As I said in the comments I think there is something wrong in your preamble as, for in-lined mathematics, the default behaviour of \frac is what you want. To demonstrate this consider the example:
\documentclass{article}
\usepackage{amsmath}% you need this!
\begin{document}
\textsf{frac}: Which of the following fractions is smaller than $\frac{1}{5}$ and
bigger than $\frac{3}{7}$?
\textsf{tfrac}: Which of the following fractions is smaller than $\tfrac{1}{5}$ and
bigger than $\tfrac{3}{7}$?
\textsf{dfrac}: Which of the following fractions is smaller than $\dfrac{1}{5}$ and
bigger than $\dfrac{3}{7}$?
For displayed equations \verb|\frac| defaults to \verb|\dfrac|:
\[ \textsf{respectively, frac, tfrac and dfrac:}
\frac{1}{5}, \tfrac{1}{5} \text{and } \dfrac{1}{5}.
\]
\end{document}
This produces:

As you see from the image, the fractions are centered in the text. Perhaps you are missing \usepackage{amsmath} or perhaps you are loading another package which redefines the default behaviour of \frac. Unless you give us a minimal working example we won't be able to tell you what the problem is.
\frac{1}{5}produces exactly what you want. So, I think that something in your tex file is changing the default behaviour. This said, if you are using the amsmath package then\tfrac{1}{5}will almost certainly work --\tfracforces the fraction to be typeset in "text" mode. There is a corresponding\dfracfor forcing the fraction into display mode. – Jun 02 '15 at 15:46