Careful! You've made a mistake! In the link you gave, terminal gives 180/sq mile as the upper limit for a COUNTRY. That is, the average of all the area in the entire kingdom. In the fields it will be much lower and in a city, much higher.
Originally Posted by miinstrel
I've seen figures in various places online saying that in medieval cities, population densities were as high as 30,000/sq mile. A couple things to keep in mind if that seems impossible. First, when people ran out of room, they built up, often adding floors above the ground that were bigger, hanging out over the street. Second, families were larger back then to keep up with infant mortality you might have six or seven people living in one or two rooms. Here is a useful site that I stumbled across just today: http://www222.pair.com/sjohn/blueroom/demog.htm It's an article that explains a lot of the math behind those calculators people are linking. (I know that it is the source of the the math for the defunct calculator terminal linked in his .pdf)
Edit: And, scrolling down the page, it looks like it is the source for the math in the Welsh Piper calculator someone linked a couple posts ago too!