var windowsize = $(window).outerWidth();
console.log('screen width:',windowsize);

var stanga= jQuery('#secondary').outerWidth();
console.log('left width:',stanga);

var dreapta= jQuery('.right').outerWidth();
console.log('width right:',dreapta);

var content= jQuery('#main-content').outerWidth();
console.log('width content',content);

var suma=stanga+dreapta+content;
console.log('total:',suma);

#main-content{width: calc(100% - 820px)}