There's really no need for sexing koi fish unless you intend to breed them, or you nurse a preoccupation for assigning appropriate names by gender. Because there are so few visible differences between boys and girls, accurately sexing koi is tricky business. Even noted koi experts get it wrong sometimes. Sexing koi before they're fully mature is a shot in the dark at best.
Viva la Difference
While some koi may spawn at 10 to 12 inches in length and a year or so of age, most aren't fully grown until they're 3 or 4 years old. Mature females may be two or more times the body thickness of males of equal length. Males typically display pointed, rather diamond-shaped pectoral fins, while females tend to have more rounded, pan-shaped fins. In the spring, male koi gill plates develop tiny tubercles, becoming very rough, like shaving stubble or sandpaper.
Always a Lady
Koi often display behavioral differences between the sexes. At feeding time, males dash to the surface, slurp up mouthfuls of grub in a frenzy and torpedo toward the bottom of the pool. The much smoother ladies have more refined table manners. The girls mosey on up lazily, gently mouth a few morsels and slowly return to the depths in an oh-so-ladylike fashion. Mature, amorous males chase gravid females during spring and early summer. Because koi don't celebrate Sadie Hawkins Day, you can easily guess which is which.