Bootstrap 5 (Updated 2021)

Bootstrap 5 is still flexbox based so vertical centering works the same way as it did in Bootstrap 4. For example, align-items-center (flex-direction: row) and justify-content-center (flex-direction: column) can used on the flexbox parent (row or d-flex).

Centering examples in Bootstrap 5

Vertical center (don't forget the parent must have a defined height!):

• my-auto for centering inside flex (.d-flex) elements
• my-auto can be used to center columns (.col-) inside row
• align-items-center to center columns (col-*) inside row

Horizontal center:

• text-center to center display:inline elements & column content
• mx-auto for centering inside flex elements
• mx-auto can be used to center columns (.col-) inside row
• justify-content-center to center columns (col-*) inside row

Bootstrap 4.3+ (Update 2019)

There's no need for extra CSS. What's already included in Bootstrap will work. Make sure the container(s) of the form are full height. Bootstrap 4 now has a h-100 class for 100% height...

Vertical center:

<div class="container h-100">
  <div class="row h-100 justify-content-center align-items-center">
    <form class="col-12">
      <div class="form-group">
        <label for="formGroupExampleInput">Example label</label>
        <input type="text" class="form-control" id="formGroupExampleInput" placeholder="Example input">
      </div>
      <div class="form-group">
        <label for="formGroupExampleInput2">Another label</label>
        <input type="text" class="form-control" id="formGroupExampleInput2" placeholder="Another input">
      </div>
    </form>   
  </div>
</div>

https://codeply.com/go/raCutAGHre

the height of the container with the item(s) to center should be 100% (or whatever the desired height is relative to the centered item)

Note: When using height:100% (percentage height) on any element, the element takes in the height of it's container. In modern browsers vh units height:100vh; can be used instead of % to get the desired height.

Therefore, you can set html, body {height: 100%}, or use the new min-vh-100 class on container instead of h-100.

Horizontal center:

• text-center to center display:inline elements & column content
• mx-auto for centering inside flex elements
• offset-* or mx-auto can be used to center columns (.col-)
• justify-content-center to center columns (col-*) inside row

Vertical Align Center in Bootstrap
Bootstrap 4 full-screen centered form
Bootstrap 4 center input group
Bootstrap 4 horizontal + vertical center full screen