$(window).width() not the same as media query

I am using Twitter Bootstrap on a project. As well as the default bootstrap styles I have also added some of my own

//My styles
@media (max-width: 767px)
{
    //CSS here
}

I am also using jQuery to change the order of certain elements on the page when the width of the viewport is less that 767px.

$(document).load($(window).bind("resize", checkPosition));

function checkPosition()
{
    if($(window).width() < 767)
    {
        $("#body-container .main-content").remove().insertBefore($("#body-container .left-sidebar"));
    } else {
        $("#body-container .main-content").remove().insertAfter($("#body-container .left-sidebar"));
    }
}

The problem I am having is that the width calculated by $(window).width() and the width calculated by the CSS doesn't seem to be the same. When $(window).width() returns 767 the css calculates it the viewport width as 751 so there seems to be a 16px different.

Does anyone know what is causing this and how I could solve the problem? People have suggested that the width of the scrollbar isn't being taken into considering and using $(window).innerWidth() < 751 is the way to go. However ideally I want to find a solution that calculates the width of the scrollbar and that is consistent with my media query (e.g where both conditions are checking against the value 767). Because surely not all browsers will have a scrollbar width of 16px?

If you don't have to support IE9 you can just use window.matchMedia() (MDN documentation).

function checkPosition() {
    if (window.matchMedia('(max-width: 767px)').matches) {
        //...
    } else {
        //...
    }
}

window.matchMedia is fully consistent with the CSS media queries and the browser support is quite good: http://caniuse.com/#feat=matchmedia

UPDATE:

If you have to support more browsers you can use Modernizr's mq method, it supports all browsers that understand media queries in CSS.

if (Modernizr.mq('(max-width: 767px)')) {
    //...
} else {
    //...
}

Check a CSS rule that the media query changes. This is guaranteed to always work.

http://www.fourfront.us/blog/jquery-window-width-and-media-queries

HTML:

<body>
    ...
    <div id="mobile-indicator"></div>
</body>

Javascript:

function isMobileWidth() {
    return $('#mobile-indicator').is(':visible');
}

CSS:

#mobile-indicator {
    display: none;
}

@media (max-width: 767px) {
    #mobile-indicator {
        display: block;
    }
}

It may be due to scrollbar, use innerWidth instead of width like

if($(window).innerWidth() <= 751) {
   $("#body-container .main-content").remove()
                                .insertBefore($("#body-container .left-sidebar"));
} else {
   $("#body-container .main-content").remove()
                                .insertAfter($("#body-container .left-sidebar"));
}

Also you can get the viewport like

function viewport() {
    var e = window, a = 'inner';
    if (!('innerWidth' in window )) {
        a = 'client';
        e = document.documentElement || document.body;
    }
    return { width : e[ a+'Width' ] , height : e[ a+'Height' ] };
}

Above code Source

$(window).width() not the same as media query

UPDATE:

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