English apostrophe rules are not difficult to master. Just remember that all possessives need an apostrophe and an "s" at the end. If the word already has an "s," it only needs an apostrophe. If the word does not already have an "s," it needs the apostrophe followed by "s."
So a name or other singular noun that ends in “s” (like “Chris”) is usually made possessive with the addition of an apostrophe plus a final “s” (as in “Chris's coat”). ... The general rule extends to proper nouns, including names ending in s, x, or z, in both their singular and plural forms.