Center an element in Bootstrap 4 Navbar

Updated for Bootstrap 4.1+

Bootstrap 4 the navbar now uses flexbox so the Website Name can be centered using mx-auto. The left and right side menus don't require floats.

<nav class="navbar navbar-expand-md navbar-fixed-top navbar-dark bg-dark main-nav">
    <div class="container">
        <ul class="nav navbar-nav">
            <li class="nav-item active">
                <a class="nav-link" href="#">Home</a>
            </li>
            <li class="nav-item">
                <a class="nav-link" href="#">Download</a>
            </li>
            <li class="nav-item">
                <a class="nav-link" href="#">Register</a>
            </li>
        </ul>
        <ul class="nav navbar-nav mx-auto">
            <li class="nav-item"><a class="nav-link" href="#">Website Name</a></li>
        </ul>
        <ul class="nav navbar-nav">
            <li class="nav-item">
                <a class="nav-link" href="#">Rates</a>
            </li>
            <li class="nav-item">
                <a class="nav-link" href="#">Help</a>
            </li>
            <li class="nav-item">
                <a class="nav-link" href="#">Contact</a>
            </li>
        </ul>
    </div>
</nav>

Navbar center with mx-auto Demo

If the Navbar only has a single navbar-nav, then justify-content-center can also be used to center.

EDIT

In the solution above, the Website Name is centered relative to the left and right navbar-nav so if the width of these adjacent navs are different the Website Name is no longer centered.

To resolve this, one of the flexbox workarounds for absolute centering can be used...

Option 1 - Use position:absolute;

Since it's ok to use absolute positioning in flexbox, one option is to use this on the item to be centered.

.abs-center-x {
    position: absolute;
    left: 50%;
    transform: translateX(-50%);
}

Navbar center with absolute position Demo

Option 2 - Use flexbox nesting

Finally, another option is to make the centered item also display:flexbox (using d-flex) and center justified. In this case each navbar component must have flex-grow:1

As of Bootstrap 4 Beta, the Navbar is now display:flex. Bootstrap 4.1.0 includes a new flex-fill class to make each nav section fill the width:

<nav class="navbar navbar-expand-sm navbar-dark bg-dark main-nav">
    <div class="container justify-content-center">
        <ul class="nav navbar-nav flex-fill w-100 flex-nowrap">
            <li class="nav-item active">
                <a class="nav-link" href="#">Home</a>
            </li>
            <li class="nav-item">
                <a class="nav-link" href="#">Download</a>
            </li>
            <li class="nav-item">
                <a class="nav-link" href="#">Register</a>
            </li>
            <li class="nav-item">
                <a class="nav-link" href="#">More</a>
            </li>
        </ul>
        <ul class="nav navbar-nav flex-fill justify-content-center">
            <li class="nav-item"><a class="nav-link" href="#">Center</a></li>
        </ul>
        <ul class="nav navbar-nav flex-fill w-100 justify-content-end">
            <li class="nav-item">
                <a class="nav-link" href="#">Help</a>
            </li>
            <li class="nav-item">
                <a class="nav-link" href="#">Contact</a>
            </li>
        </ul>
    </div>
</nav>

Navbar center with flexbox nesting Demo

Prior to Bootstrap 4.1.0 you can add the flex-fill class like this...

.flex-fill {
   flex:1
}

As of 4.1 flex-fill is included in Bootstrap.

---

Bootstrap 4 Navbar center demos
More centering demos
Center links on desktop, left align on mobile

Related:
How to center nav-items in Bootstrap?
Bootstrap NavBar with left, center or right aligned items
How move 'nav' element under 'navbar-brand' in my Navbar

In Bootstrap 4, there is a new utility known as .mx-auto. You just need to specify the width of the centered element.

Ref: http://v4-alpha.getbootstrap.com/utilities/spacing/#horizontal-centering

Diffferent from Bass Jobsen's answer, which is a relative center to the elements on both ends, the following example is absolute centered.

Here's the HTML:

<nav class="navbar bg-faded">
  <div class="container">
    <ul class="nav navbar-nav pull-sm-left">
      <li class="nav-item">
        <a class="nav-link" href="#">Link 1</a>
      </li>
      <li class="nav-item">
        <a class="nav-link" href="#">Link 2</a>
      </li>
      <li class="nav-item">
        <a class="nav-link" href="#">Link 3</a>
      </li>
      <li class="nav-item">
        <a class="nav-link" href="#">Link 4</a>
      </li>
    </ul>
    <ul class="nav navbar-nav navbar-logo mx-auto">
      <li class="nav-item">
        <a class="nav-link" href="#">Brand</a>
      </li>
    </ul>
    <ul class="nav navbar-nav pull-sm-right">
      <li class="nav-item">
        <a class="nav-link" href="#">Link 5</a>
      </li>
      <li class="nav-item">
        <a class="nav-link" href="#">Link 6</a>
      </li>
    </ul>
  </div>
</nav>

And CSS:

.navbar-logo {
  width: 90px;
}