The US cup is a unit of volume in the US customary system. One cup is 8 fluid ounces, 16 tablespoons or 48 teaspoons, and half a pint. NIST lists it as 236.5882 mL (0.2365882 L).
Conversion
mL = cups × 236.588
cups = mL ÷ 236.588
For US nutrition labeling, 21 CFR 101.9(b)(5)(viii) defines a cup as 240 mL, about 1.4% more than the exact value.
Example
Two and a half cups are 2.5 × 236.588 = 591.5 mL, or 2.5 × 8 = 20 fl oz, or 2.5 × 16 = 40 tablespoons.
Cup of volume vs cup of weight
A cup measures volume, so what it weighs depends on the ingredient. The cups to grams guide lists examples, and how many ounces in a cup separates fluid from weight ounces. See how many tablespoons in a cup for fractions of a cup. Other countries use cups of different sizes, so a recipe from outside the US may not match.