Vertically centering Bootstrap modal window

Question

I would like to center my modal on the viewport (middle) I tried to add some css properties

 .modal { position: fixed; top:50%; left:50%; }   

I'm using this example http://jsfiddle.net/rniemeyer/Wjjnd/

I tried

$("#MyModal").modal('show').css(
    {
        'margin-top': function () {
            return -($(this).height() / 2);
        },
        'margin-left': function () {
            return -($(this).width() / 2);
        }
    })

Answer 1

This does the job : http://jsfiddle.net/sRmLV/1140/

It uses a helper-div and some custom css. No javascript or jQuery required.

HTML (based on Bootstrap's demo-code)

<button class="btn btn-primary btn-lg" data-toggle="modal" data-target="#myModal">Launch demo modal</button>

<!-- Modal -->
<div class="modal fade" id="myModal" tabindex="-1" role="dialog" aria-labelledby="myModalLabel" aria-hidden="true">
    <div class="vertical-alignment-helper">
        <div class="modal-dialog vertical-align-center">
            <div class="modal-content">
                <div class="modal-header">
                    <button type="button" class="close" data-dismiss="modal"><span aria-hidden="true">&times;</span><span class="sr-only">Close</span>

                    </button>
                     <h4 class="modal-title" id="myModalLabel">Modal title</h4>

                </div>
                <div class="modal-body">...</div>
                <div class="modal-footer">
                    <button type="button" class="btn btn-default" data-dismiss="modal">Close</button>
                    <button type="button" class="btn btn-primary">Save changes</button>
                </div>
            </div>
        </div>
    </div>
</div>

CSS

.vertical-alignment-helper {
    display:table;
    height: 100%;
    width: 100%;
    pointer-events:none; /* This makes sure that we can still click outside of the modal to close it */
}
.vertical-align-center {
    /* To center vertically */
    display: table-cell;
    vertical-align: middle;
    pointer-events:none;
}
.modal-content {
    /* Bootstrap sets the size of the modal in the modal-dialog class, we need to inherit it */
    width:inherit;
    max-width:inherit; /* For Bootstrap 4 - to avoid the modal window stretching full width */
    height:inherit;
    /* To center horizontally */
    margin: 0 auto;
    pointer-events: all;
}

Answer 2

Because gpcola's answer didn't work for me, I edited a bit so its works now. I used "margin-top" instead of transform. Also, i use the "show" instead of "shown" event because after it gave me a very bad jump of positioning (visible when you have bootstrap animations on). Be sure to set the display to "block" before positioning, otherwise $dialog.height() will be 0 and the modal will not be centered completely.

(function ($) {
    "use strict";
    function centerModal() {
        $(this).css('display', 'block');
        var $dialog  = $(this).find(".modal-dialog"),
        offset       = ($(window).height() - $dialog.height()) / 2,
        bottomMargin = parseInt($dialog.css('marginBottom'), 10);

        // Make sure you don't hide the top part of the modal w/ a negative margin if it's longer than the screen height, and keep the margin equal to the bottom margin of the modal
        if(offset < bottomMargin) offset = bottomMargin;
        $dialog.css("margin-top", offset);
    }

    $(document).on('show.bs.modal', '.modal', centerModal);
    $(window).on("resize", function () {
        $('.modal:visible').each(centerModal);
    });
}(jQuery));