# Problem getting my equation alignment / numbering right

I've struggled to get my alignment look nice for quite a time now. After a lot of googling I have finally achieved almost what I want:

Now the problem is the equation numbers (1 and 3 in the example), which are not vertically centered to the equations. Please help me getting them right! MWE:

\documentclass[12pt,a4paper]{article}
\usepackage{mathtools}
\usepackage{amsmath, amsfonts, amssymb, siunitx}

\begin{document}
\nocite{*}
\begin{alignat}{2}
&\phantom{U_{\mathrm{HETKELLINEN}}}&f
&= \frac{1}{T}\notag\\
&\Leftrightarrow
&T &=\frac{1}{f}=\SI{0,001}{\second}\\
%
& &\hat{U}
&= \sqrt{2} \cdot U = \SI{14,1421}{\volt}\\
%
& &U_{\mathrm{HETKELLINEN}}
&= \hat{U} \cdot \sqrt{2 \cdot \pi \cdot f \cdot t}\notag\\
& & &= \SI{14,1421}{\volt} \cdot \sqrt{2 \cdot \pi \cdot \SI{1}{\kilo\hertz} \cdot \SI{0,0011}{\second}}\notag\\
&&&= \SI{8,3125}{\volt}
\end{alignat}
\end{document}
• Welcome. It is a rare case that somebody posts a working and minimal example to reproduce a problem. Unfortunately, i am not sure where you expect the numbers to be. Can you elaborate? – Johannes_B Feb 19 '17 at 10:17
• Maybe this could help? tex.stackexchange.com/questions/13396/… – Philipp Imhof Feb 19 '17 at 10:22
• @philipp-imhof No, that example doesn't help. Or at least I cannot make it work with that long left side text, it messes up the alignment between the equations. – msimila Feb 19 '17 at 11:01

You can use split:

\documentclass[12pt,a4paper]{article}
\usepackage{amsmath, amssymb, siunitx}
\usepackage{mathtools}

\begin{document}

\begin{align}
\begin{split}
f &= \frac{1}{T}\\
\Leftrightarrow\hphantom{U_{\mathrm{HETKELLINEN}}}
T &= \frac{1}{f}=\SI{0,001}{\second}
\end{split}
\\
%
\hat{U} &= \sqrt{2} \,U = \SI{14,1421}{\volt}\\
%
\begin{split}
U_{\mathrm{HETKELLINEN}}
&= \hat{U} \sqrt{2\pi ft}\\
&= \SI{14,1421}{\volt} \cdot \sqrt{2\pi \cdot \SI{1}{\kilo\hertz}
\cdot \SI{0,0011}{\second}}\\
&= \SI{8,3125}{\volt}
\end{split}
\end{align}
\end{document}

• Thank you! Now equation 3 is numbered correctly, but 1 is not. I cannot get the split to work with equation 1. Is there some trick with that \Leftrightarrow? The line that makes problem in my code seems to be &\Leftrightarrow T &= \frac{1}{f} &=\SI{0,001}{\second} The equation number position is defined by my school, cannot change that. – msimila Feb 19 '17 at 10:55
• @msimila Fixed as you wished – egreg Feb 19 '17 at 11:01
• Thank you! Perfect! I messed up with the &-signs. So difficult to count to three.. ;) – msimila Feb 19 '17 at 11:04