Set bootstrap modal body height by percentage

Question

I am trying to make a modal with a body that will scroll when the content becomes too large. However, I want the modal to be responsive to the screen size. When I set the max-height to 40% it has no effect. However, if I set the max-height to 400px it works as expected, but is not responsive. I am sure I am just missing something simple, but I can't seem to get it to work.

Here is an example

Doesn't work:

.modal-body {
    max-height:40%; 
    overflow-y: auto;
}

Works:

.modal-body {
    max-height:400px; 
    overflow-y: auto;
}

Answer 1

This worked for me

.modal-dialog,
.modal-content {
    /* 80% of window height */
    height: 80%;
}

.modal-body {
    /* 100% = dialog height, 120px = header + footer */
    max-height: calc(100% - 120px);
    overflow-y: scroll;
}

Fiddle: http://jsfiddle.net/mehmetatas/18dpgqpb/2/