Calculate the square meters / foot of a level

I was wondering how to calculate the square foot of a level. I'm toying around with Jeremy's http://thebuildingcoder.typepad.com/blog/2012/10/room-polygon-and-furniture-picker-in-svg.html and would like to try and create some kind of svg floorplan. But to get the dimensions right for a single room i would need to know the full square meters / foot of level. Im having a hard time getting my head around it, but my guess would be to create a temperary plane by using the dimensions of the room on that floor. Any input would be appriciated.