formule for tiling

hello evry1
l work for an company and l get a calculation work for few big tiling projeckt.
l just wonder if there is a formule for marjin but it must not in prosent.
l mean hight-lengt=area and hight and lengt from tiles and exact net=m2 and gross=m2 formule.
thanks

does this help at all
http://www.excelforum.com/excel-form...h-formula.html