The Bitterness Calculator takes into account both the time of boil and the wort gravity during the boil. The numbers are my own, based partly on data and partly on experience. Your experience and brewing practices may be different than mine so here's the official disclaimer: Your Mileage May Vary (YMMV). Have fun with this and don't forget to let me know what you think of it.
Send feedback to: gtinseth (at) yahoo (dot) com