The pH of distilled (pure) water is equal to 7.
The concentration of OH-(hydroxide) and H3O+ (hydronium) is equal to 14:
[H3O+]+[OH-]=10^-14
Since we know pH is the -log of hydronium concentration, we can use our value of 7 pH and do. the opposite log (10^-7) so therefore the concentrations of both will be equal to 10^-7