How to vertically align Bootstrap v4 modal dialogs

Simply add modal-dialog-centered class along with model-dialog as below

<head>
  <link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css" integrity="sha384-ggOyR0iXCbMQv3Xipma34MD+dH/1fQ784/j6cY/iJTQUOhcWr7x9JvoRxT2MZw1T" crossorigin="anonymous">
  <script src="https://code.jquery.com/jquery-3.3.1.slim.min.js" integrity="sha384-q8i/X+965DzO0rT7abK41JStQIAqVgRVzpbzo5smXKp4YfRvH+8abtTE1Pi6jizo" crossorigin="anonymous"></script>
  <script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.7/umd/popper.min.js" integrity="sha384-UO2eT0CpHqdSJQ6hJty5KVphtPhzWj9WO1clHTMGa3JDZwrnQq4sF86dIHNDz0W1" crossorigin="anonymous"></script>
  <script src="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/js/bootstrap.min.js" integrity="sha384-JjSmVgyd0p3pXB1rRibZUAYoIIy6OrQ6VrjIEaFf/nJGzIxFDsf4x0xIM+B07jRM" crossorigin="anonymous"></script>
</head>
 <button class="btn btn-success" data-toggle="modal" data-target="#MyModal">Launch Modal</button>     
<div class="modal align-middle" id="MyModal">
    <div class="modal-dialog modal-dialog-centered">
        <div class="modal-content">
            <div class="modal-header">
                <h5 class="modal-title">Modal Title</h5>
                <button class="close" data-dismiss="modal">&times;</button>
            </div>
            <div class="modal-body">Lorem Ipsum is simply dummy text of the                        printing and typesetting industry</div> 
            <div class="modal-footer">
                <button class="btn btn-info" data-dismiss="modal">Close</button>
            </div>                
        </div>
    </div>
</div>

Update, as of Beta 3, [docs]:

Add .modal-dialog-centered to .modal-dialog to vertically center the modal.

Original answer:

SCSS:

.modal-dialog {
  min-height: calc(100vh - 60px);
  display: flex;
  flex-direction: column;
  justify-content: center;
  overflow: auto;
  @media(max-width: 768px) {
    min-height: calc(100vh - 20px);
  }
}

or unprefixed CSS:

.modal-dialog {
    min-height: calc(100vh - 60px);
    display: flex;
    flex-direction: column;
    justify-content: center;
    overflow: auto;
}
@media(max-width: 768px) {
  .modal-dialog {
    min-height: calc(100vh - 20px);
  }
}

Note 1: Please note fully prefixed CSS gradually becomes obsolete as browser support for certain properties changes. The right way of getting the updated prefixed CSS is to:

  • copy/paste the unprefixed CSS into Autoprefixer.
  • set the filter in the bottom box to the desired setting (for max. browser support use > 0%).
  • get the latest code from the box on the right.

Note 2: This answer was added in early stages of v4 (alpha 3 or 4), which is now currently in beta. You can safely replace the CSS part of this answer by adding the following classes to .modal-dialog: 

h-100 d-flex flex-column justify-content-center my-0

..., as pointed out by @Androbaut in the comment below. You will still need the JavaScript (see below) to close the modal window on click tap below/above the modal window.

jQuery (needed to close modal on click/tap above/below):

$('.modal-dialog').on('click tap', function(e){
  if ($(e.target).hasClass('modal-dialog')) {
    $('.modal').modal('hide');
  }
})

That's it.

Working snippet, fully-prefixed CSS and markup using different modal sizes:

$('.modal-dialog').on('click tap', function(e){
  if ($(e.target).hasClass('modal-dialog')) {
  	$('.modal').modal('hide');
  }
})
.modal-dialog {
  min-height: -webkit-calc(100vh - 60px);
  min-height: -moz-calc(100vh - 60px);
  min-height: calc(100vh - 60px);
  display: -webkit-box;
  display: -webkit-flex;
  display: -moz-box;
  display: -ms-flexbox;
  display: flex;
  -webkit-box-orient: vertical;
  -webkit-box-direction: normal;
  -webkit-flex-direction: column;
     -moz-box-orient: vertical;
     -moz-box-direction: normal;
      -ms-flex-direction: column;
          flex-direction: column;
  -webkit-box-pack: center;
  -webkit-justify-content: center;
     -moz-box-pack: center;
      -ms-flex-pack: center;
          justify-content: center;
  overflow: auto; 
}
@media (max-width: 768px) {
  .modal-dialog {
    min-height: -webkit-calc(100vh - 20px);
    min-height: -moz-calc(100vh - 20px);
    min-height: calc(100vh - 20px);   
  }
}

/* you don't need the CSS below this line. It's mainly cosmetic and for aligning the modal launch buttons */

.modal-content {
  display: -webkit-box;
  display: -webkit-flex;
  display: -moz-box;
  display: -ms-flexbox;
  display: flex;
  -webkit-box-orient: vertical;
  -webkit-box-direction: normal;
  -webkit-flex-direction: column;
     -moz-box-orient: vertical;
     -moz-box-direction: normal;
      -ms-flex-direction: column;
          flex-direction: column; }
.modal-content > * {
  -webkit-box-flex: 0;
  -webkit-flex: 0 0 auto;
     -moz-box-flex: 0;
      -ms-flex: 0 0 auto;
          flex: 0 0 auto; 
}
.modal-content > *.modal-body {
  -webkit-box-flex: 1;
  -webkit-flex-grow: 1;
     -moz-box-flex: 1;
      -ms-flex-positive: 1;
          flex-grow: 1; 
}

#Modal_2 .modal-content {
  min-height: 50vh; 
}

#Modal_3 .modal-content {
  min-height: 85vh; 
}

#Modal_4 .modal-content {
  min-height: 200vh; 
}

.full-page-center {
  display: -webkit-box;
  display: -webkit-flex;
  display: -moz-box;
  display: -ms-flexbox;
  display: flex;
  -webkit-box-pack: center;
  -webkit-justify-content: center;
     -moz-box-pack: center;
      -ms-flex-pack: center;
          justify-content: center;
  -webkit-box-align: center;
  -webkit-align-items: center;
     -moz-box-align: center;
      -ms-flex-align: center;
          align-items: center;
  min-height: 100vh; 
}
.full-page-center button {
  margin: 15px; 
}
@media (max-width: 768px) {
  .full-page-center {
    -webkit-flex-wrap: wrap;
        -ms-flex-wrap: wrap;
            flex-wrap: wrap;   
  }
  .full-page-center button {
    display: block;
    min-width: 100%;
    margin: 10px 15px;
  }
  .full-page-center::after {
    display: none;
    -webkit-box-flex: 0;
    -webkit-flex-grow: 0;
       -moz-box-flex: 0;
        -ms-flex-positive: 0;
            flex-grow: 0;
  }
}
<link href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-alpha.5/css/bootstrap.min.css" rel="stylesheet"/>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<script src="https://npmcdn.com/[email protected]/dist/js/tether.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-alpha.5/js/bootstrap.min.js"></script>


<div class="container full-page-center">
  <button type="button" class="btn btn-primary btn-lg" data-toggle="modal" data-target="#Modal_1">
    Tiny modal
  </button>
  <button type="button" class="btn btn-default btn-lg" data-toggle="modal" data-target="#Modal_2">
    Normal modal
  </button>
  <button type="button" class="btn btn-success btn-lg" data-toggle="modal" data-target="#Modal_3">
    Large modal
  </button>
  <button type="button" class="btn btn-warning btn-lg" data-toggle="modal" data-target="#Modal_4">
    Very large modal
  </button>
</div>
<div class="modal fade" id="Modal_1" tabindex="-1" role="dialog" aria-labelledby="modalLabel_1" aria-hidden="true">
  <div class="modal-dialog" role="document">
    <div class="modal-content">
      <div class="modal-header">
        <button type="button" class="close" data-dismiss="modal" aria-label="Close">
          <span aria-hidden="true">&times;</span>
        </button>
        <h4 class="modal-title" id="modalLabel_1">Tiny modal</h4>
      </div>
      <div class="modal-body">
        I am cute...
      </div>
      <div class="modal-footer">
        <button type="button" class="btn btn-secondary" data-dismiss="modal">Close</button>
        <button type="button" class="btn btn-primary">Save changes</button>
      </div>
    </div>
  </div>
</div>
<div class="modal fade" id="Modal_2" tabindex="-1" role="dialog" aria-labelledby="modalLabel_2" aria-hidden="true">
  <div class="modal-dialog" role="document">
    <div class="modal-content">
      <div class="modal-header">
        <button type="button" class="close" data-dismiss="modal" aria-label="Close">
          <span aria-hidden="true">&times;</span>
        </button>
        <h4 class="modal-title" id="modalLabel_2">Dull modal</h4>
      </div>
      <div class="modal-body">
        I am normal...
      </div>
      <div class="modal-footer">
        <button type="button" class="btn btn-secondary" data-dismiss="modal">Close</button>
        <button type="button" class="btn btn-primary">Some action</button>
      </div>
    </div>
  </div>
</div>
<div class="modal fade" id="Modal_3" tabindex="-1" role="dialog" aria-labelledby="modalLabel_3" aria-hidden="true">
  <div class="modal-dialog" role="document">
    <div class="modal-content">
      <div class="modal-header">
        <button type="button" class="close" data-dismiss="modal" aria-label="Close">
          <span aria-hidden="true">&times;</span>
        </button>
        <h4 class="modal-title" id="modalLabel_3">Don't call me fat</h4>
      </div>
      <div class="modal-body">
        Call me "oversized".
      </div>
      <div class="modal-footer">
        <button type="button" class="btn btn-secondary" data-dismiss="modal">Close</button>
        <button type="button" class="btn btn-success">Some action</button>
      </div>
    </div>
  </div>
</div>
<div class="modal fade" id="Modal_4" tabindex="-1" role="dialog" aria-labelledby="modalLabel_4" aria-hidden="true">
  <div class="modal-dialog" role="document">
    <div class="modal-content">
      <div class="modal-header">
        <button type="button" class="close" data-dismiss="modal" aria-label="Close">
          <span aria-hidden="true">&times;</span>
        </button>
        <h4 class="modal-title" id="modalLabel_4">Huge modal</h4>
      </div>
      <div class="modal-body">
        Comments, anyone?
      </div>
      <div class="modal-footer">
        <button type="button" class="btn btn-secondary" data-dismiss="modal">Close</button>
        <button type="button" class="btn btn-warning">Some action</button>
      </div>
    </div>
  </div>
</div>

If you find any bugs or shortcomings please let me know. I will take the time to improve the answer and keep it useful. Help with this task is welcome.

Tags:

Vertical Alignment

Modal Dialog

Flexbox

Bootstrap 4

Bootstrap Modal

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