Based on <a href='https://math.stackexchange.com/a/739906/37806'>https://math.stackexchange.com/a/739906/37806</a>.<br>
Assuming <input id=len type=number value=6 min=4 max=150 style='width: 50px'> chars and one of each category: upper, lower, digit, special.<br>

<div id=out></div>

<script>
	n_lower = 26;
	n_upper = 26;
	n_digit = 10;
	n_special = 33;
	n_total = 95;

	pow = Math.pow;
	function update() {
		n = pow(n_total, len.value);

		n -= pow(n_total - n_lower, len.value);
		n -= pow(n_total - n_upper, len.value);
		n -= pow(n_total - n_digit, len.value);
		n -= pow(n_total - n_special, len.value);

		n += pow(n_total - n_lower - n_upper, len.value);
		n += pow(n_total - n_lower - n_digit, len.value);
		n += pow(n_total - n_lower - n_special, len.value);
		n += pow(n_total - n_upper - n_digit, len.value);
		n += pow(n_total - n_upper - n_special, len.value);
		n += pow(n_total - n_digit - n_special, len.value);

		n -= pow(n_lower, len.value);
		n -= pow(n_upper, len.value);
		n -= pow(n_digit, len.value);
		n -= pow(n_special, len.value);

		out.innerText = (Math.round(n/1e8)/10).toString() + " billion possible combinations or "
			+ (Math.round(Math.log(n)/Math.log(2)*10)/10).toString() + " bits of entropy.";
	}

	update();

	len.onkeyup = len.onchange = update;
</script>
<noscript>
(javascript is off)
</noscript>