There are different types of "cups" used for measuring volume, but commonly:

• 1 US customary cup = approximately 236.588 milliliters (mL)
• 1 metric cup = exactly 250 milliliters (mL)
• For nutrition labeling in the US, 1 cup is sometimes defined as 240 milliliters (mL)

So generally, for most cooking and everyday purposes, 1 cup is about 236.6 mL if using US customary measurements, or 250 mL if using metric cups.