.w-25 to .w-100, .h-25 to .h-100 and -auto size an element relative to its parent (a percentage height needs a parent of definite height), and .mw-100 and .mh-100 cap it. .vw-100, .vh-100 and .min-vh-100 use the viewport: .d-flex flex-column min-vh-100 on <body> plus .mt-auto on the footer keeps a footer at the bottom of short pages. Prefer .w-100 to .vw-100, because 100vw includes the desktop scroll bar and overflows the page.