<div class="contenedor-modal">
  <button type="button" class="btn btn-primary btn-lg" data-toggle="modal" data-target="#miModal">Abrir modal</button>
</div>

<div class="modal fade" id="miModal" tabindex="-1" role="dialog" aria-labelledby="myModalLabel" 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="myModalLabel">Esto es un modal</h4>
      </div>
      <div class="modal-body">
        Texto del modal
      </div>
    </div>
  </div>
</div>
.contenedor-modal {
  position: absolute;
  width: 100%;
  height: 100%;
  text-align: center;
}

.contenedor-modal button {
  position: relative;
  top: 40%;
}

External CSS

  1. https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap.min.css

External JavaScript

  1. https://code.jquery.com/jquery-2.2.4.min.js
  2. https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/js/bootstrap.min.js