How to center a div with Bootstrap2?

besides shrinking the div itself to the size you want, by reducing span size like so... class="span6 offset3", class="span4 offset4", etc... something as simple as style="text-align: center" on the div could have the effect you're looking for

you can't use span7 with any set offset and get the span centered on the page (Because total spans = 12)

Bootstrap's spans are floated to the left. All it takes to center them is override this behavior. I do this by adding this to my stylesheet:

.center {
     float: none;
     margin-left: auto;
     margin-right: auto;
}

If you have this class defined, just add it to the span and you're good to go.

<div class="span7 center"> box </div>

Note that this custom center class must be defined after the bootstrap css. You could use !important but that isn't recommended.

Bootstrap3 has the .center-block class that you can use. It is defined as

.center-block {
  display: block;
  margin-left: auto;
  margin-right: auto;
}

Documentation here.