\documentclass{article} \usepackage{xparse} \let\RealFrac\frac \RenewDocumentCommand\frac{mg}{% \IfNoValueTF{#2} {\RealFrac{1}{#1}} {\RealFrac{#1}{#2}}% } \begin{document} One arg: \verb|$\frac{10}$| prints $\frac{10}$. Two args: \verb|$\frac{2}{30}$| prints $\frac{2}{30}$. \end{document}