How to center form in bootstrap 3

There is a simple way of doing this in Bootstrap. Whenever I need to make a div center in a page, I divide all columns by 3 (total Bootstrap columns = 12, divided by 3 >>> 12/3 = 4). Dividing by four gives me three columns. Then I put my div in middle column. And all this math is performed by this way:

<div class="col-md-4 col-md-offset-4">my div here</div>

col-md-4 makes one column of 4 Bootstrap columns. Let's say it's the main column. col-md-offset-4 adds one column (of width of 4 Bootstrap column) to both sides of the main column.

A simple way is to add

.center_div{
    margin: 0 auto;
    width:80% /* value of your choice which suits your alignment */
}

to you class .container.Add width:xx % to it and you get perfectly centered div!

eg :

<div class="container center_div">

but i feel that by default container is centered in BS!