US cups to US teaspoons (cup to tsp)

One US cup is exactly 48 US teaspoons. 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: 0.25 cup x 48 = 12 tsp.

Going the other way, one US teaspoon is 0.0208333333 US cups.

For mental arithmetic: Multiply by 48.

Forty-eight teaspoons to a cup: three teaspoons to a tablespoon, sixteen tablespoons to a cup. The number is only useful at the small end, where it answers the real question: a quarter cup is 12 teaspoons and an eighth is 6. Below that, measuring by teaspoon is how a recipe expresses a quantity too small for a cup to resolve, which is also why baking powder and salt are always given in spoons and never in fractions of a cup.

Precision is worth matching to the input. A measurement given as 0.0625 cup carries one or two significant figures, and reporting its conversion to six implies a precision the original never had.