How to detect page zoom level in all modern browsers?

Didn't test this for IE, but if you make an element elem with

min-width: 100%

then

window.document.width / elem.clientWidth

will give you your browser zoom level (including the document.body.style.zoom factor).

This is for Chrome, in the wake of user800583 answer ...

I spent a few hours on this problem and have not found a better approach, but :

There are 16 'zoomLevel' and not 10

When Chrome is fullscreen/maximized the ratio is window.outerWidth/window.innerWidth, and when it is not, the ratio seems to be (window.outerWidth-16)/window.innerWidth, however the 1st case can be approached by the 2nd one.

So I came to the following ...

But this approach has limitations : for example if you play the accordion with the application window (rapidly enlarge and reduce the width of the window) then you will get gaps between zoom levels although the zoom has not changed (may be outerWidth and innerWidth are not exactly updated in the same time).

var snap = function (r, snaps)
{
    var i;
    for (i=0; i < 16; i++) { if ( r < snaps[i] ) return i; }
};
var w, l, r;
w = window.outerWidth, l = window.innerWidth;
return snap((w - 16) / l,
            [ 0.29, 0.42, 0.58, 0.71, 0.83, 0.95, 1.05, 1.18, 1.38, 1.63, 1.88, 2.25, 2.75, 3.5, 4.5, 100 ],
);

And if you want the factor :

var snap = function (r, snaps, ratios)
{
    var i;
    for (i=0; i < 16; i++) { if ( r < snaps[i] ) return eval(ratios[i]); }
};
var w, l, r;
w = window.outerWidth, l = window.innerWidth;
return snap((w - 16) / l,
            [ 0.29, 0.42, 0.58, 0.71, 0.83, 0.95, 1.05, 1.18, 1.38, 1.63, 1.88, 2.25, 2.75, 3.5, 4.5, 100 ],
            [ 0.25, '1/3', 0.5, '2/3', 0.75, 0.9, 1, 1.1, 1.25, 1.5, 1.75, 2, 2.5, 3, 4, 5 ]
);

I have this solution for mobile only (tested with Android):

jQuery(function($){

zoom_level = function(){

    $("body").prepend('<div class="overlay" ' +
                'style="position:fixed; top:0%; left:0%; ' +
                'width:100%; height:100%; z-index:1;"></div>');

    var ratio = $("body .overlay:eq(0)").outerWidth() / $(window).width();
    $("body .overlay:eq(0)").remove();

    return ratio;
}

alert(zoom_level());

});

If you want the zoom level right after the pinch move, you will probably have to set a little timeout because of the rendering delay (but I'm not sure because I didn't test it).

This answer is based on comments about devicePixelRatio coming back incorrectly on user1080381's answer.

I found that this command was coming back incorrectly in some instances too when working with a desktop, Surface Pro 3, and Surface Pro 4.

What I found is that it worked on my desktop, but the SP3 and SP4 were giving different numbers from each other and the desktop.

I noticed though that the SP3 was coming back as 1 and a half times the zoom level I was expecting. When I took a look at the display settings, the SP3 was actually set to 150% instead of the 100% I had on my desktop.

So, the solution to the comments should be to divide the returned zoom level by the scale of the machine you are currently on.

I was able to get the scale in the Windows settings by doing the following:

ManagementObjectSearcher searcher = new ManagementObjectSearcher("SELECT * FROM Win32_DesktopMonitor");
double deviceScale = Convert.ToDouble(searcher.Get().OfType<ManagementObject>().FirstOrDefault()["PixelsPerXLogicalInch"]);
int standardPixelPerInch = 96;
return deviceScale / standardPixelPerInch;

So in the case of my SP3, this is how this code looks at 100% zoom:

devicePixelRatio = 1.5
deviceScale = 144
deviceScale / standardPixelPerInch = 1.5
devicePixelRatio / (deviceScale / standardPixelPerInch) = 1

Multiplying by the 100 in user1080381's original answer then would give you a zoom of 100 (%).

Have it currently working however still need to separate by browser. Tested successfully on Chrome(75) and Safari(11.1) (haven't found way for FF as of yet). It also gets the zoom value correct on retina display and calculations are triggered on resize event.

    private pixelRatio() {
      const styleString = "(min-resolution: 2dppx), (-webkit-min-device-pixel-ratio: 1.5),(-moz-min-device-pixel-ratio: 1.5),(min-device-pixel-ratio: 1.5)";
      const chromeRatio = (Math.round((this.window.outerWidth / this.window.innerWidth)*100) / 100);
      const otherRatio = (Math.round(window.devicePixelRatio * 100) / 100);
      const resizeValue = (this.isChrome()) ? chromeRatio : otherRatio;

      return resizeValue || (this.window.matchMedia && this.window.matchMedia(styleString).matches ? 2 : 1) || 1;
    }


  private isChrome():boolean {
    return (!!this.window.chrome && !(!!this.window.opera || this.window.navigator.userAgent.indexOf(' Opera') >= 0))
  }

  private chrome() {
    const zoomChrome = Math.round(((this.window.outerWidth) / this.window.innerWidth)*100) / 100;
    return {
      zoom: zoomChrome,
      devicePxPerCssPx: zoomChrome1 * this.pixelRatio()
    };
  }

This detects scale.

html code:

<body id="blog">

js:

function scale(){
return Math.round(100/(d.getElementById('blog').offsetWidth/d.getElementById('blog').getBoundingClientRect().width))/100;
}

Zoom factor and scale are not to be confused with each other. Users control "zoom"; "scale" is a CSS transformation. 11 years and 10 months ago, however, understack noted:

Basically I want to know the dimension of a DIV at 100% zoom.

This does that. I use it in conjunction with @media queries to detect in JavaScript device scaling, i.e. 360x720 might apply 0.5 scaling; 720x360, 0.75.

var d=document;

I used a different approach, i created a dom element with a fixed height and width of 100px, position fixed and opacity of 0, basically a hidden element.

I then used dom-to-image to capture this element as an image which might sound like overkill but i wanted a bullet proof solution and we were already using this package anyways, then validated the output image width, if it came back with 110, the zoom is at 110%, it's very accurate.

const ORIGINAL_ZOOM_SIZE = 100;
async function isValidateZoomLevel() {
    const base64Render = await domToImage({
      ref: hiddenElementReference,
    });
    const pixleRatio = Math.round(window.devicePixelRatio);
    return new Promise((resolve, reject) => {
      const img = new Image();
      img.onload = () => resolve(ORIGINAL_ZOOM_SIZE === img.width / pixleRatio && ORIGINAL_ZOOM_SIZE === (img.height / pixleRatio));
      img.onerror = reject;
      img.src = base64Render; 
    });
  }

This may or may not help anyone, but I had a page I could not get to center correctly no matter what Css tricks I tried so I wrote a JQuery file call Center Page:

The problem occurred with zoom level of the browser, the page would shift based upon if you were 100%, 125%, 150%, etc.

The code below is in a JQuery file called centerpage.js.

From my page I had to link to JQuery and this file to get it work, even though my master page already had a link to JQuery.

<title>Home Page.</title>
<script src="Scripts/jquery-1.7.1.min.js"></script>
<script src="Scripts/centerpage.js"></script>

centerpage.js:

// centering page element
function centerPage() {
    // get body element
    var body = document.body;

    // if the body element exists
    if (body != null) {
        // get the clientWidth
        var clientWidth = body.clientWidth;

        // request data for centering
        var windowWidth = document.documentElement.clientWidth;
        var left = (windowWidth - bodyWidth) / 2;

        // this is a hack, but it works for me a better method is to determine the 
        // scale but for now it works for my needs
        if (left > 84) {
            // the zoom level is most likely around 150 or higher
            $('#MainBody').removeClass('body').addClass('body150');
        } else if (left < 100) {
            // the zoom level is most likely around 110 - 140
            $('#MainBody').removeClass('body').addClass('body125');
        }
    }
}


// CONTROLLING EVENTS IN jQuery
$(document).ready(function() {
    // center the page
    centerPage();
});

Also if you want to center a panel:

// centering panel
function centerPanel($panelControl) {
    // if the panel control exists
    if ($panelControl && $panelControl.length) {
        // request data for centering
        var windowWidth = document.documentElement.clientWidth;
        var windowHeight = document.documentElement.clientHeight;
        var panelHeight = $panelControl.height();
        var panelWidth = $panelControl.width();

        // centering
        $panelControl.css({
            'position': 'absolute',
            'top': (windowHeight - panelHeight) / 2,
            'left': (windowWidth - panelWidth) / 2
        });

        // only need force for IE6
        $('#backgroundPanel').css('height', windowHeight);
    }
}

This is question was posted like ages back, but today when i was looking for the same answer "How to detect zoom in and out event", i couldn't find one answer that would fit all the browsers.

As on now : For Firefox/Chrome/IE8 and IE9 , the zoom in and out fires a window.resize event. This can be captured using:

$(window).resize(function() {
//YOUR CODE.
});

A workaround for FireFox 16+ to find DPPX (zoom level) purely with JavaScript:

var dppx = (function (precision) {
  var searchDPPX = function(level, min, divisor) {
    var wmq = window.matchMedia;
    while (level >= min && !wmq("(min-resolution: " + (level/divisor) + "dppx)").matches) {
      level--;
    }
    return level;
  };

  var maxDPPX = 5.0; // Firefox 22 has 3.0 as maximum, but testing a bit greater values does not cost much
  var minDPPX = 0.1; // Firefox 22 has 0.3 as minimum, but testing a bit smaller values does not cost anything
  var divisor = 1;
  var result;
  for (var i = 0; i < precision; i++) {
    result = 10 * searchDPPX (maxDPPX, minDPPX, divisor);
    maxDPPX = result + 9;
    minDPPX = result;
    divisor *= 10;
  }

  return result / divisor;
}) (5);

The problem lies in the types of monitor used, a 4k monitor vs standard monitor. Chrome is by far the smartest at being able to tell us what the zoom level is just by using window.devicePixelRatio, apparently it can tell what the pixel density is and reports back the same number to matter what.

Other browsers, not so much. IE and Edge are probably the worst, as they handle zoom levels much differently. To get the same size text on a 4k monitor, you have to select 200%, instead of 100% on a standard monitor.

As of May 2018, here's what I have to detect zoom levels for a few of the most popular browsers, Chrome, Firefox, and IE11. I have it tell me what the zoom percentage is. For IE, it reports 100% even for 4k monitors that are actually at 200%, but the text size is really the same.

Here's a fiddle: https://jsfiddle.net/ae1hdogr/

If anyone would care to take a stab at other browsers and update the fiddle, then please do so. My main goal was to get these 3 browsers covered to detect if people were using a zoom factor greater than 100% to use my web application and display a notice suggesting a lessor zoom factor.

here it does not change!:

<html>
 <head>
  <title></title>
 </head>
<body>
 <div id="xy" style="width:400px;">
  foobar
 </div>
 <div>
  <button onclick="alert(document.getElementById('xy').style.width);">Show</button>
 </div>
</body>
</html>

create a simple html file, click on the button. regardless of what zoom level: it will show you the width of 400px (at least with firefox and ie8)

function supportFullCss3()
{
    var div = document.createElement("div");
    div.style.display = 'flex';
    var s1 = div.style.display == 'flex';
    var s2 = 'perspective' in div.style;

    return (s1 && s2);
};

function getZoomLevel()
{
    var screenPixelRatio = 0, zoomLevel = 0;

    if(window.devicePixelRatio && supportFullCss3())
        screenPixelRatio = window.devicePixelRatio;
    else if(window.screenX == '0')
        screenPixelRatio = (window.outerWidth - 8) / window.innerWidth;
    else
    {
        var scr = window.frames.screen;
        screenPixelRatio = scr.deviceXDPI / scr.systemXDPI;
    }

    //---------------------------------------
    if (screenPixelRatio <= .11){ //screenPixelRatio >= .01 &&
      zoomLevel = "-7";
    } else if (screenPixelRatio <= .25) {
      zoomLevel = "-6";
    }else if (screenPixelRatio <= .33) {
      zoomLevel = "-5.5";
    } else if (screenPixelRatio <= .40) {
      zoomLevel = "-5";
    } else if (screenPixelRatio <= .50) {
      zoomLevel = "-4";
    } else if (screenPixelRatio <= .67) {
      zoomLevel = "-3";
    } else if (screenPixelRatio <= .75) {
      zoomLevel = "-2";
    } else if (screenPixelRatio <= .85) {
      zoomLevel = "-1.5";
    } else if (screenPixelRatio <= .98) {
      zoomLevel = "-1";
    } else if (screenPixelRatio <= 1.03) {
      zoomLevel = "0";
    } else if (screenPixelRatio <= 1.12) {
      zoomLevel = "1";
    } else if (screenPixelRatio <= 1.2) {
      zoomLevel = "1.5";
    } else if (screenPixelRatio <= 1.3) {
      zoomLevel = "2";
    } else if (screenPixelRatio <= 1.4) {
      zoomLevel = "2.5";
    } else if (screenPixelRatio <= 1.5) {
      zoomLevel = "3";
    } else if (screenPixelRatio <= 1.6) {
      zoomLevel = "3.3";
    } else if (screenPixelRatio <= 1.7) {
      zoomLevel = "3.7";
    } else if (screenPixelRatio <= 1.8) {
      zoomLevel = "4";
    } else if (screenPixelRatio <= 1.9) {
      zoomLevel = "4.5";
    } else if (screenPixelRatio <= 2) {
      zoomLevel = "5";
    } else if (screenPixelRatio <= 2.1) {
      zoomLevel = "5.2";
    } else if (screenPixelRatio <= 2.2) {
      zoomLevel = "5.4";
    } else if (screenPixelRatio <= 2.3) {
      zoomLevel = "5.6";
    } else if (screenPixelRatio <= 2.4) {
      zoomLevel = "5.8";
    } else if (screenPixelRatio <= 2.5) {
      zoomLevel = "6";
    } else if (screenPixelRatio <= 2.6) {
      zoomLevel = "6.2";
    } else if (screenPixelRatio <= 2.7) {
      zoomLevel = "6.4";
    } else if (screenPixelRatio <= 2.8) {
      zoomLevel = "6.6";
    } else if (screenPixelRatio <= 2.9) {
      zoomLevel = "6.8";
    } else if (screenPixelRatio <= 3) {
      zoomLevel = "7";
    } else if (screenPixelRatio <= 3.1) {
      zoomLevel = "7.1";
    } else if (screenPixelRatio <= 3.2) {
      zoomLevel = "7.2";
    } else if (screenPixelRatio <= 3.3) {
      zoomLevel = "7.3";
    } else if (screenPixelRatio <= 3.4) {
      zoomLevel = "7.4";
    } else if (screenPixelRatio <= 3.5) {
      zoomLevel = "7.5";
    } else if (screenPixelRatio <= 3.6) {
      zoomLevel = "7.6";
    } else if (screenPixelRatio <= 3.7) {
      zoomLevel = "7.7";
    } else if (screenPixelRatio <= 3.8) {
      zoomLevel = "7.8";
    } else if (screenPixelRatio <= 3.9) {
      zoomLevel = "7.9";
    } else if (screenPixelRatio <= 4) {
      zoomLevel = "8";
    } else if (screenPixelRatio <= 4.1) {
      zoomLevel = "8.1";
    } else if (screenPixelRatio <= 4.2) {
      zoomLevel = "8.2";
    } else if (screenPixelRatio <= 4.3) {
      zoomLevel = "8.3";
    } else if (screenPixelRatio <= 4.4) {
      zoomLevel = "8.4";
    } else if (screenPixelRatio <= 4.5) {
      zoomLevel = "8.5";
    } else if (screenPixelRatio <= 4.6) {
      zoomLevel = "8.6";
    } else if (screenPixelRatio <= 4.7) {
      zoomLevel = "8.7";
    } else if (screenPixelRatio <= 4.8) {
      zoomLevel = "8.8";
    } else if (screenPixelRatio <= 4.9) {
      zoomLevel = "8.9";
    } else if (screenPixelRatio <= 5) {
      zoomLevel = "9";
    }else {
      zoomLevel = "unknown";
    }

    return zoomLevel;
};