Set Position of Bootstrap Modal Popup

@media screen and (min-width: 768px) {
    .modal-dialog {
        left: 50%;
        right: auto;
        width: 624px;
    }
}
.modal-dialog{
position: absolute;
left: 50%;
//now you must set a margin left under zero - value is a half width your window
margin-left: -312px;
// this same situation is with height - example
height: 500px;
top: 50%;
margin-top: -250px;
}