What is the percentage of silver in a silver half dollar?

All half dollars minted up to 1964 are 90% pure silver. Those from 1965 to 1970 have a core made of 80% copper and 20% silver, with outer layers that have the opposite percentages for an overall content of 40% silver. All halves minted after 1970 (except those included in silver proof sets) are copper-nickel and contain no silver.

US Mint "Prestige" proof sets include 90% silver Kennedy Halves, although all others versions of the coin including standard proofs are made of copper-nickel.