Good post about units. px pixels absolute value em relative from parent 1, 1.2, 2, 5... rem relative from root(html) font-size 1rem, 0.8rem default html font size is 16px do not depend on other elements good for zooming % relative from parent's same property for margin % is taken from parent's width for line-height % is taken from current font-size for width & height % is taken from parent's w/h for width & height with position:fixed % is taken from window's w/h for width & height with position:absolute % % is taken from window's w/h if there is no parent for top right bottom left % is taken from parents height & width (as understood) vw & vh 1% from window width & height 50% means the half of the viewport width & height vmin & vmax min & max from vw & vh vmin is 1% of viewport's smaller side vmax is 1% of viewport's larger side scalable for screen sizes good for phones ex & ch Smth like em , not used widely, just forget it. mm, cm, pt, pc Outdated, also forget it.