It depends on the date.
- Indian Head pennies dated 1864 to 1909 weigh 3.11 grams.
- The Lincoln, Wheat Ears Reverse penny weighs 3.11 grams except for steel cents made only in 1943 that weigh 2.67 grams.
- The Lincoln, Memorial Reverse penny (1959 to mid 1982) weighs 3.11 grams.
- The Lincoln, Memorial Reverse penny (mid 1982 to present) weighs 2.5 grams.
It's not easy to tell the 1982s apart, so to figure out how many cents weigh a pound (or any other amount) you'd have to decide which date range you wanted to use and eliminate all 1982 coins. A pound is 453.6 gm so just divide that by the weight of the penny date that you decided to use.
See Related Links, below, for a list of all current specifications for other U.S. coins
Read more, below about the various pennies.