How to globally change the spacing around equations?

Question

How could I go about setting the 'above' and 'below' vspaces around the equation environment?

If the same could be done for the subequations or gather environments, this would be a bonus.

Answer 1

You can adjust the values of \abovedisplayskip, \belowdisplayskip, \abovedisplayshortskip, \belowdisplayshortskip. The shortskip versions are used in the situation where a short text line comes before the displayed equation: if the text ends before the displayed equation starts, it's good to add less vertical space.

Since document classes often define those skips in \normalsize you could redefine this macro, patch it, or simply add to it, because just setting the values in the preamble would not work then.

\documentclass{article}
\usepackage{amsmath}
\makeatletter
\g@addto@macro\normalsize{%
  \setlength\abovedisplayskip{40pt}
  \setlength\belowdisplayskip{40pt}
  \setlength\abovedisplayshortskip{40pt}
  \setlength\belowdisplayshortskip{40pt}
}
\makeatother
\begin{document}
text
\begin{gather}
  1 + 1 = 2
\end{gather}
text
\begin{equation}
  1 + 1 = 2
\end{equation}
\end{document}

The same can be done without \g@addto@macro, so without any @ and not requiring \makeatletter and \makeatother, by using \expandafter:

\expandafter\def\expandafter\normalsize\expandafter{%
    \normalsize
    \setlength\abovedisplayskip{40pt}
    \setlength\belowdisplayskip{40pt}
    \setlength\abovedisplayshortskip{40pt}
    \setlength\belowdisplayshortskip{40pt}
}

I prefer the way of adding to \normalsize, because both redefining and patching require knowing the exact definition of \normalsize used by the document class. Bot ways I described earlier in the LaTeX Community forum, in an answer to Can't set vertical whitespace in the preamble.

More information is available in the excellent mathmode documentation.

Answer 2

As an update of Stefan's answer.

Your questions should be closed as duplicated You will find a lot of question/answeres here. E.g.:

Different spacing around equation and align

Before showing some other approaches here a very important fact:

Don't miss the glue! The length \abovedisplayskip etc. can be defined with a glue and so you allow LaTeX to setup the space more flexible. Please read this question/answer: What is glue stretching?

However I want to show an other possibility to setup the length. First of all you can use the command \AtBeginDocument. The font will be setup at the beginning of the document and so the hook will executed the stuff later:

\AtBeginDocument{%
 \abovedisplayskip=12pt plus 3pt minus 9pt
 \abovedisplayshortskip=0pt plus 3pt
 \belowdisplayskip=12pt plus 3pt minus 9pt
 \belowdisplayshortskip=7pt plus 3pt minus 4pt
}

The command \g@addto@macro is an internal macro. Packages like etoolbox or the newer one xpatch are providing more robust versions of the command \g@addto@macro. Show instead of using the internal command and without special handling of the @ you can use:

\usepackage{etoolbox}
\apptocmd\normalsize{%
 \abovedisplayskip=12pt plus 3pt minus 9pt
 \abovedisplayshortskip=0pt plus 3pt
 \belowdisplayskip=12pt plus 3pt minus 9pt
 \belowdisplayshortskip=7pt plus 3pt minus 4pt
}{}{}

or

\usepackage{xpatch}
\xapptocmd\normalsize{%
 \abovedisplayskip=12pt plus 3pt minus 9pt
 \abovedisplayshortskip=0pt plus 3pt
 \belowdisplayskip=12pt plus 3pt minus 9pt
 \belowdisplayshortskip=7pt plus 3pt minus 4pt
}{}{}

---

That is work with gather here a simple example. You should play with the lengths.:

\documentclass{article}
\usepackage{kantlipsum}
\usepackage{xpatch}
\xapptocmd\normalsize{%
 \abovedisplayskip=12pt plus 3pt minus 9pt
 \abovedisplayshortskip=0pt plus 3pt
 \belowdisplayskip=12pt plus 3pt minus 9pt
 \belowdisplayshortskip=7pt plus 3pt minus 4pt
}{}{}
\usepackage{amsmath}
\begin{document}
\kant[1]
\begin{gather}
  1 + 1 = 2
\end{gather}
\kant[1]
\begin{gather}
  1 + 1 = 2
\end{gather}
\kant[1]
\begin{subequations}
\begin{equation}
  1 + 1 = 2
\end{equation}
\end{subequations}
\kant[1]
\kant[1]
\end{document}