# How do I set the margins of a centered math expression? [duplicate]

Possible Duplicate:
How to globally change the spacing around equations?

For example,

this is a paragraph
$\frac{1}{2}$
this is a paragraph


as result, I want something like

this is a paragraph

$\frac{1}{2}$

this is a paragraph


If I understand correctly, you want to adjust the vertical spacing which is set by the four length parameters

\abovedisplayskip
\abovedisplayshortskip
\belowdisplayskip
\belowdisplayshortskip


The default values depend on the current size for example the standard classes define \normalsize as

\renewcommand\normalsize{%
\@setfontsize\normalsize\@xpt\@xiipt
\abovedisplayskip 10\p@ \@plus2\p@ \@minus5\p@
\abovedisplayshortskip \z@ \@plus3\p@
\belowdisplayshortskip 6\p@ \@plus3\p@ \@minus3\p@
\belowdisplayskip \abovedisplayskip
\let\@listi\@listI}


so if you go \setlength\abovedisplayskip{2in plus 1in} after \normalsize you will get more space.