article {width: 46%;}
left-sidebar {width: 25%;}
right-sidebar {width: 25%;}
/* 100% - 46% - 25% * 2 = 4% */
/* ОШИБКА */
article {margin-rigth: 2%;}
left-sidebar {margin-rigth: 2%;}
.article-wrp {width: calc(46% + 2%);}
/* 48% - 100%
2% - x
x = 2 * 100 / 48 = 4,166
*/
.article {width: calc(100% - 4,16%);}
Пример расчета inline-block