How to convert css pixels to css3 scale (x,y)

I have a div whose initial width and height are set by the user. When a user zooms in the browser(ctrl+ or ctrl-) the initial width and height of that div changes, say a user zooms in 175%, the aspect ratio of that div stays the same because the width and height is adjusted. Is there a way to replicate this by dynamically setting transform: scale(x,y). I have tried several thing but can't seem to find a solid solution.