You should only ever use % when making a fluid layout. If anything, always stick to px and em. Think of it like this. Percentage is constantly changing proportions (although not technically true for font size etc.) but for a div, based on the size of your browser window, % will adapt to that window size, whereas px is absolute in size. if the height is 100px, it will always be 100px. If the div is 33% in height, that will cover one-third of the height of your window always.