\documentclass[a4j,11pt,papersize]{jsarticle}

\usepackage{calc}
\newcounter{uho}
\newlength{\hoge}

\begin{document}

\section{ほげほげ}

\setcounter{uho}{3 + 5 * 4 - \value{page}}
\setlength{\hoge}{\widthof{$f + g$} + \value{section}pt * \real{2.5}}

\theuho\par
\the\hoge

\end{document}