If you don't already know your Havaianas size (the Havaianas Brazilian size is printed in a circle just below the Havaianas logo) then it's best to use the fit guide below. Generally, Havaianas run bigger than the US size that is printed on them as all Havaianas are printed with a men's size, not a women's size. Some Havaianas now also have a men's size ("M" after the US size) and women's ("W" after the US size) printed on them, however, these are still inaccurate sizes. The "W" does not mean it's a wide shoe.
Below is a general guide and actual sizes may vary.
Black & Black Diamond Cluster