How to vertically align Bootstrap v4 modal dialogs

Question

Vertically center modal dialogues in Bootstrap 4.

Note: The requirements below have been added to make it clear I am looking for a prop

Answer 1

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;
  }
}

  
    Tiny modal
  
  
    Normal modal
  
  
    Large modal
  
  
    Very large modal
  


  

    

      

        
          ×
        
        
Tiny modal
      
      

        I am cute...
      
      

        Close
        Save changes
      
    
  


  

    

      

        
          ×
        
        
Dull modal
      
      

        I am normal...
      
      

        Close
        Some action
      
    
  


  

    

      

        
          ×
        
        
Don't call me fat
      
      

        Call me "oversized".
      
      

        Close
        Some action
      
    
  


  

    

      

        
          ×
        
        
Huge modal
      
      

        Comments, anyone?
      
      

        Close
        Some action
      
    
  

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.