Ounces to cups (fl oz to cup)

One US fluid ounce is exactly 0.125 US cups. That ratio belongs to the customary system itself and is much older than any link to metric: the 1959 international yard and pound agreement later fixed the yard and the pound against the meter and the kilogram, but it left the ratios between customary units (feet to the yard, ounces to the pound, cups to the gallon) exactly as they were.

Worked through: 12 fl oz x 0.125 = 1.5 cup.

Going the other way, one US cup is 8 US fluid ounces.

For mental arithmetic: Divide by 8.

Divide by 8. The direction that turns up when a container is labeled in fluid ounces and a recipe is written in cups: a 12 fl oz can is 1.5 cups, a 16 fl oz bottle is exactly 2, and a 32 fl oz carton is 4. Worth knowing that the US legal cup used on nutrition labels is 240 ml rather than the customary 236.588, so a label's 'cup' and a measuring cup's differ by about 1.4%. That is immaterial for cooking, and it is the reason label servings look rounder than they should.

Precision is worth matching to the input. A measurement given as 1 fl oz carries one or two significant figures, and reporting its conversion to six implies a precision the original never had.