There's no built-in way, I'm afraid. You can do something like this:

var width = ( 100 * parseFloat($('.largeField').css('width')) / parseFloat($('.largeField').parent().css('width')) ) + '%';

You could put styles you need to access with jQuery in either:

1. the head of the document directly
2. in an include, which server side script then puts in the head

Then it should be possible (though not necessarily easy) to write a js function to parse everything within the style tags in the document head and return the value you need.

You could access the document.styleSheets object:

<style type="text/css">
.largeField {
width: 65%;
}
</style>
<script type="text/javascript">
var rules = document.styleSheets[0].rules || document.styleSheets[0].cssRules;
for (var i=0; i < rules.length; i++) {
var rule = rules[i];
if (rule.selectorText.toLowerCase() == ".largefield") {
alert(rule.style.getPropertyValue("width"));
}
}
</script>

You can use the css(width) function to return the current width of the element.

ie.

var myWidth = $("#myElement").css("width");

See also: http://api.jquery.com/width/ http://api.jquery.com/css/

I have a similar issue in Getting values of global stylesheet in jQuery, eventually I came up with the same solution as above.

Just wanted to crosslink the two questions so others can benefit from later findings.

Most easy way

$('.largeField')[0].style.width


// >>> "65%"

A jQuery plugin based on Adams answer:

(function ($) {


$.fn.getWidthInPercent = function () {
var width = parseFloat($(this).css('width'))/parseFloat($(this).parent().css('width'));
return Math.round(100*width)+'%';
};


})(jQuery);


$('body').html($('.largeField').getWidthInPercent());​​​​​

Will return '65%'. Only returns rounded numbers to work better if you do like if (width=='65%'). If you would have used Adams answer directly, that hadn't worked (I got something like 64.93288590604027). :)

This is most definitely possible!

You must first hide() the parent element. This will prevent JavaScript from calculating pixels for the child element.

$('.parent').hide();
var width = $('.child').width();
$('.parent').show();
alert(width);

See my example.

Now... I wonder if I'm first to discover this hack:)

Update:

One-liner

element.clone().appendTo('body').wrap('<div style="display: none"></div>').css('width');

It will leave behind a hidden element before the </body> tag, which you may want to .remove().

See an example of one-liner.

I'm open to better ideas!

Building on timofey's excellent and surprising solution, here is a pure Javascript implementation:

function cssDimensions(element) {
var cn = element.cloneNode();
var div = document.createElement('div');
div.appendChild(cn);
div.style.display = 'none';
document.body.appendChild(div);
var cs = window.getComputedStyle
? getComputedStyle(cn, null)
: cn.currentStyle;
var ret = { width: cs.width, height: cs.height };
document.body.removeChild(div);
return ret;
}

Hope it's helpful to someone.

Late, but for newer users, try this if the css style contains a percentage:

$element.prop('style')['width'];

There's nothing in jQuery, and nothing straightforward even in javascript. Taking timofey's answer and running with it, I created this function that works to get any properties you want:

// gets the style property as rendered via any means (style sheets, inline, etc) but does *not* compute values
// domNode - the node to get properties for
// properties - Can be a single property to fetch or an array of properties to fetch
function getFinalStyle(domNode, properties) {
if(!(properties instanceof Array)) properties = [properties]


var parent = domNode.parentNode
if(parent) {
var originalDisplay = parent.style.display
parent.style.display = 'none'
}
var computedStyles = getComputedStyle(domNode)


var result = {}
properties.forEach(function(prop) {
result[prop] = computedStyles[prop]
})


if(parent) {
parent.style.display = originalDisplay
}


return result
}

Convert from pixels to percentage using cross multiplication.

Formula Setup:

(element_width_pixels/parent_width_pixels) = (element_width_percentage / 100)

2.) element_width_percentage = (100 * element_width_pixels) / parent_width_pixels

The actual code:

<script>


var $width_percentage = (100 * $("#child").width()) / $("#parent").width();


</script>

A late response but wanted to add on for anyone 2020+ who stumbles across this. Might be more for niche cases but I wanted to share a couple options.

If you know what the initial % value is you can also assign these values to variables in the :root of the style sheet. i.e

:root {
--large-field-width: 65%;
}


.largeField {
width: var(--large-field-width);
}

When you want to access this variable in JS you then simply do the following:

let fieldWidth = getComputedStyle(document.documentElement).getPropertyValue('--large-field-width');
// returns 65% rather than the px value. This is because the % has no relative
// size to the root or rather it's parent.

The other option would be to assign the default styling at the start of your script with:

element.style.width = '65%'

It can then be accessed with:

let width = element.style.width;

I personally prefer the first option but it really does depend on your use case. These are both technically inline styling but I like how you can update variable values directly with JS.