让divs保持在父div中，并有空白

i've been looking around to fix this, i havent seen a good answer on why this happens and how i can fix it.. basically i have a div that i set to 100% width and height, inside i have like a tabs section like on a broswer, i added a margin and padding to the main area, and when i set the tabs div to full width it sticks out some pixels, whats the correct way to deal with child divs sticking out of parents divs when playing with %'s and margins/paddingi've been looking around to fix this, i havent