#container {
    width: 800px;
}

#fixed {
    position: fixed;
    width: 100%;
}

<div id="container">
    <div id="fixed">Sitename</div>
    <p>
        blaat
    </p>
</div>

#container {
    width: 400px;
    border: 1px solid red;
}

#fixed {
    position: fixed;
    width: inherit;
    border: 1px solid green;
}

<div class="head">

<div id="fx">123</div>

</div>

<div class="head">
    <div class="area_for_fix">
        <div id="fx">123</div>
    </div>
</div>

.head{
        min-width:960px;
        width:960px;
        nax-width:1400px;
        height:300px;
        border: 1px solid #000;
        **position:relative;**
        margin:0 auto;
        margin-top:50px;
        padding:20px;
        text-align:right;
        height:1500px;
    }

    .area_for_fix{

        **position:absolute;**
        right:0;
        width:150px;
        height:200px;
        background-color:red;
    }


    #fx{
        width:150px;
        height:75px;
        background-color:#ccc;
        **position:fixed;**




    }

-webkit-transform: translateZ(0);

var new_width = $('#container').width();
$('#fixed').width(new_width);