When did they stop making pennies with copper?

US Pennies were made of 95% copper (with the exception of the 1943 steel cent) from 1864-1982. Midway through 1982, the cent switched to copper plated zinc. So any penny dated 1983-present is just copper plated zinc. However a coin dated 1982 could either be 95% copper or copper plated zinc. A copper coin should weigh about 3.11 grams while the zinc pennies are lighter at about 2.5 grams.