Convert from em to px and vice versa.

empx.js

function unit( scope ) {
    return parseInt( window.getComputedStyle( scope || document.body ).fontSize, 10 );
}

function pxToEm( px, scope ) {
    return ( parseInt( px, 10 ) / unit( scope ) ).toFixed( 8 ) + 'em';
}

function emToPx( em, scope ) {
    return Math.round( parseFloat( em ) * unit( scope ) ) + 'px';
}