Designing a web page for mobile browser, should I calculate the browser bar and android navigation bar?

I was designing a logging page for a mobile webpage, and then I started wondering about the real size inside of an artboard. If I have an artboard with 640px height, do I need to markdown 2 spaces for the browser bar and for the Android navigation bar, and then use the rest of the space between the browser bar and the navigation - or I should use all 640px without marking down the browser and Android navigation bar?