bootstrape vertically center modal

1<!-- Button trigger modal -->
2<button type="button" class="btn btn-primary" data-toggle="modal" data-target="#exampleModalCenter">
3  Launch demo modal
4</button>
5
6<!-- Modal -->
7<div class="modal fade" id="exampleModalCenter" tabindex="-1" role="dialog" aria-labelledby="exampleModalCenterTitle" aria-hidden="true">
8  <div class="modal-dialog modal-dialog-centered" role="document">
9    <div class="modal-content">
10      <div class="modal-header">
11        <h5 class="modal-title" id="exampleModalCenterTitle">Modal title</h5>
12        <button type="button" class="close" data-dismiss="modal" aria-label="Close">
13          <span aria-hidden="true">&times;</span>
14        </button>
15      </div>
16      <div class="modal-body">
17        ...
18      </div>
19      <div class="modal-footer">
20        <button type="button" class="btn btn-secondary" data-dismiss="modal">Close</button>
21        <button type="button" class="btn btn-primary">Save changes</button>
22      </div>
23    </div>
24  </div>
25</div>

1Vertically centered
2Add .modal-dialog-centered to .modal-dialog to vertically center the modal.
3
4<!-- Button trigger modal -->
5<button type="button" class="btn btn-primary" data-toggle="modal" data-target="#exampleModalCenter">
6  Launch demo modal
7</button>
8
9<!-- Modal -->
10<div class="modal fade" id="exampleModalCenter" tabindex="-1" role="dialog" aria-labelledby="exampleModalCenterTitle" aria-hidden="true">
11  <div class="modal-dialog modal-dialog-centered" role="document">
12    <div class="modal-content">
13      <div class="modal-header">
14        <h5 class="modal-title" id="exampleModalCenterTitle">Modal title</h5>
15        <button type="button" class="close" data-dismiss="modal" aria-label="Close">
16          <span aria-hidden="true">&times;</span>
17        </button>
18      </div>
19      <div class="modal-body">
20        ...
21      </div>
22      <div class="modal-footer">
23        <button type="button" class="btn btn-secondary" data-dismiss="modal">Close</button>
24        <button type="button" class="btn btn-primary">Save changes</button>
25      </div>
26    </div>
27  </div>
28</div>