Pixel to MM equation?

Question

Is there a reliable equation to work out pixel size to MM? Or is that not possible cross device?

We are working with a bespoke system that delivers content to many

Answer 1

Based on the @Dawa answer above, this is my solution:

var mmToPx = function(mm) {
    var div = document.createElement('div');
    div.style.display = 'block';
    div.style.height = '100mm';
    document.body.appendChild(div);
    var convert = div.offsetHeight * mm / 100;
    div.parentNode.removeChild(div);
    return convert;
};

Just note that the 100mm height, will give you a better precision factor. The calculation will be instant and the div will not be visible. No need for jQuery