When the numerator = 1, the denominator is singular:
one-half
one-third
one-quarter OR one-fourth
Note that denominators of 5 or higher are all ordinal numbers:
one-fifth
one-sixth
one-seventh
one-eighth
one-ninth
one-tenth
...and so on
When the numerator is greater than one, the denominator is pluralized by adding -s, with the