1/8 inch, 3/8 inches. Singular vs plural is not a matter of measure but rather the number of units being referenced. In the matter of fractions, while both are less than one (1) inch, 1/8 is one of eight parts, therefore, a singular "inch" should be used, whereas anything more than one of eight (i.e. two, three, four... of eight) is plural and thus "inches" is appropriate. 2/8 inches = 1/4 inch. In similar manner for decimals, 0.1 inch, 0.2 inches, 0.11 inches.