Where  Vm: adsorbed gas volume for one monolayer (ml ) 
                    N:  Avogadro’s number (6.024×1023）
　　              :  area occupied by each nitrogen molecule (0.162nm2 )
　　              W:  powder weight (g)           
　　              （note：At STP, 1 mol gas has a volume of 22.4L or 22400ml ） 

       Plug in N and to the above equation，we obtain;