[Tex/LaTex] Equation errors – I want to use units in the equation

Question

I´m trying to get some simple equations into my document. However it seems my used units are not equation friendly.

\documentclass[a4paper,DIV10,12pt,headsepline,headings=small]{scrreprt}
\usepackage{lmodern}
%\usepackage[ngerman]{babel}
\usepackage[english]{babel}
\usepackage{cite}
\usepackage[margin=10pt,font=small,labelfont=bf,
labelsep=endash,singlelinecheck=off]{caption}
\usepackage{longtable} 
\usepackage{threeparttable}
\usepackage{booktabs}
\usepackage{graphicx} 
\usepackage{float}
\usepackage{parskip}
\usepackage{setspace}
\usepackage{graphicx}
\onehalfspacing
\pagestyle{headings}
\setcounter{secnumdepth}{3} 
\setcounter{tocdepth}{3}
\usepackage{tocloft}
\usepackage{ragged2e}
\usepackage{array} 
\usepackage{amsmath}
\usepackage {picins}

\begin{gather}
Thickness&=\frac{100 $\mu$g/$\mathrm{cm}^{2}$}{8.38 $\mu$g/$\mathrm{cm}^{3}$} \\[1ex]
Thickness&=0.199 $\mu$m
\end{gather}

The equation should give me something like:
Thickness = (100 µg/cm²) / (8.38 µg /cm³) [without brackets of course]

It seems that my unit definitions are not working properly in equations. There is no problem while using them in a normal text.

I really appreciate any hints!

(hope I used the correct tags)

Answer 1

Use a specialized package such as siunitx that make quite easy to input numbers and units; I'll show three ways to get the same output:

\documentclass[a4paper,DIV10,12pt,headsepline,headings=small]{scrreprt}
\usepackage[utf8]{inputenc}
\usepackage{lmodern}
\usepackage{amsmath}
\usepackage{textgreek}
\usepackage{siunitx}
\sisetup{text-micro=\textmu,math-micro=\mathmu} % better micro symbol

\begin{document}
\begin{align}
\mathit{Thickness}&=\frac{\SI{100}{\micro\gram/\centi\meter^{2}}}{\SI{8.38}{\micro\gram/\centi\meter^{3}}} \\[1ex]
\mathit{Thickness}&=\SI{0.199}{\micro\meter}
\end{align}
\begin{align}
\mathit{Thickness}&=\frac{\SI{100}{\ug/\cm^{2}}}{\SI{8.38}{\ug/\cm^{3}}} \\[1ex]
\mathit{Thickness}&=\SI{0.199}{\um}
\end{align}
\begin{align}
\mathit{Thickness}&=\frac{\SI{100}{μg/cm^{2}}}{\SI{8.38}{μg/cm^{3}}} \\[1ex]
\mathit{Thickness}&=\SI{0.199}{μm}
\end{align}
\end{document}