How to calculate number of characters of monospace font that fit into div

I have code like this:

div {
    font-family: monospace;
    font-size: 12px;
    line-height: 14px;
}

$(function() {
    var div = $('div');
    var space = $('<span>&nbsp;</span>').appendTo(div);
    var num_chars = div.width() / space.width();
    var chars = '';
    for (var i=0; i<num_chars; ++i) {
        chars += 'd';
    }
    div.html(chars);
    space.remove();
});

but number of characters is way to small to fill the width of the div. Anyone know how to calculate the number of characters that will fill one line of a div?