Carton of juice in the UK, although I'm in the south it might be different up north...

US: a "carton of juice" is usually a quart (about a liter) of juice that comes in a cardboard box. They are usually not shelf-stable and need to be kept in the refrigerator or else they will spoil. a "juice box" is about 200mL of juice that you give to children for snacks or their lunch boxes. (Sometimes adults drink them.) They are shelf stable and can last months without refrigeration.

I'm from the northeastern US, and I agree with Don. In my area juice boxes are always single-serving packages that are shelf-stable. Cartons of juice are NOT shelf-stable; they must be kept cold or else they may spoil within a few hours.

I would say carton of juice in the U.S. A juice box is small carton containing a single serving and might commonly be placed in a child's lunch box. It usually comes with a straw.

Carton of juice in the UK, although I'm in the south it might be different up north...