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

i use this simple function

function pix2mm(val,dpi){
    return (val/0.0393701)/dpi;
}

test outputs it 300,600,900 DPI

var r = pix2mm(100,300);  // converting 100 pixels it 300 DPI 
console.log(r);  // output : 8.4 mm
var r1 = pix2mm(100,600);  // converting 100 pixels it 600 DPI
console.log(r1);  // output : 4.2 mm
var r2 = pix2mm(100,900);  // converting 100 pixels it 900 DPI
console.log(r2);  // output : 2.8 mm

DEMO : https://jsfiddle.net/xpvt214o/29044/