A flood so large that experts believe it has a 1 percent, or one out of 100, chance of occurring in any given year. Nevertheless, the term refers to the flood's size, not how often it occurs. Several 100-year floods can occur within the same year or within a few short years. Experts designate the size of a 100-year flood for specific areas; the actual size will vary from one place to another. Other measures include 10-year floods, with a 10 percent chance of occurring in any given year; 50-year floods, with a 2 percent chance of occurring in any given year; and 500-year floods, with a two-tenths of 1 percent chance.