US cups to US quarts (cup to qt)

One US cup is exactly 0.25 US quarts. 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: 6 cup x 0.25 = 1.5 qt.

Going the other way, one US quart is 4 US cups.

To convert without a calculator, multiply by 0.25. Rounding that to 0.25 is within 0% for most purposes.

Four cups to a quart, four quarts to a gallon, two cups to a pint. The US volume system is almost entirely built on repeated doubling, which is why it works in a kitchen without arithmetic.

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