Oct 2016: Fixed. Parentheses in jsbin demo were missing from answer. Oops.
Chrome, Firefox, IE9+. Live Demo on jsbin
var h = document.documentElement,
b = document.body,
st="scrollTop",
sh="scrollHeight";
var percent = (h[st]||b[st]) / ((h[sh]||b[sh]) - h.clientHeight) * 100;
As function:
function getScrollPercent() {
var h = document.documentElement,
b = document.body,
st="scrollTop",
sh="scrollHeight";
return (h[st]||b[st]) / ((h[sh]||b[sh]) - h.clientHeight) * 100;
}
If you prefer jQuery
(original answer):
$(window).on('scroll', function(){
var s = $(window).scrollTop(),
d = $(document).height(),
c = $(window).height();
var scrollPercent = (s / (d - c)) * 100;
console.clear();
console.log(scrollPercent);
})
html{ height:100%; }
body{ height:300%; }
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>