Plurals

Q. I am editing a textbook for English students in Brazil. One of the exercises presents a recipe for pumpkin pie. Students are told the pie filling contains 1 1/2 cup pumpkin, 1 1/2 cup sugar, and so on. I seem to remember that anything greater than 1 should be plural. Am I correct? In other words, should the recipe read 1 1/2 cups?

A. Yes, make those measures plural—but that’s way too much sugar.

[This answer relies on the 17th edition of CMOS (2017) unless otherwise noted.]