documentclass[a5paper,twoside]{article}
usepackage{tikz}
usepackage{geometry}

geometry{a5paper}
geometry{portrait}
geometry{inner=18mm,outer=10mm,top=10mm,bottom=10mm}
pagestyle{empty}

begin{document}

section{graph}
begin{tikzpicture}
  draw [step=0.5cm,very thin, gray] (0,0) grid (12.5,17.5);
end{tikzpicture}

end{document}

section{graph}
newdimenspaceleft
spaceleft=dimexprtextheight-pagetotal-14ptrelax
% not sure why I need to subtract 14pt here.  Without it, the content is
% pushed down to the next page.
begin{tikzpicture}
  draw [step=0.5cm,very thin, gray] (0,0) grid (textwidth,spaceleft);
end{tikzpicture}

pgfmathparse{textwidth - mod(textwidth,0.5cm)}
Textwidth: thetextwidth\
Modified width: pgfmathresult pt