Bootstrap Center Vertical and Horizontal Alignment

I have a page where only form exists and I want form to be placed in the center of the screen.

<div class="container">
  <div class="row justify-content-center align-items-center">
    <form>
      <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>

The justify-content-center aligns the form horizontally, but I can't figure out how to align it vertically. I have tried to use align-items-center and align-self-center, but it doesn't work.

What am I missing?

DEMO

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

This work for me:

<section class="h-100">
  <header class="container h-100">
    <div class="d-flex align-items-center justify-content-center h-100">
      <div class="d-flex flex-column">
        <h1 class="text align-self-center p-2">item 1</h1>
        <h4 class="text align-self-center p-2">item 2</h4>
        <button class="btn btn-danger align-self-center p-2" type="button" name="button">item 3</button>
      </div>
    </div>
  </header>
</section>

flexbox can help you. info here

<div class="d-flex flex-row justify-content-center align-items-center" style="height: 100px;">
    <div class="p-2">
     1
    </div>
    <div class="p-2">
     2
    </div>
</div>

Bootstrap has text-center to center a text. For example

<div class="container text-center">

You change the following

<div class="row justify-content-center align-items-center">

to the following

<div class="row text-center">

You need something to center your form into. But because you didn't specify a height for your html and body, it would just wrap content - and not the viewport. In other words, there was no room where to center the item in.

html, body {
  height: 100%;
}
.container, .row.justify-content-center.align-items-center {
  height: 100%;
  min-height: 100%;
}