How many grams are in a cup of pecans?

100 to 120 grams.

A cup belongs to the cooking measures of a "volume".

A cup is 8 U.S. fluid ounces or one half U.S. liquid pint, that is about 237 mL.

A cup is 8 Imperial fluid ounces or one half fluid pint, that is about 227 mL.

A cup is about 250 mL (metric).