carton. You would use a quart to measure the capacity of a gallon.

Hope this helps :)

Answer 2

Many containers and cartons are measured in millimeters; a teaspoon is traditionally 5 mL. So as a unit to measure capacity: Yes, they would be good.

However in a 2 liter bottle (common size) there are 2000/5 = 400 teaspoons.
While this is easy enough to write on a label- indeed, soft cans are often 355 mL, so there is no real difference bar of that unit- it is extremely difficult to measure out 400 teaspoons accuralty. As a way to fill that capacity (i.e. "measure it out"): No they are terrible. <span />