Default

The scale when the page is loaded is defined by the viewport meta tag

Maximum, minimum

The maximum-scale, minimum-scale, and user-scalable properties are defined by the viewport meta tag

Calculation

You can calculate that factor by dividing screen.width by window.innerWidth

onResize = function () {
     zoomFactor=window.innerWidth/screen.width; 
     console.log("The Zooming factor is "+zoomFactor);
}
onResize();

window.onresize = function () {
     zoomFactor=window.innerWidth/screen.width; 
     console.log("The Zooming factor is "+zoomFactor);
}