center a row using Bootstrap 3

Why not using the grid system?

The bootstrap grid system consist of 12 columns, so if you use the "Medium" columns it will have a 970px width size.

Then you can divide it to 3 columns (12/3=4) so use 3 divs with "col-md-4" class:

<div class="col-md-4"></div>
<div class="col-md-4"></div>
<div class="col-md-4"></div>

Each one will have 323px max width size. Keep the first and the last empty and use the middle one to get your content centerd:

<div class="col-md-4"></div>
<div class="col-md-4">
   <p>Centered content.</p>
</div>
<div class="col-md-4"></div>

What you are doing is not working, because you apply the margin: auto to the full-width column.

Wrap it in a div and center that one. E.g:

<div class="i-am-centered">
  <div class="row">...</div>
</div>

.

.i-am-centered { margin: auto; max-width: 300px;}

http://www.bootply.com/93751

Its a cleaner solution anyway, as it is more expressive and as you usually don't want to mess with the grid.